Dell EMC SC Series Storage: Synchronous Replication and Live

Dell EMC SC Series Storage: Synchronous
Replication and Live Volume
Abstract
This document provides descriptions and use cases for the Dell EMC™
SC Series data protection and mobility features of synchronous
replication and Live Volume.
December 2017
Dell EMC Technical White Paper
Revisions
Revisions
Date
Description
May 2014
Merged synchronous replication and Live Volume documents; updated for Enterprise
Manager 2014 R2 and SCOS 6.5
July 2014
vSphere HA PDL update
November 2015
Updated for SCOS 6.7
July 2016
Updated for SCOS 7.1 and Dell Storage Manager 2016 R2
October 2016
Minor updates
February 2017
Updated guidance on MPIO settings for Windows Server and Hyper-V
July 2017
Minor updates
December 2017
Minor updates to section 3.4.1
Acknowledgments
Authors: Jason Boche, Marty Glaser, Mike Matthews, Dan Tan, Mark Tomczik, and Henry Wong
The information in this publication is provided “as is.” Dell Inc. makes no representations or warranties of any kind with respect to the information in this
publication, and specifically disclaims implied warranties of merchantability or fitness for a particular purpose.
Use, copying, and distribution of any software described in this publication requires an applicable software license.
© 2014–2017 Dell Inc. or its subsidiaries. All Rights Reserved. Dell, EMC, Dell EMC and other trademarks are trademarks of Dell Inc. or its subsidiaries.
Other trademarks may be trademarks of their respective owners.
Dell believes the information in this document is accurate as of its publication date. The information is subject to change without notice.
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Table of contents
Table of contents
Revisions.............................................................................................................................................................................2
Acknowledgments ...............................................................................................................................................................2
Table of contents ................................................................................................................................................................3
Executive summary.............................................................................................................................................................5
1
2
Introduction to synchronous replication ........................................................................................................................6
1.1
Features of SC Series synchronous replication .................................................................................................6
1.2
Synchronous replication requirements ...............................................................................................................7
Data replication primer .................................................................................................................................................8
2.1
3
4
5
6
Synchronous replication features ...............................................................................................................................12
3.1
Modes of operation ...........................................................................................................................................12
3.2
Minimal recopy..................................................................................................................................................14
3.3
Asynchronous replication capabilities ..............................................................................................................14
3.4
Multiple replication topologies ..........................................................................................................................15
3.5
Live Volume ......................................................................................................................................................17
3.6
Dell Storage Manager/Enterprise Manager recommendations ........................................................................17
3.7
Dell Storage Manager/Enterprise Manager DR recovery .................................................................................18
3.8
Support for VMware vSphere Site Recovery Manager ....................................................................................18
Synchronous replication use cases ............................................................................................................................19
4.1
Overview ...........................................................................................................................................................19
4.2
High consistency...............................................................................................................................................19
4.3
High availability .................................................................................................................................................21
4.4
Remote database replicas ................................................................................................................................24
4.5
Disaster recovery ..............................................................................................................................................25
Live Volume overview ................................................................................................................................................34
5.1
Reference architecture .....................................................................................................................................34
5.2
Proxy data access ............................................................................................................................................36
5.3
Live Volume connectivity requirements ............................................................................................................37
5.4
Replication and Live Volume attributes ............................................................................................................39
Data Progression and Live Volume ............................................................................................................................43
6.1
7
3
Primary and secondary Live Volume ................................................................................................................43
Live Volume and MPIO ..............................................................................................................................................44
7.1
8
Replication methods ...........................................................................................................................................8
MPIO policies for Live Volume .........................................................................................................................44
VMware vSphere and Live Volume ............................................................................................................................45
8.1
MPIO .................................................................................................................................................................45
8.2
Single-site MPIO configuration .........................................................................................................................46
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Table of contents
8.3
Multi-site MPIO configuration ...........................................................................................................................48
8.4
VMware vMotion and Live Volume ...................................................................................................................48
8.5
vSphere Metro Storage Cluster ........................................................................................................................49
8.6
Live Volume Failover Automatically..................................................................................................................49
8.7
vMSC storage presentation ..............................................................................................................................50
8.8
Tiebreaker service ............................................................................................................................................52
8.9
Common automatic failover scenarios .............................................................................................................52
8.10 Detailed failure scenarios .................................................................................................................................56
8.11 Live Volume Restore Automatically ..................................................................................................................58
8.12 VMware DRS/HA and Live Volume ..................................................................................................................59
8.13 vMSC and Live Volume Requirements ............................................................................................................63
8.14 VMware and Live Volume Managed Replication..............................................................................................63
9
Live Volume support for Microsoft Windows/Hyper-V ................................................................................................65
9.1
MPIO .................................................................................................................................................................65
9.2
Round Robin .....................................................................................................................................................65
9.3
Round Robin with Subset .................................................................................................................................65
9.4
Failover Only.....................................................................................................................................................65
9.5
Uniform server mappings with Live Volume and Round Robin ........................................................................66
9.6
Hyper-V and Live Volume.................................................................................................................................67
9.7
SCVMM/SCOM and Performance and Resource Optimization (PRO) ............................................................68
9.8
Live Volume and Cluster Shared Volumes .......................................................................................................68
9.9
Live Volume Automatic Failover for Microsoft ..................................................................................................69
9.10 Live Volume with SQL Server...........................................................................................................................73
10 Live Volume with Linux / UNIX ...................................................................................................................................75
10.1 Live Volume and Synchronous Replication ......................................................................................................75
10.2 Live Volume Managed Replication ...................................................................................................................76
10.3 Use cases .........................................................................................................................................................76
11 Use cases ...................................................................................................................................................................84
11.1 Zero-downtime SAN maintenance and data migration.....................................................................................84
11.2 Storage migration for virtual machine migration ...............................................................................................85
11.3 Disaster avoidance ...........................................................................................................................................86
11.4 On-demand load distribution ............................................................................................................................87
11.5 Cloud computing ...............................................................................................................................................88
11.6 Replay Manager and Live Volume ...................................................................................................................89
A
Technical support and additional resources...............................................................................................................90
A.1
4
Related resources ............................................................................................................................................90
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Executive summary
Executive summary
Preventing the loss of data or transactions requires a reliable method of continuous data protection. In the
event of a disaster or unplanned outage, applications and services must be made available at an alternate
site as quickly as possible. A variety of data mobility methods, including asynchronous replication, can
accomplish the task of providing offsite replicas. Synchronous replication sets itself apart from the other
methods by guaranteeing transactional consistency between the production site and the recovery site.
While remote replicas have traditionally provided a data protection strategy for disaster recovery, the disaster
itself and the execution of a disaster recovery (DR) plan involves a period of downtime for organizations.
Replicas along with storage virtualization can provide other types of data mobility that fit a broader range of
proactive high availability use cases without an outage.
This guide focuses on two of the main data protection and mobility features available with Dell EMC™ SC
Series storage: synchronous replication and Live Volume. In this paper, each feature is discussed and use
cases are highlighted where these technologies fit independently or together.
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Introduction to synchronous replication
1
Introduction to synchronous replication
While SC Series storage supports both asynchronous and synchronous replication, this document focuses
primarily on synchronous replication.
By definition, synchronous replication ensures data is written and committed to both the replication source
and destination volumes in real time. The data is essentially written to both locations simultaneously. In the
event that the data cannot be written to either of the locations, the write I/O will not be committed to either
location, ensuring transactional consistency, and a write I/O failure will be issued to the storage host and
application where the write request originated. The benefit synchronous replication provides is guaranteed
consistency between replication sites resulting in zero data loss in a recovery scenario.
Dell Storage advises customers to understand the types of replication available, their applications, and their
business processes before designing and implementing a data protection and availability strategy.
1.1
Features of SC Series synchronous replication
Mode migration: Existing replications may be migrated to an alternate type without rebuilding the replication
or reseeding data.
Live Volume support: Live Volumes may leverage any available type of replication offered with SC Series
storage including both modes of synchronous (high consistency or high availability) and asynchronous.
Live Volume Managed Replication: Live Volume allows an additional synchronous or asynchronous
replication to a third SC Series array that can be DR activated using Dell Storage Manager (DSM) or
Enterprise Manager (EM).
Preserve Live Volume (Manual Failover): In the event an unplanned outage occurs impacting availability of
a primary Live Volume, the secondary Live Volume can be promoted to the primary Live Volume role
manually using DSM or EM.
Live Volume Failover Automatically: In the event an unplanned outage occurs impacting availability of a
primary Live Volume, the secondary Live Volume can be promoted to the primary Live Volume role
automatically.
Live Volume Restore Automatically: After Live Volume Automatic Failover has occurred, Live Volume pairs
may be automatically repaired after the impacted site becomes available.
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Introduction to synchronous replication
1.2
Synchronous replication requirements
Replicating volumes between SC Series systems requires a combination of software, licensing, storage, and
fabric infrastructure. The following sections itemize each requirement.
1.2.1
Dell Storage Manager/Enterprise Manager
Enterprise Manager (EM) was rebranded as Dell Storage Manager (DSM) in 2016. DSM 2016 R2 or newer is
required to leverage all available replication and Live Volume features.
1.2.2
Storage Center OS
Storage Center OS (SCOS) 7.1 or newer is required to leverage all available replication and Live Volume
features.
1.2.3
Licensing
Replication licensing, which includes synchronous replication and asynchronous replication, is required for
each SC Series array participating in volume replication. Additionally, a Live Volume license for each array is
required for all Live Volume features.
1.2.4
Supported replication transport
SC Series systems support array-based replication using either Fibre Channel or iSCSI connectivity. A
dedicated network is not required but a method of isolation for performance and/or security should be
provided. Synchronous replication typically requires more bandwidth and less latency than asynchronous
replication due to sensitivity of applications and end users where the impacts of high latency will be felt.
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Data replication primer
2
Data replication primer
Data replication is one of many options that exist to provide data protection and availability. The practice of
replication evolved out of a necessity to address a number of matters such as substantial data growth,
shrinking backup windows, more resilient and efficient disaster recovery solutions, high availability, mobility,
globalization, cloud, and regulatory requirements. The common requirement is to maintain multiple copies of
data and make them highly available and easily accessible. Traditional backup methods satisfied early data
protection requirements, but this feasibility diminished as data sets and other availability constraints grew.
Vanishing backup windows, ecommerce, and exponential growth of transactions brought about the need for
continuous data protection (CDP). Replicas are typically used to provide disaster recovery or high availability
for applications and data, to minimize or eliminate loss of transactions, to provide application and data locality,
or to provide a disposable data set that can be internally developed or tested. At a higher level, data
protection translates to guarding the reputation of an organization by protecting end-user data.
2.1
Replication methods
There are a number of replication approaches, but two methods stand out as highly recognized today:
asynchronous and synchronous. SC Series arrays support a flexible variety of replication methods that fall in
the category of asynchronous or synchronous.
2.1.1
Synchronous
Synchronous replication guarantees data consistency (zero data loss) between the replication source and
destination. This is achieved by ensuring write I/O commitments at the replication source and destination
before a successful write acknowledgement is sent back to the storage host and the requesting application. If
the write I/O cannot be committed at the source or destination, the write will not be committed at either
location to ensure consistency. Furthermore, a write failure is sent back to the storage host and its
application. Application error handling will then determine the next appropriate step for the pending
transaction. By itself, synchronous replication provides CDP. Coupled with hardware redundancy, application
clustering, and failover resiliency, continuous availability for applications and data can be achieved.
Because of the method used in synchronous replication to ensure data consistency, any issues impacting the
source or destination storage, or the replication link inbetween, will adversely impact applications in terms of
latency (slowness) and availability. This applies to Live Volumes built on top of synchronous replications as
well. For this reason, appropriate performance sizing is paramount for the source and destination storage, as
well as the replication bandwidth and any other upstream infrastructure that the storage is dependent on.
Figure 1 demonstrates the write I/O pattern sequence with synchronous replication:
1.
2.
3.
4.
5.
6.
The application or server sends a write request to the source volume.
The write I/O is mirrored to the destination volume.
The mirrored write I/O is committed to the destination volume.
The write commit at the destination is acknowledged back to the source.
The write I/O is committed to the source volume.
Finally, the write acknowledgement is sent to the application or server.
The process is repeated for each write I/O requested by the application or server.
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Data replication primer
Synchronous replication write I/O sequence
2.1.2
Asynchronous
Asynchronous replication accomplishes the same data protection goal in that data is replicated from source
storage to destination storage. However, the manner and frequency that the data is replicated differs from
synchronous replication. Instead of committing a write at both replication source and destination
simultaneously, the write is committed only at the source and an acknowledgement is then sent to the storage
host and application. The accumulation of committed writes at the source volume are replicated to the
destination volume in one batch at scheduled intervals and committed to the destination volume.
Aside from replicating the active snapshot (Replay) (semi-synchronous replication is discussed in section
2.1.3), Asynchronous replication in SC Series storage is tied to the source volume replication schedule. When
a snapshot (Replay) is created on the source volume, and that volume is configured for asynchronous
replication, the new snapshot is replicated to the destination volume. Snapshots on a volume may be created
automatically according to a schedule or manually created from a variety of integration tools. Regardless, all
snapshots occur on a per-volume basis. As a result, volumes may adhere to their own independent replication
schedule, or they may share a replication schedule with other volumes leveraging the same snapshot profile.
This type of replication is also referred to as a point-in-time replication, which is a type of asynchronous
replication that specifically leverages volume snapshots. Because asynchronously replicated transactions are
not required to wait for write committals at the replica destination volume, the replication link and/or
destination storage will not contribute to application or transaction latency at the source volume.
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Data replication primer
Figure 2 demonstrates the write I/O pattern sequence with respect to asynchronous replication.
1. The application or server sends a write request to the source volume.
2. The write I/O is committed to the source volume.
3. Finally, the write acknowledgement is sent to the application or server.
The process is repeated for each write I/O requested by the application or server.
4. Periodically, a batch of write I/Os that have already been committed to the source volume are
transferred to the destination volume.
5. The write I/Os are committed to the destination volume.
6. A batch acknowledgement is sent to the source.
Asynchronous replication write I/O sequence
2.1.3
Semi-synchronous
With SC Series storage, semi-synchronous replication behaves like synchronous replication in that application
transactions are immediately sent to the replication destination storage (assuming that the replication link and
destination storage have the bandwidth to support the current rate of change). The difference is that the write
I/O is committed at the source volume and an acknowledgement is sent to the storage host and application
without a guarantee that the write I/O was committed at the destination storage. Semi-synchronous replication
is configured in Dell Storage Manager (DSM) or Enterprise Manager (EM) by creating asynchronous
replication between two volumes and checking the box for Replicate Active Snapshot (DSM) or Replicate
Active Replay (EM). A snapshot (EM) or Replay (DSM) is a SC Series storage term that describes frozen
data. The Active Snapshot (DSM) or Active Replay (EM) refers to newly written or updated data that has not
yet been frozen in a snapshot. Semi-synchronous offers a synchronous-like recovery point objective (RPO)
without application latency, but the RPO and loss of data in an unplanned outage scenario cannot be
guaranteed.
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Data replication primer
Figure 3 demonstrates the write I/O pattern sequence with semi-synchronous replication.
1. The application or server sends a write request to the source volume.
2. The write I/O is committed to the source volume.
3. The write acknowledgement is sent to the application or server.
The process is repeated for each write I/O requested by the application or server.
For each write I/O that completes that process, there is an independent and parallel process:
a. The write request is sent to the destination.
b. The write I/O is committed to the destination.
c. The write acknowledgement of the mirror copy is sent to the source array.
The commits at the source and destination volumes are not guaranteed to be in lockstep with each other.
Semi-synchronous replication write I/O sequence
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Synchronous replication features
3
Synchronous replication features
SC Series storage supports a wide variety of replication features. Each feature is outlined in the following
sections.
3.1
Modes of operation
A number of evolutionary improvements have been made to enhance synchronous replication with SC Series
arrays. Among these improvements are choice in replication mode on a per-volume basis. Synchronous
replication can be configured in one of two modes: high consistency or high availability.
3.1.1
Legacy
Synchronous replications created prior to SCOS 6.3 are identified as legacy after upgrading to SCOS 6.3 and
newer. Legacy synchronous replications cannot be created in SCOS 6.3 or newer and do not possess the
newer synchronous replication features currently available. To upgrade a legacy synchronous replication to
synchronous high consistency or synchronous high availability replication, a legacy synchronous replication
must be deleted and recreated after both source and destination SC Series arrays have SCOS 6.3 or newer
installed. Deleting and recreating a synchronous replication will result in data inconsistency between the
replication source and destination volumes until 100 percent of the initial and journaled replication is
completed.
3.1.2
High consistency
Synchronous high consistency mode rigidly follows the storage industry specification of synchronous
replication outlined earlier and shown in Figure 1. The mechanisms involved with this method of replication
will guarantee data consistency between the replication source and destination volumes unless an
administrator pauses the replication for maintenance or other reasons. Latency can impact applications at the
source volume if the replication link or replication destination volume is unable to absorb the amount of data
being replicated or the rate of change. Furthermore, if write transaction data cannot be committed to the
destination volume, the write will not be committed on the source volume and in effect, a transaction involving
a write fails. An accumulation of write failures will likely result in an application failure or outage when a
tolerance threshold is crossed. For these reasons, application latency and high availability are important
points to consider in a storage design proposing synchronous replication in high consistency mode.
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Synchronous replication features
3.1.3
High availability
Synchronous high availability mode bends the rules of synchronous replication by relaxing the requirements
associated with high consistency mode. While the replication link and the replica destination storage are able
to absorb the write throughput, high availability mode performs like high consistency mode (described in
section 3.1.2 and illustrated in Figure 1). Data is consistently committed at both source and destination
volumes and excess latency in the replication link or destination volume will be observed as application
latency at the source volume.
The difference between high consistency and high availability mode is that data availability will not be
sacrificed for data consistency. What this means is that if the replication link or the destination storage either
becomes unavailable or exceeds a latency threshold, the SC Series array will automatically remove the dual
write committal requirement at the destination volume. This allows application write transactions at the source
volume to continue, with no downstream latency impacts, instead of write I/O being halted or slowed, which is
the case with high consistency mode and legacy synchronous replication. This relaxed state is referred to as
being “out of date”. If and when an SC Series array enters the out-of-date state, inconsistent write I/O will be
journaled at the source volume. When the destination volume becomes available within a tolerable latency
threshold, journaled I/O at the source volume is flushed to the destination volume where it will be committed.
During this process, incoming application writes continue to be written to the journal. After all journaled data is
committed to the destination volume, the source and destination will be in sync and the data on both volumes
will be consistent. When the source and destination volumes are in sync, downstream latency will return
within the application at the source volume. Similar to the high consistency mode, application latency and
data consistency are important points to consider in a design that incorporates synchronous replication in high
availability mode.
High availability mode synchronous replication in an out-of-date state
3.1.4
Mode migration
In SCOS 6.5 or newer, replications may be migrated from one mode to another without manually having to
destroy the replication and destination replica volumes, and then rebuild. This includes migrations such as
asynchronous to synchronous high consistency, synchronous high consistency to synchronous high
availability, or synchronous high availability to asynchronous. Leveraging the mode migration feature can
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Synchronous replication features
save significant time and replication bandwidth. It also reduces the data availability risk exposure associated
with the time taken to destroy and rebuild a replica volume. Lastly, this method preserves predefined DR
settings in Dell Storage Manager or Enterprise Manager that are tied to restore points and replica volumes.
For all of these reasons, individually or combined, it is recommended to take full advantage of this feature.
Note: This feature is compatible with all replication modes except legacy synchronous replication.
3.2
Minimal recopy
As discussed in section 3.1.3, synchronous replications configured in high availability mode allow write
access to the source volume if the destination volume becomes unavailable or falls behind. While out of date,
a journalizing mechanism shown in Figure 4 tracks the write I/O that makes the source and destination
volumes inconsistent. Prior to SCOS 6.3 with legacy replication, journaling was not performed and if the
destination volume became unavailable and then later available, all data on the source volume needed to be
re-replicated to the destination to get back in sync. However, with the minimal recopy feature, only the
changed data contained in the journal is replicated to the destination volume in order to bring the source and
destination volumes back in sync. This dramatically reduces the recovery time and data inconsistency risk
exposure as well as the replication link bandwidth consumed to recover. Minimal recopy is also employed in
high consistency mode should the destination volume become unavailable during initial synchronization or an
administrator invoked a pause operation on the replication.
Flushing journaled writes to the destination volume to regain volume consistency
3.3
Asynchronous replication capabilities
Synchronous replication has seen numerous improvements over time and includes key features that were
previously associated only with asynchronous replication.
3.3.1
Snapshots (Replays) and consistency groups
The most notable asynchronous feature is the replication of SC Series snapshots (Replays). In the past, only
the active snapshot or active Replay data was replicated from source to destination. With snapshots
automatically replicated to the destination site, customers have more flexibility in recovery options with many
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Synchronous replication features
historical restore points to choose from. By virtue of having snapshot functionality, synchronous replication
can be integrated with consistency groups and Replay-Manager-created Replays across volumes to enable
snapshot interval consistency across replicated volumes. In high consistency mode, snapshot/Replay
consistency will be guaranteed. In high availability mode, snapshot/Replay consistency is highly likely.
3.3.2
Pause
Synchronous replications configured in either high consistency or high availability modes can be paused
without impacting availability of applications relying on the replication source volume. Pausing replication can
facilitate multiple purposes. For example, it can be used to relieve replication link bandwidth utilization. In
designs where replication bandwidth is shared, other processes can temporarily be given burstable priority.
Pausing may also be preferred in anticipation of a scheduled replication link or fabric outage.
3.4
Multiple replication topologies
Dell extends synchronous replication support beyond just a pair of SC Series volumes residing in the same or
different sites. A choice of two topologies or a hybrid combination of both is available.
3.4.1
Mixed topology
The mixed topology, also known as 1-to-N (N=2 as of SCOS 6.5), allows a source volume to be replicated to
two destination volumes where one replication is synchronous or asynchronous and the additional replications
are asynchronous. The maximum number of additional replications is set by the value of N. This topology is
useful when data must be protected in multiple locations. If data recovery becomes necessary, a flexible
choice of locations is available for recovery.
Mixed topology
If the volume replication source becomes unavailable, volume replication stops.
The source volume of a replication becomes unavailable and replication stops
For recovery purposes, the replica can be activated and mapped by Dell Storage Manager to a storage host
(for instance, at a disaster recovery site).
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Synchronous replication features
Furthermore in a mixed topology, a replica volume may be configured to replicate to another one of the
replicas (asynchronous or synchronous) without having to reseed a majority of the data both volumes would
already have before the original source volume became unavailable. This may be useful where two or more
disaster recovery sites exist.
After DR activation, a replica volume can be replicated to another replica with efficiency
3.4.2
Cascade topology
The cascade topology allows asynchronous replications to be chained to synchronous or asynchronous
replication destination volumes. This topology is useful in providing immediate reprotection for a recovery site.
Similar to the mixed topology, it provides a flexible choice of locations for data recovery or business
continuation practices. It could also be used as a means of providing replicas of data in the same data center
or a remote site. Copies of Microsoft® SQL Server® or Oracle® databases for parallel test, dev, or QA
environments are popular examples of this.
Cascade topology
3.4.3
Hybrid topology
A hybrid topology can also be created by combining mixed and cascade topology types. This configuration is
adaptable to virtually any replica or data protection needs a business may require.
Hybrid topology
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3.5
Live Volume
The Live Volume feature, which is discussed in detail later in this document, is built on replication. In versions
of SCOS prior to 6.5, Live Volume was supported only with asynchronous replication. With SCOS 6.5 and
newer, Live Volume is designed to work in conjunction with asynchronous and synchronous replication types.
In addition, Live Volume supports many of the current synchronous replication features such as modes of
operation and mode migration.
3.5.1
Preserve Live Volume
In SCOS 6.5 or newer, recovering data from a secondary Live Volume, when the primary Live Volume is
unavailable, is faster, easier, and more flexible. Secondary Live Volumes may be promoted to the primary
Live Volume role that preserves volume identity and storage host mappings. Alternatively, data on a
secondary Live Volume may be recovered by creating a new a View Volume and then mapping that View
Volume to one or more storage hosts.
3.5.2
Live Volume Failover Automatically
SCOS 6.7 introduced automatic failover for Live Volumes. Depending on the nature of the unplanned outage,
the recovery process is similar the Preserve Live Volume feature except that it is completely automated and
occurs within a matter of seconds, and at scale.
3.5.3
Live Volume Restore Automatically
When an unplanned outage occurs, Live Volume Restore Automatically provides high availability for
configured volumes. However, at this point, volume availability is at risk should a second unplanned event
impact the remaining system. If the original outage is minimal in scope and the site can be brought back
online, the Live Volume Restore Automatically feature repairs the Live Volume back to its original redundant
state with no administrator intervention required.
3.5.4
Live Volume Managed Replication
A Live Volume Managed Replication is an additional replication and replica volume that uses the primary Live
Volume as its replication source. The Live Volume Managed Replication may be synchronous or
asynchronous depending on the Live Volume configuration. To maintain data integrity and consistency, when
a Live Volume swap role occurs automatically or manually, the Live Volume Managed Replication persistently
follows the primary Live Volume as its source of replication.
Live Volume Managed Replication before and after swap role
3.6
Dell Storage Manager/Enterprise Manager recommendations
Dell Storage Manager (DSM) or Enterprise Manager (EM) periodically checks the status of replication and
records the progress of completeness. In the event of a failure at the source site, DSM/EM provides a safe
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Synchronous replication features
recommendation on the use of the destination replica. When using high consistency synchronous replication,
data between source and destination must be consistent for DSM/EM to advise it is safe to use the
destination replica for recovery.
When using high availability synchronous replication (or high consistency with the ability to pause replication),
the data between source and destination volumes may or may not be consistent depending on whether the
replication was in sync or out of date at the time of the failure. If at the time of failure replication was in sync,
DSM/EM will advise that the destination replica volume is data consistent and safe to use for recovery.
Conversely, if the synchronous replication was out of date, this means journaled transactions at the source
volume likely have not been replicated to the destination and the destination replica is not data consistent and
not recommend for use. At this point, the data recovery options would be to use a data consistent snapshot (if
using DSM) or Replay (if using EM) as the recovery point or continue with using the inconsistent replica. In
either case, the most recent transactions will have been lost at the destination but recovering from a snapshot
or Replay will provide a precise point in time as the recovery point.
3.7
Dell Storage Manager/Enterprise Manager DR recovery
Synchronous replication volumes are supported in the scope of the DSM or EM predefined disaster recovery
and DR activation features. Those that have used this feature with asynchronously replicated volumes in the
past can extend the same disaster recovery test and execution processes to synchronously replicated
volumes. DSM/EM and its core functionality is freely available to SC Series customers, making it an attractive
and affordable tool for improving recovery time objectives. Note that DR settings cannot be predefined for
Live Volumes, nor can Live Volume restore points be test activated.
3.8
Support for VMware vSphere Site Recovery Manager
Standard asynchronous or synchronous (either mode) replication types can be leveraged by VMware®
vSphere® Site Recovery Manager protection groups, recovery plans, and reprotection.
SRM version 6.1 support for stretched storage with Live Volume has been added in DSM 2016 R1. Supported
configurations are Synchronous High Availability or Asynchronous replication with non-uniform storage
mapping to hosts. For more information on use cases and integrating stretched storage with SRM, please see
the Site Recovery Manager Administration documentation provided by VMware.
Uniform storage mapping, Managed Replication, Consistency Groups, Live Volume Auto Role Swap, and Live
Volume Auto Failover are not supported with SRM.
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4
Synchronous replication use cases
Replicating data can be a valuable and useful tool, but replication by itself serves no purpose without tying it
to a use case to meet business goals. The following sections highlight sample use cases for synchronous
replication.
4.1
Overview
Array-based replication is typically used to provide upper tier application high availability or disaster recovery,
a data protection process to enable image or file-level backup and recovery, or a development tool to
generate copies of data in near or remote locations for application development or testing purposes. For
many business use cases, asynchronous replication provides a good balance of meeting recovery point
objective (RPO) and recovery time objective (RTO) service level agreements without a cost-prohibitive
infrastructure such as dark fibre, additional networking hardware, or additional storage. This is why
asynchronous replication is often used between data centers where longer distances are involved.
However, there are an increasing number of designs where a strong emphasis is placed on the prevention of
data loss. Regardless of where the need originates, the method of replication that satisfies zero transaction
loss is synchronous. The next few sections highlight examples of synchronous replication with a focus on high
consistency for zero data loss or high availability for relaxed data consistency requirements.
4.2
High consistency
The primary need for synchronous replication is preventing data loss or guaranteeing data consistency
between the source and destination replica volume. Synchronous replication provides the same data
protection benefits for both proactive and reactive use cases. Refer to section 3 for more detail on
synchronous high consistency replication operational characteristics.
4.2.1
VMware and Hyper-V
When virtualized, server workloads in the data center are encapsulated into a small set of files that represent
the virtual BIOS, virtual hardware resources, and the virtual disks that provide read and write access to data.
The I/O profile is dependent on the virtual machine role and the applications and services running within it.
Virtual machines work particularly well with replication because their compute resources are portable and
hardware independent by nature. Their inherent mobility, combined with storage replication, allows them to
easily migrate from one site to another, with comparatively little effort required to bring them online at the
destination site. Virtual machines may be relocated for load balancing or disaster avoidance/recovery
purposes. Whatever the reason for relocation, high consistency synchronous replication will ensure that the
contents of the virtual machine at the source and destination match. In the event the vSphere or Microsoft®
Hyper-V® virtual machine needs to be migrated to a host or cluster of hosts at the destination site, data
consistency of the virtual machine being brought up at the destination site is guaranteed. Disaster recovery is
covered in more detail in section 4.5.
For more information on configuring a pre-defined DR plan for Hyper-V, refer to the Enterprise Manager DR
Plan for Hyper-V Demo Video on Dell TechCenter.
Note that Dell Storage Manager/Enterprise Manager DR plans cannot be predefined with Live Volumes.
Predefined DR plans are supported with regular (asynchronous or synchronous) volume replications or with a
managed (cascaded or hybrid) asynchronous replication from a Live Volume.
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High consistency synchronous with consolidated vSphere or Hyper-V sites
A replication link or destination volume issue in St. Paul results in a VM outage in Minneapolis
4.2.2
Microsoft SQL Server and Oracle Database/Oracle RAC
Database servers and clusters in critical environments are often designed to provide highly available, large
throughput, and low latency access to data for application tier servers and sometimes directly to application
developers or end users. Database servers differ from virtual machines in that protection of the database
volumes is paramount while protection of the operating system is not required for data recovery. However,
booting from SAN and replicating that SAN volume to a remote site with compatible hardware can drastically
improve RTO. Similar to virtual environments, the critical data may be spread across multiple volumes. When
designing for performance, application, or instance isolation, this is often the case. Unless the replication is
paused, consistency between volumes is guaranteed in high consistency mode because the write order at the
destination will mirror the write order at the source, otherwise the write will not happen in either location (see
Figure 15). This is the fundamental premise of high consistency mode detailed in section 3.1.2.
High consistency synchronous with databases
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A replication link or destination volume issue in St. Paul results in database outage in Minneapolis
To summarize, there are high consistency use cases that can be integrated with virtualization as well as
database platforms. The key benefit being provided is data consistency and zero transaction loss. Keep in
mind that the infrastructure supporting synchronous replication between sites must perform sufficiently. In the
case of high consistency, the supporting infrastructure must be highly redundant and immune to outages for
slowness or an outage of the replication link or the destination site is reflected equally at the source site
where the end user applications are running.
An important factor when considering the type of replication to be implemented is that the infrastructure
required to keep two sites well connected, particularly at greater distances, often comes at a premium.
Stakeholders may be skeptical about implementing a design where a failure at the secondary site or a failure
of the connection between sites can have such a large impact on application availability and favor
asynchronous replication over synchronous replication. However, with the high availability synchronous
replication offered with SC series storage, customers have additional flexibility compared to legacy
synchronous replication.
4.3
High availability
Many organizations prefer asynchronous replication for its cost effectiveness and its significant reduction in
risk of an application outage, should the destination storage become unavailable. The high availability
synchronous replication mode in SC Series arrays provide data consistency throughout normal uptime
periods. However, if unexpected circumstances arise resulting in a degradation or outage of the replication
link or destination storage, latency or loss of production application connectivity at the replication source is not
at risk. While this deviates slightly from the industry-recognized definition of synchronous replication, it adds
flexibility that is not found in high consistency synchronous by blending desirable features of both
synchronous and asynchronous replication. In addition, SC Series storage automatically adapts to shifting
destination replica availability. Refer to section 3 for more detail on high availability synchronous replication
operational characteristics.
4.3.1
VMware and Hyper-V
Encapsulated virtual machines are replicated in a data consistent manner as they are when using high
consistency mode replication. The difference of behavior comes into play if the replication link (or the
destination replica volume) becomes burdened with excess latency or is unavailable. Instead of failing writes
from the hypervisor, the writes are committed and journaled at the source volume, allowing applications to
continue functioning but at the expense of a temporary lack of data consistency while the destination volume
is unavailable.
In the following examples, note that using high availability mode in place of high consistency mode does not
automatically allow the design to be stretched over further distances without consideration to application
latency. High availability mode is still a form of synchronous replication and should not be confused with
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asynchronous replication. Adding significant distance between sites generally results in latency increases
which will still be observed in the applications at the source side for as long as the high availability replication
is in sync.
Finally, if virtual machines are deployed in a configuration that spans multiple volumes, consider using Replay
Manager or consistency groups. Replay Manager is covered in section 11.6.
High availability synchronous with consolidated vSphere or Hyper-V sites
A replication link or destination volume issue in St. Paul results in no VM outage in Minneapolis
4.3.2
Microsoft SQL Server and Oracle Database/Oracle RAC
As discussed in section 4.3.1, the behavioral delta between high availability and high consistency modes is
minimal until extreme latency or an outage impacts the destination volume availability. If high availability
synchronous replication falls into an out-of-date state, write I/O at the source volume is journaled and the
destination volume becomes inconsistent. In terms of recovery, this may or may not be acceptable. A feature
in Dell Storage Manager (DSM) or Enterprise Manager (EM) advises customers on whether or not the active
snapshot or Replay on the destination volume is safe to recover from at a data consistency level. In the event
that DSM or EM detects the data is not consistent, the recommendation is to revert to the most recent
consistent snapshot or Replay associated with the destination volume (another new feature in synchronous
replication: replication of snapshots/Replays).
For storage hosts with data confined to a single volume, special considerations are not necessary. However,
if the host has application data spread across multiple volumes (for example, a VM with multiple virtual
machine disk files, or a database server with instance or performance isolation of data, logs, and other files)
then it becomes critical to ensure snapshot/Replay consistency for the replicated data that will be used as a
restore point. Ensuring all volumes of a dataset are quiesced and then snapped at precisely 8:00, for
example, provides a data consistent restore point across volumes supporting the dataset. This
snapshot/Replay consistency is accomplished with Replay Manager (especially recommended for Microsoft
products through VSS integration) or by containerizing volumes by use of consistency groups.
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To create consistency across snapshots/Relays using consistency groups, a snapshot/Replay profile is
created with a snapshot/Replay Creation Method of Consistent (Figure 18). This profile is then applied to all
volumes containing the dataset. For virtual machines, the volumes would contain the virtual disks
representing the c: drive, d: drive, or Linux mount points such as / or /tmp. For Microsoft SQL Server
database servers, the volumes may represent system databases, application databases, transaction logs,
and tempdb. For Oracle databases, the dataset must contain all volumes contining any part of the database
(data, index, data dictionary, temporary files, control files, online redo logs, and optionally offline redo logs).
For Oracle RAC, OCR files or voting disks can be added to the dataset. For either database platform,
separate volumes for hot dumps, archived redo logs, or boot from SAN may exist but typically would not need
to be included in a consistency group with the key database files.
Creating a consistency group in Dell Storage Manager (top) or Enterprise Manager (bottom)
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Another method of capturing consistency in snapshots/Replays across volumes (and perhaps more useful for
customers with Microsoft Windows, SQL Server, Exchange, Hyper-V, or VMware vSphere) would be to use
Replay Manager. Replay Manager has the underlying storage integration and VSS awareness required to
create application consistent Replays, across volumes if necessary, which can then be replicated
synchronously (either mode) or asynchronously.
Once data is frozen with consistency across volumes using Replay Manager or consistency groups, those
Replays will be replicated to the destination volume where they can serve as historical restore points for high
availability mode recovery, disaster recovery, or remote replicas which will be discussed in the coming
sections.
High availability synchronous with databases
A replication link or destination volume issue in St. Paul results in no database outage in
Minneapolis
4.4
Remote database replicas
One practice commonly found in organizations with Microsoft SQL Server or Oracle database technologies is
to create copies of databases. There are several reasons to clone databases, and most of them stem from
the common principle of minimal or no disruption to the production database, and thus to the application and
end users. To identify a few examples, at least one separate copy of a database is maintained for application
developers to develop and test code against. A separate database copy is maintained for DBA staff to test
index changes, queries, and for troubleshooting areas such as performance. A copy of the production
database may be maintained for I/O intensive queries or reporting. SC Series storage snapshots/Replays and
View Volumes are a natural tactical fit for fulfilling database replica needs locally on the same SC Series
array.
However, if the replica is to be stored on a different array, whether or not it is in the same building or
geographic region, replication or portable volume must be used to seed the data remotely, and replication
should be used to refresh the data as needed. For the purposes of developer or DBA testing, asynchronous
replication may be timely enough. However, for reporting purposes, synchronous replication will ensure the
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data is up to date when the reporting database is refreshed using the replicated volumes. The choice of
providing zero data loss through high consistency mode or a more flexible high availability mode should be
decided ahead of time with the impacts of each mode well understood.
SC Series snapshots/Replays, as well as asynchronous and synchronous replication, are natively space and
bandwidth efficient on storage and replication links respectively. Only the changed data is frozen in a
snapshot/Replay and replicated to remote SC Series arrays. In the following figure, notice the use of high
availability synchronous replication within the Minneapolis datacenter. Although the two arrays are well
connected, the risk of internal reporting database inconsistency does not warrant a production outage for the
organization.
Database replicas distributed in a mixed topology
4.5
Disaster recovery
With data footprints growing exponentially, backup and maintenance windows shrinking along with the cost of
storage, and the impact of downtime gnawing on the conscience of businesses, migrating to online storagebased data protection strategies is trending for a variety of organizations. Legacy processes which dumped
data to tape were once cost effective and acceptable, but the convergence of key decision-making factors has
prompted a shift from yesterday’s nearline storage to the more affordable and efficient online storage of
today. Data replication within or between sites is the ubiquitous backbone for much more scalable data
protection strategies. With replication in place as the data mover, an assortment of vendor and platform
provided tools and methods can be coupled to replication to form a manual or electronically documented and
reliable recovery process. The SC Series support for multiple replication topologies really comes into play in
the disaster recovery conversation because it adds a lot of flexibility for customers wanting to provide data
protection for multiple or distributed site architectures. Before getting into platform-specific examples, two
fundamental disaster recovery metrics need to be understood as they will be referenced throughout business
continuation planning discussions.
Recovery point objective (RPO): This is the acceptable amount of data loss measured by time or the
previous point in time at which data is recovered from. An RPO is negotiated with business units and
predefined in a disaster recovery plan. In terms of replication, the keys to achieving an RPO target are
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choosing the appropriate replication type, making sure replication is current (as opposed to out of date or
behind), and knowing the tools and processes required to recovery from a given restore point.
Recovery time objective (RTO): This is the elapsed recovery time allowed to bring up a working production
environment. Just like RPO, RTO is negotiated with business units and predefined in a disaster recovery plan
and may also be included in a service level agreement (SLA). The keys to achieving targeted RTO may vary
from data center to data center but they all revolve around process efficiency and automation tools wherever
possible. Replication is a quintessential contributor to meeting RTO, especially at large scale.
By leveraging replication, aggressive RPOs and RTOs can be targeted. Data footprint and rate of change
growth may be continuous, but feasible RPO and RTO goals do not linearly diminish as long as the replication
infrastructure (this includes network, fabric, and storage) can scale to support the amount of data being
replicated and the rate of change.
4.5.1
Hyper-V and VMware
As discussed in previous sections, replicating the file objects that form the construct of a virtual machine takes
advantage of the intrinsic encapsulation and portability attributes of a VM. Along with hardware
independence, these attributes essentially mean the VM can be moved to any location where a supported
hypervisor exists, and a VM or group of VMs are quickly and easily registered and powered on depending on
the hypervisor and the automation tools used to perform the cutover. Compare this to legacy methods of
disaster recovery in which physical or virtual servers are built from the ground up at the recovery site,
applications needed to be installed and configured, and then large amounts of data needed to be restored
from tape. Instead, at the time a disaster is declared, virtual machines and their configured applications are
essentially ready to be added to the hypervisor’s inventory and powered on. Virtualization and replication
shave off massive amounts of recovery time, which helps achieve targeted RTO. When the VMs are powered
on, their application data payload from the most recently completed replication is already present, meeting the
RPO component of the DR plan.
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Virtualization and replication combined meet aggressive RTOs and RPOs
4.5.2
Microsoft SQL Server and Oracle Database/Oracle RAC
Aside from infrastructure servers such as Microsoft Active Directory®, LDAP, DNS, WINS, and DHCP,
database servers are among the first assets to be recovered. Typically, databases are classified as Tier 1
infrastructure (Tier 1 assets included in a DR plan receive first recovery priority) and database servers are the
first tier that must be brought online in a multi-tier application architecture. The next online is the application
tier servers, and then the application front end (on either client desktops or a load-balanced web portal).
RTO is a paramount metric to meet in testing and executing a live business continuation plan. In a DR plan,
all steps are predefined and executed in order according to the plan; some steps may be carried out in
parallel. Successfully recovered database servers are a required dependency beginning early in the DR plan.
This includes bringing up application and web servers that have a critical tie to the database server. The more
databases a shared database server hosts, the broader the impact because the number of dependent
application and front end tiers fan out.
Industry analysis reflects data growing at alarming rates across many verticals. Providing performance and
capacity is not a challenge with current technology, but protecting the data is. Data growth drives changes in
technology and strategy so that SLAs, RTOs, and RPOs can still be maintained even though they were
defined when data was a fraction of the size it is today. Restoring 10 TB of data from tape is probably not
going to satisfy a 24-hour RTO. Data growth on tapes means that there is a growing number of sequentialaccess, long-seek time tapes for restoration. This diminishes the chances of meeting RTO, and increases the
chances that one bad tape will cause data recovery to fail. Data replication is a major player in meeting RTO.
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Intra-volume consistency is extremely important in a distributed virtual machine disk or database volume
architecture. Comparing the synchronous replication modes, high consistency guarantees data consistency
between sites across all high consistency replicated volumes. Unfortunately, this is at the cost of destination
site latency, or worse, downtime of the production application if the destination volume becomes unavailable
or exceeds latency thresholds.
Outside of use cases that require the textbook definition of synchronous replication, high availability mode (or
asynchronous) may be a lot more attractive for DR purposes. This mode offers data consistency in the proper
conditions, as well as some allowance for latency while in sync. However, consistency is not guaranteed if
production application uptime is jeopardized should the destination volume become unavailable.
Because consistency cannot be guaranteed in high availability mode, it is important to implement VSSintegrated Replay Manager Replays or consistency groups with high availability synchronous replication
where a multiple volume relationship exists (this is commonly found in both SQL Server and Oracle
environments). While this will not guarantee active snapshot/Replay consistency across volumes, the next set
of frozen snapshots/Replays that have been replicated to the remote array should be consistent across
volumes.
4.5.3
Preparing and executing volume and data recovery
With the appropriate hypervisor, tools, and automation in the DR plan, powering on a virtual machine is
relatively simple. Likewise, preparing the volumes for use and getting the database servers up and running
requires a quick process compared to legacy methods. This is especially true when replicating database
server boot from SAN (BFS) volumes with similar hardware at the DR site.
When choosing to access volumes at the DR site, it is important to consider the purpose. Is this a validation
test of the DR plan, or is this an actual declared disaster? As an active replication destination target
(regardless of asynchronous, synchronous, mode, or topology), destination volumes cannot be mounted for
read/write use to a storage host at the DR site. (For circumstances involving Live Volume, refer to section 7).
To perform a test of the DR plan, present view volumes from the snapshots/Replays of each test volume to
the storage hosts. Snapshots/Replays and view volumes are available for both asynchronous and
synchronous replications in either high consistency or high availability mode. Snapshots/Replays and view
volumes are beneficial during DR testing because replication continues between the source and destination
volumes to maintain RPO in case an actual disaster occurs during the test. Conversely, if a disaster is being
declared and the Activate Disaster Recovery feature is invoked, then replication from source to destination
needs to be halted (if it has not been already by the disaster) if the active volume at the destination site is
intended for data recovery in the DR plan.
Dell Storage Manager (DSM) or Enterprise Manager (EM)is a unified management suite available to SC
Series and FS8600 customers. DSM/EM has disaster recovery features built into it that can create, manage,
and monitor replications, as well as automate the testing and execution of a predefined DR plan.
DSM/EM bundles DR automation tools to help meet RTO requirements
For virtualization and database use cases alike, DSM/EM is used to create asynchronous or synchronous
replications. These replications predefine the destination volumes that will be presented to storage hosts for
disaster recovery.
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Note: Destination replica volumes are for DR purposes only and should not be used actively in a Microsoft or
VMware vSphere Metro Storage Cluster design.
In most cases, predefined destination volumes are data volumes. Where physical hosts are involved, boot
from SAN volumes can also be included in the pre-defined DR plan to quickly and effortlessly recover
physical hosts and applications as opposed to rebuilding and installing applications from scratch. Rebuilding
takes significant time, is error prone, and may require subject matter expert knowledge of platforms,
applications, and the business depending on how well detailed the build process is in the DR plan. Confusion
or errors during test or DR execution lead to high visibility failure. Detailed and current DR documentation
provides clarity at the DR site. Process inconsistency and errors are mitigated by automation or closely
following DR documentation. With these points in mind, the benefit of automating a DR plan with DSM/EM (or
a similar tool) is clear. From the moment a disaster is declared by the business, the RTO is in jeopardy;
automation of tasks saves time and provides process consistency.
Predefining a DR plan in Dell Storage Manager/Enterprise Manager
Once the volumes are prepared and presented manually or automatically by DSM/EM or API scripting, the
process of data recovery continues. For SQL Server and Oracle database servers, databases are attached
and various scripts are run to prepare the database server and applications for production use (such as to
resync login accounts). For VMware vSphere and Hyper-V hosts, VM datastores are now visible to the hosts
and VMs need to be added to the inventory so that they can be allocated as compute and storage resources
by the hypervisor and then powered on.
In Hyper-V 2008 R2, the configuration file for each virtual machine must be generated with the correct number
of processors, memory, network card, and attached virtual machine disk files. This is a process that is
documented or scripted prior to the DR event. Hyper-V 2012/R2 includes a virtual machine import wizard that
is able to import the existing virtual machine configuration located on replicated Dell storage, rather than
generating a new configuration for each VM from scratch. Once the VMs are added to inventory in Hyper-V
2008 R2 or Hyper-V 2012/R2, they can be powered on. All versions of VMware vSphere have the same
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capability as Hyper-V 2012/R2 in that once datastores are presented to the vSphere hosts, the datastores can
be browsed and the virtual machine configuration file that is located in each VM folder can be added to
inventory and then powered on. A manual DR process, especially in an environment with hundreds or
thousands of virtual machines, quickly eats into RTO. The automation of discovering and adding virtual
machines to inventory is covered in the next section.
VMware vSphere Site Recovery Manager is a disaster recovery and planned migration tool for virtual
machines. It bolts onto an existing vSphere environment and leverages Dell certified storage and array based
replication. Both synchronous and asynchronous replication are supported as well as each of their native
features. Stretched storage with SRM 6.1 is also supported by Live Volume in specific configurations. With
this support, customers can strive for RPOs that are more aggressive and maintain compatibility with third
party automation tools, like SRM, to maintain RTOs in large VMware virtualized environments.
For vSphere environments, SRM invokes the commands necessary for tasks (such as managing replication,
creating snapshots/Replays, creating view volumes, and presenting and removing volumes from vSphere
hosts) to be performed at the storage layer without removing DSM/EM from the architecture. The storagerelated commands from SRM flow to the Storage Replication Adapter (SRA) and then to the DSM/EM server.
For this reason, an DSM/EM server needs to remain available at the recovery site for the automation to be
carried out. Aside from SRM, in a heterogeneous data center, DSM/EM or API scripting would be needed to
carry out the DR automation for Hyper-V or physical hosts.
Beyond the scope of storage, SRM automates other processes of DR testing, DR recovery, and planned
migrations, making it a major contributor to meeting RTO goals. SRM takes care of important, timeconsuming tasks such as adding virtual machines to inventory at the DR site, modifying TCP/IP address
configurations, VM dependency, power-on order, and reprotection of virtual machines.
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VMware vSphere Site Recovery Manager and SC Series active/active architecture
The DSM or EM server must be available to perform DR testing or an actual DR cutover for an automated DR
solution involving SC Series storage replication. This means making sure that at least one DSM/EM server
resides at the recovery site so that it can be engaged when needed for DR plan execution. The DSM/EM
server labeled as a physical server in Figure 22, also represents a virtualization candidate if it is already
powered on and not required to automate the recovery of the vSphere infrastructure where it resides.
4.5.4
Topologies and modes
Replication of data between or within data centers is the fastest, most efficient, and automated method for
moving large amounts of data in order to provide replica data, data protection, and business continuation in
the event of a disaster. This section provides examples of the different topologies and modes available with
synchronous replication in addition to appropriate uses of asynchronous replication.
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DR within a campus, standard topology, Fibre Channel replication
Metro DR site, standard topology, Fibre Channel replication
Metro and remote DR sites, mixed topology (1-to-N), Fibre Channel and iSCSI replication
Intra campus and metro DR sites, cascade topology, Fibre Channel and iSCSI replication
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Intra-campus, metro, and remote DR sites; hybrid topology; Fibre Channel and iSCSI replication
The examples in Figure 26 through Figure 30 serve to represent physical hosts or virtual machines. Some
notable details in the examples are:





33
A 1U DSM or EM server is racked at each recovery site to facilitate DR testing and cutover
automation through DSM/EM, VMware Site Recovery Manager, or both. The DSM/EM shown is for
logical representation only. DSM/EM may be a virtual machine.
SC Series storage supports Fibre Channel and iSCSI-based replication.
SC Series storage supports asynchronous replication as well as multiple modes of synchronous
replication.
Either mode of synchronous replication latency will impact production applications relying on the
replication source volume. The infrastructure design should be sized for adequate replication
bandwidth, controllers, and storage at the recovery site to efficiently absorb throughput.
The examples provide adequate hardware and data center redundancy at the recovery site when
implementing high consistency synchronous replication. Aside from a pause operation, unavailability
of a destination replica volume leads to unavailability of the source volume and will result in a
production application outage.
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Live Volume overview
5
Live Volume overview
Live Volume is a high availability and data mobility feature for SC Series storage that builds on the Dell Fluid
Data architecture. It enables non-disruptive data access, data migration, and stretched volumes between SC
Series arrays. It also provides the storage architecture and automatic failover required for VMware vSphere
Metro Storage Cluster certification (vMSC) with SCOS 6.7 and newer. SCOS 7.1 extended Live Volume
automatic failover (LV-AFO) support to Microsoft Server 2012/Hyper-V 2012 and newer clusters.
5.1
Reference architecture
Live Volume migration
Live Volume is a software-defined high availability solution that is integrated into the SC Series controllers. It
is designed to operate in a production environment that allows storage hosts and applications to remain
operational during planned volume migration or data movement regardless of the geographical distance
between arrays. The automatic failover and restore features available for vSphere Metro Storage Cluster
integration with SCOS 6.7 and newer, and for Microsoft environments with SCOS 7.1 and newer, provides
high availability for applications and services during planned or unplanned events in the data center.
Live Volume increases operational efficiency, eliminates the need for planned storage outages, and enables
planned migrations as well as disaster avoidance and disaster recovery. The Live Volume feature provides
powerful options:







34
Allows storage to follow mobile virtual machines and applications in virtualized environments within a
data center, campus, or across larger distances
Supports automatic or manual mechanisms to migrate virtual machine storage as virtual machines
are migrated within or across hypervisor clusters
Zero application downtime occurs for planned maintenance outages
Enables all data to be moved non-disruptively between arrays to achieve full planned or unplanned
site shutdown without downtime
Provides on-demand load balancing; Live Volume enables data to be relocated as desired to
distribute workload between arrays
Stretches Microsoft clustered volumes between geographically disperse locations
Allows VMware vSphere and Microsoft Clusters to see the same disk signature on the volume
between data centers and allows the volume to be clustered across arrays
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Live Volume overview

Supports asynchronous or synchronous replication and included features such as:
-



Snapshots/Replays
High consistency and high availability modes
Mode migration
DR activation for Live Volume Managed Replications
Supports an additional asynchronous or synchronous Live Volume replication to a third array created
and dynamically managed by Live Volume
Provides automatic or manual Live Volume failover and restore in the event of an unplanned outage
at a primary Live Volume site
Includes VMware vSphere Metro Storage Cluster (vMSC) certification with SCOS 6.7 and newer
Live Volume is designed to fit into existing physical and virtual environments without disruption or significant
changes to existing configurations or workflow. Physical and virtual servers see a consistent, unchanging
virtual volume. Volume presentation is consistent and transparent before, during, and after migration. The
Live Volume role swap process can be managed automatically or manually and is fully integrated into the
array and Dell Storage Manager (DSM) or Enterprise Manager (EM). Live Volume operates asynchronously
or synchronously and is designed for application high availability during planned migration, resource
balancing, disaster avoidance, and disaster recovery use cases.
A Live Volume can be created between two SC Series arrays residing in the same data center or between
two well-connected datacenters with Fibre Channel or iSCSI replication connectivity.
Using DSM/EM, a Live Volume and an optional Live Volume Managed Replication can be created from a new
volume, an existing volume, or an existing replication. For more information on creating a Live Volume, see
the appropriate Dell Storage Manager or Enterprise Manager User Guide provided with the DSM/EM
software.
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Live Volume overview
Creating a replica as a Live Volume
5.2
Proxy data access
An SC Series Live Volume is comprised of a pair of replication enabled volumes: a primary Live Volume and
a secondary Live Volume. A Live Volume can be accessed through either array supporting the Live Volume
replication. However, the primary Live Volume role can only be active on one of the available arrays. All read
and write activity for a Live Volume is serviced by the array hosting the primary Live Volume. If a server is
accessing the Live Volume through uniform or non-uniform paths to the secondary Live Volume array, the I/O
is passed through the Fibre Channel or iSCSI replication link to the primary Live Volume system.
In Figure 33, a mapped server is accessing a Live Volume by proxy access through the secondary Live
Volume system to the primary Live Volume system. This type of proxy data access requires the replication
link between the two arrays to have enough bandwidth and minimum latency to support the I/O operations
and latency requirements of the application data access.
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Live Volume overview
Proxy data access through the Secondary Live Volume
5.3
Live Volume connectivity requirements
Live Volume connectivity requirements vary depending on intended use. For example, there are different
requirements for using Live Volume to migrate a workload depending on whether or not the virtual machines
are powered on during the migration.
From the Live Volume perspective and outside of automatic failover with vSphere Metro Storage Cluster or
Microsoft Server/Hyper-V clusters, there are no firm restrictions on bandwidth or latency. However, to proxy
data access from one SC Series array to another requires the Live Volume arrays to be connected through a
high-bandwidth, low-latency replication link. Some operating systems and applications require disk latency
under 10 ms for optimal performance. However, performance impact may not be effectively realized until disk
latency reaches 25 ms or greater. Some applications are more latency sensitive. This means that if average
latency in the primary data center to the storage is 5 ms for the volume and the connection between the two
data centers averages 30 ms of latency, the storage latency writing data to the primary Live Volume from the
secondary Live Volume proxy across the link is probably going to be 35 ms or greater. While this may be
tolerable for some applications, it may not be tolerable for others.
If the Live Volume proxy communication or synchronous replication is utilized, it is strongly recommended to
leverage site-to-site replication connectivity which offers consistent bandwidth and the least amount of
latency. The amount of bandwidth required for the connectivity is highly dependent on the amount of changed
data that requires replication, as well as the amount of other traffic on the same wire. If a site is not planning
to proxy data access between arrays with asynchronous replication, then latency is not as much of a concern.
It is recommended to use dedicated VLANs or fabrics to isolate IP-based storage traffic from other types of
general-purpose LAN traffic, especially when spanning data centers. While this is not a requirement for Live
Volume, it is a general best practice for IP-based storage.
For hypervisor virtualization products such as VMware vSphere, Microsoft Hyper-V, and Citrix XenServer, a
site must have at least a 1 GB connection with 10 ms or less latency between servers to support vMotion
Metro or live migration activities. Standard VMware vMotion requires 5 ms or less latency between the source
and destination host.
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Live Volume overview
5.3.1
High bandwidth, low latency
For an inter-datacenter, campus environment (or within a 60-mile radius as an example), high-speed fiber
connectivity is possible. While inter-datacenter and campus environments may be able to run fiber speeds of
up to 16 Gb using Multi-mode fiber connectivity, single-mode fiber connectivity of up to 1 Gb using dark fibre
can assist with connecting data centers together that may be up to 60 miles apart as an example. Minimal
latency is especially key when implementing Live Volume on top of synchronous replication (in either mode).
This type of connectivity is highly recommended for live migrating virtual machine workloads between arrays
and required for synchronous Live Volume with automatic failover used in vSphere Metro Storage Cluster
configurations or Microsoft Windows Server/Hyper-V clusters.
5.3.2
Low bandwidth, high latency
If a site is planning on using Live Volume on a low-bandwidth, high-latency replication link, it is recommended
to control swap role activities manually by shutting down the application running at site A, perform a Live
Volume swap role, and then bring the application up at the remote site. This scenario prevents any storage
proxy traffic from going across the link, as well as providing a pause in replication I/O for the link allowing the
replication to catch up so that a Live Volume swap role can occur as quickly as possible. Manual swap role
activities can be controlled by deselecting the Automatically Swap Roles option on the Live Volume
configuration. If the replication connection has characteristics of high latency, asynchronous replication is
recommended for Live Volume so that applications are not adversely impacted by replication latency. Note
that Live Volume automatic failover supports synchronous replication in high availability mode only.
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Live Volume overview
5.4
Replication and Live Volume attributes
Once a Live Volume is created, additional attributes can be viewed and modified by editing the replication
properties of the Live Volume. To modify the Live Volume settings, select Replication & Live Volumes from
Dell Storage Manager (DSM) or Enterprise Manager (EM), and then select Live Volumes as depicted in
Figure 34.
Live Volume settings
5.4.1
Replication information
Live Volume is built on standard SC Series storage replicated volumes in which each replicated volume can
individually be configured as asynchronous, synchronous high availability, or synchronous high consistency.
Note that Live Volume automatic failover requires synchronous high availability mode. More information about
these attributes can be found throughout this document and in the Dell Storage Manager or Enterprise
Manager Administrator’s Guide.
Type: This refers to asynchronous or synchronous replication.
Sync Mode: If the replication type is synchronous, Sync Mode describes the mode of synchronous replication
that may be either high consistency or high availability. Sync Mode is not displayed for asynchronous Live
Volumes.
Sync Status: If the replication type is synchronous, Sync Status describes the current state of the
synchronous replication as Current or Out Of Date. When Sync Status is Current, synchronous replication is
in sync. This means that both the source and destination volumes are consistent and the cumulative latency
to the secondary Live Volume will be observed at the primary Live Volume application. An out-of-date status
indicates that the data on the source and destination volumes is not consistent. Changed or inconsistent data
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Live Volume overview
is tracked in a journal where the primary Live Volume resides until the two volumes are once again Current.
Sync Status is not displayed for asynchronous Live Volumes.
Deduplication: The Deduplication feature replicates only the changed portions of the Replay history on the
source volume, rather than all data captured in each Replay. While this is a more processor-intensive activity,
it may reduce the amount of replication traffic and bandwidth required. If sufficient bandwidth is present on the
connection, Dell Storage recommends disabling Deduplication for Live Volumes in order to preserve controller
CPU time for other processes.
Replicate Active Snapshot/Replay: It is recommend that Replicate Active Snapshot/Replay is enabled for
asynchronous Live Volumes. This ensures that data is replicated in real time as quickly as possible which
decreases the amount of time required to perform a Live Volume Swap role. For Live Volumes configured
with either mode of synchronous replication, the Active Snapshot/Replay is effectively replicated in real time
providing the replication is in sync (HA mode) and not paused by an administrator (HA and HC modes).
Replicate Storage to Lower Tier: The Replicate Storage to Lowest Tier feature is automatically enabled for
a new Live Volume. Disable this option to replicate data to Tier 1 on the destination array. Many users
perform the initial Live Volume replication to the lowest tier, and then deselect this option once the initial
replication completes. This strategy aids in preserving Tier 1 storage capacity, which is useful when using
15K spindles or SSD drives. For more information on Data Progression with Live Volume, see the section,
Data Progression and Live Volume.
QoS Nodes: A pair of QoS Nodes depicts the desired egress traffic shaping to be applied when replicating
from the primary to the secondary Live Volume. Although labeled a secondary QoS Node, it does not provide
ingress traffic shaping. Instead, it provides egress traffic shaping after a role swap occurs and it becomes the
primary Live Volume. QoS Nodes apply to replication traffic only. The Live Volume proxy traffic between the
arrays is not governed by QoS Nodes. If the link between the Live Volume storage controllers is shared by
other traffic, it may be necessary to throttle the replication traffic using QoS Nodes to prevent it from flooding
the replication link. However, throttling synchronous replication traffic will produce latency for applications
which are dependent on the Live Volume. For this reason, replication links and QoS Nodes should be sized
appropriately taking into account the amount of data per Live Volume, rate of change, application latency
requirements, and any other applications or services that may be sharing the replication link. This is
especially important when using synchronous replication. Note that vSphere Metro Storage Cluster latency
between sites should not exceed 10 ms.
For instance, if a 20 Gbps replication link exists between data centers that is shared by all intra-data-center
traffic, a replication QoS could be set at 10 Gbps and thereby limits the amount of bandwidth used by
replication traffic to half of the pipe capacity. This allows the other non-Live-Volume replication traffic to
receive a reasonable share of the replication pipe, but could cause application latency if synchronous
replication traffic exceeds 1,000 MBps.
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Live Volume overview
Creating a Live Volume with QoS nodes
As a best practice, common QoS Nodes should not be shared between a single SC Series source and
multiple SC Series destinations, particularly where Live Volume Managed Replications are in use. For
instance, if volume A is replicating to volume B synchronously to support a Live Volume and volume A is also
replicating to volume C asynchronously to support a Live Volume Managed Replication, an independent QoS
Node should be created and used for each of these replications.
5.4.2
Live Volume information
A Live Volume provides additional attributes that control the behavior of the Live Volume. Those attributes are
listed below.
Automatically Swap Roles: When Automatically Swap Roles is selected, the Live Volume will be
automatically swapped to the array with the most I/O load as long as it meets the conditions for a swap. The
Live Volume logic gathers I/O samples to determine the primary access to the Live Volume (from either
servers accessing it directly on the primary Live Volume array, or servers accessing from a secondary Live
Volume array). Samples are taken every 30 seconds and automated role swap decisions are based on the
last ten samples (five minutes worth). This occurs continuously on the primary Live Volume array (it does not
start once the 30-minute delay timer expires).
The autoswap design is meant to make intelligent decisions on the autoswap movement of Live Volume
primary systems while preventing role swap movement from occurring rapidly back and forth between arrays.
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Min Amount Before Swap: The first aspect is the amount of data accessed from a secondary system. If
there is light, infrequent access to a Live Volume from a secondary array, does it make sense to move the
primary to that system? If so, set this value to a very small value. The criteria for this aspect are defined by
the Min Amount Before Swap attribute for the Live Volume. The value specifies an amount of (read/write
access)/second per sample value. If a sample shows that the secondary array access exceeds this value, this
sample/aspect has been satisfied.
Min Secondary Percent Before Swap: The second aspect is the percentage of total access of a Live
Volume from the secondary array on a per-sample basis. The criteria for this aspect are defined by the Min
Secondary Percent for Swap attribute for the Live Volume. If a sample shows the secondary array accessed
the Live Volume more than the defined setting for this aspect, this sample/aspect has been satisfied. The
default setting for this option is 60 percent. The SC Series array takes samples every 30 seconds and keeps
the most recent ten samples (five minutes' worth) for analysis. This means that the secondary Live Volume
has to have more I/O than the primary system for six out of ten samples (60 percent).
Min Time As Primary Before Swap
Each Live Volume has a TimeAsPrimary (default setting of 30 minutes) timer that will prohibit an autoswap
from occurring after a role swap has completed. This means that following a role swap of a Live Volume
(either auto or user specified), the SC Series array waits the specified amount of time before analyzing data
for autoswap conditions to be met again. The purpose of this is to prevent thrashing of autoswap in
environments where the primary access point could be dynamic or when a Live Volume is shared by
applications that can be running on servers both at the primary and secondary sites.
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Data Progression and Live Volume
6
Data Progression and Live Volume
Data Progression lifecycles are storage profiles that are managed independently on each SC Series array
configured with Live Volume. If a Live Volume is not replicated to the lowest tier on the destination array, data
ingestion follows the volume-based storage profile. The Data Progression lifecycle on the destination array
then moves data based on the destination storage profile. If a Live Volume is replicated to the lowest tier on
the destination array, data ingestion occurs in the lowest tier of storage. The Data Progression lifecycle on the
destination array then moves data based on the destination storage profile.
6.1
Primary and secondary Live Volume
If a primary Live Volume is located on volume A and is not being replicated to the lowest tier on the
destination volume B, the data will progress down on the destination array to the next tier or RAID level every
12 days. This is because the data on the destination array is never actually being read. All reads take place
on the primary Live Volume array.
For instance, if an array has two tiers of storage and the storage profiles write data at RAID 10 on Tier 1,
snapshot/Replay data at Tier 1 is at RAID 5, and Tier 3 is at RAID 5. The blocks of data that were written
during the day will progress from Tier 1, RAID 10 to Tier 1, RAID 5 on the first night.
If a primary Live Volume is frequently swapped between the Live Volume arrays, then the Data Progression
pattern will be determined by how often the data is accessed on both systems.
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Live Volume and MPIO
7
Live Volume and MPIO
By using Live Volume with a storage host that has access to both SC Series arrays in a Live Volume
configuration, multiple paths can be presented to the server through each SC Series controller. Live Volume
data access from the secondary system is proxied to the primary array by replication ports. For this reason,
special consideration should be taken to understand and optimize I/O paths for the Live Volume.
7.1
MPIO policies for Live Volume
For Live Volume arrays where a server has volume access through both the primary and secondary Live
Volume controllers (uniform storage presentation), it may be desirable to configure the MPIO policy to prevent
primary data access through the secondary Live Volume array. Instead, they would be reserved as failover
paths only. These types of MPIO policies are typically Failover Only, Fixed. Examples include asynchronous
replication link slowness or a QoS node that adds too much latency to proxied traffic.
A Round Robin MPIO policy may also be used with Live Volume. There may be slight deviations based on the
storage host, but in general, Round Robin utilizes all available paths to a device.
Standalone servers or clustered servers accessing shared storage may use uniform storage presentation.
Uniform storage presentation provides host to Live Volume storage paths through both arrays. This means
that half of the I/O will go through the primary Live Volume and the other half with be proxied through the
replication link through the secondary Live Volume. Automatic role swap should be disabled when using
Round Robin in conjunction with uniform storage presentation.
Clustered servers accessing shared storage may also use non-uniform storage presentation. Non-uniform
means that each cluster node will access a Live Volume through one array or the other, but not both. Nonuniform typically applies to a stretched cluster configuration where each node in the cluster accesses the Live
Volume through paths that are local in fabric proximity only. Each cluster node would have read/write access
to either the primary or the secondary Live Volume, but not both simultaneously. For the cluster nodes which
have access to the primary Live Volume, their front-end I/O will remain local in proximity. For the cluster
nodes which have access to the secondary Live Volume, their front end I/O will be proxied to the primary Live
Volume through the replication link. Implementing Round Robin with non-uniform presentation is typically
preferred to provide automated port and fabric balance but fixed paths may also be used. Automatic role
swap would be preferred with non-uniform storage presentation so that a role swap will automatically follow
the vMotion of virtual machines between sites.
Regardless of storage presentation, MPIO path selection, and role swap policies, synchronous replication
latency between arrays will impact applications. Additional information on configuring Live Volume MPIO can
be found in each of the sections of this document devoted to a specific application, such as VMware vSphere,
Windows/Hyper-V, Linux, Solaris, and AIX.
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VMware vSphere and Live Volume
8
VMware vSphere and Live Volume
When vSphere and Live Volume combine, they can provide VMware-certified, large-scale application and
service mobility, high availability, planned maintenance, resource balancing, disaster avoidance, and disaster
recovery options for virtual environments.
8.1
MPIO
vSphere ships with three MPIO policies: Fixed, Round Robin, and Most Recently Used (MRU should not be
used with SC Series storage). When mapping a Live Volume uniformly through both the primary and
secondary SC Series arrays to a vSphere host (in a single data center, for instance), the MPIO policy on the
host should be set to Fixed with the preferred path set to the primary Live Volume SC Series controller.
Fixed policy
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VMware vSphere and Live Volume
The following figure depicts a Round Robin policy on a uniformly presented Live Volume between two
SC Series arrays. The Round Robin I/O traffic pattern in a uniform Live Volume storage presentation may be
sufficient for applications but not optimal because 50 percent of the I/O traffic has to traverse the Live Volume
Replication proxy link. This is because SC Series reports all available paths to both the primary and the
secondary as optimal to the Round Robin PSP.
Round Robin policy
8.2
Single-site MPIO configuration
In a single-site or uniform configuration, multiple vSphere hosts may be zoned to both arrays. If a Live Volume
is mapped over both arrays to the vSphere hosts, then the volume can participate in an MPIO configuration
involving both arrays. In a Live Volume configuration, the optimal I/O path is through the primary Live Volume.
In this scenario, using the vSphere Fixed MPIO policy and Live Volume automatic role swap ensures the
majority of front-end I/O traffic is going through the primary Live Volume array. The preferred path is always
used in this policy unless the primary path fails, at which time the Fixed policy migrates I/O to one of the other
available paths in the policy.
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VMware vSphere and Live Volume
As depicted in Figure 38, a Live Volume replication exists between SC Series A and SC Series B. Two
vSphere hosts are mapped to the Live Volume on each array. The Primary Live Volume is located on SC
Series A, so the Fixed Preferred path (Figure 36) on each vSphere host is configured to use a path to SC
Series A as the preferred path.
Uniform Live Volume replication
If planned downtime will impact SC Series A, the preferred path for the vSphere hosts could be configured for
SC Series B. When the Live Volume is configured for automatic role swap, this change in preferred paths will
trigger the Live Volume to swap roles making SC Series B the Primary Live Volume controller, so that SC
Series A can be taken offline without a disruption to virtual machines on that Live Volume.
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8.3
Multi-site MPIO configuration
In a multi-site or non-uniform stretched cluster configuration, the vSphere hosts are zoned and mapped only
to their local corresponding array (see Figure 39).
Non-uniform multi-site MPIO configuration
In this configuration, the MPIO policy for the primary Live Volume can be configured as either Fixed or Round
Robin as the Live Volume mappings are non-uniform and do not include multiple SC Series arrays within the
same data center. Enabling Live Volume automatic role swap would be optimal in this configuration where
vMotion is used to migrate virtual machines between sites. Read and write I/O stemming from VMware Host B
to the secondary Live Volume is proxied to the Primary Live Volume controller through the asynchronous or
synchronous replication link(s) between the arrays. Automatic role swap allows SC Series storage to optimize
the I/O after vMotion so that all or the majority is flowing through the primary Live Volume.
8.4
VMware vMotion and Live Volume
In a non-uniform storage presentation model, one method of controlling the location of the primary Live
Volume is to use vMotion and configure Live Volume automatic role swap (see Figure 39). vSphere Host A is
mapped to SC Series A and vSphere Host B is mapped to SC Series B. When a virtual machine running on a
Live Volume is migrated from Host A to Host B, Live Volume with automatic role swap observes that the
storage access has shifted from SC Series A to SC Series B. The SC Series array automatically swaps the
Secondary Live Volume to become the Primary Live Volume. Once this occurs, the virtual machine disk I/O
traverses a local path to the Primary Live Volume on SC Series B (using the Fixed or Round Robin MPIO
policy) instead of going through the Secondary Live Volume proxy across the replication link. The result
evades an inevitably higher cost in terms of increased latency. For stretched clusters or data centers,
consider the vMotion and Metro vMotion latency requirements between hosts. vMotion requires a round-trip
latency of 5 ms or less between hosts on the vMotion network. vSphere Metro Storage Cluster boosts the
allowable latency from 5 ms to 10 ms round-trip latency on the vMotion network between sites.
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VMware vSphere and Live Volume
8.5
vSphere Metro Storage Cluster
vSphere Metro Storage Cluster (vMSC) is a specific VMware-certified design and an implementation of
vSphere, compute, storage, and network infrastructure. The design is typically a stretched cluster
configuration over varying distances to provide high availability for virtualized workloads. This is achieved
through continuous availability of compute and storage resources provided by a combination of hardware
redundancy and automatic failover capabilities built into the vSphere hypervisor and the Dell SC series Live
Volume architecture. With the configuration of the Live Volume automatic failover feature, and by meeting
other solution requirements, SC Series storage is certified by VMware in supporting vSphere Metro Storage
Cluster in uniform, non-uniform, Fibre Channel, and iSCSI designs.
8.6
Live Volume Failover Automatically
Configuring Live Volume for synchronous high availability replication and automatic failover (Figure 40) is a
key requirement for vMetro Storage Cluster deployments. Live Volume and replication may be configured on
a per-volume basis using the SC Series vSphere web client plug-in or DSM or EM. The Failover Automatically
feature for Live Volume is configured using DSM or EM. Live Volume automatic failover is currently supported
with vSphere 5.5 or 6.0 VMFS datastores and virtual mode RDMs, with support for physical mode RDMs
added with the release of SCOS 7.1.
Live Volume configured for Synchronous High Availability and Failover Automatically enabled
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8.7
vMSC storage presentation
vSphere Metro Storage Cluster supports uniform or non-uniform storage presentation and fabric designs.
Within the vMSC entity, one presentation model should be implemented consistently for all vSphere hosts in
the stretched cluster. Mixing storage presentation models within a stretched cluster is not recommended. It
may work but it has not been formally or thoroughly tested.
8.7.1
vMSC uniform storage presentation
Uniform presentation (Figure 41) tends to be a more common design, which provides host-to-Live-Volume
storage paths through both arrays. This means that half of the I/O will go through the primary Live Volume
and the other half with be proxied through the replication link through the secondary Live Volume. Uniform
presentation helps provide a better level of uptime for virtual machines if Live Volume automatic failover
occurs because of an unplanned storage or fabric outage. Automatic role swap should be disabled when
using the Round Robin PSP in conjunction with uniform storage presentation.
Site 1
Uniform
Site 2
High Availability
vSphere Stretched Cluster
Fibre Channel
or iSCSI
Fibre Channel
or iSCSI
Live Volume
VMFS Datastores
Synchronous H igh
Availability Replication
Primary
Secondary
Secondary
Primary
Fibre Channel
or iSCSI
Tiebreaker
Site
Storage
Center
Storage
Center
TCP/ IP
© VMware, Inc.
TCP/ IP
vMSC uniform storage presentation
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8.7.2
vMSC non-uniform storage presentation
Non-uniform means that each vSphere Metro Storage Cluster host will access a Live Volume through one
array or the other, but not both. Each cluster node has read/write access to either the primary or the
secondary Live Volume, but not both simultaneously. For the cluster nodes which have access to the primary
Live Volume, their front-end I/O remains local in proximity. For the cluster nodes which have access to the
secondary Live Volume, their front-end I/O is proxied to the primary Live Volume using the replication link.
Implementing Round Robin with non-uniform presentation is typically preferred to provide automated port and
fabric balance but fixed paths may also be used. Automatic role swap would be preferred with non-uniform
storage presentation so that a role swap will automatically follow the vMotion of virtual machines between
sites.
Site 1
Non- Uniform
Site 2
High Availability
vSphere Stretched Cluster
Fibre Channel
or iSCSI
Fibre Channel
or iSCSI
Live Volume
VMFS Datastores
Synchronous H igh
Availability Replication
Primary
Secondary
Secondary
Primary
Fibre Channel
or iSCSI
Tiebreaker
Site
Storage
Center
Storage
Center
TCP/ IP
© VMware, Inc.
TCP/ IP
vMSC non-uniform storage presentation
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8.8
Tiebreaker service
A tiebreaker service is built into Dell Storage Manager/Enterprise Manager data collector, and acts as a
quorum to facilitate Live Volume automatic failover. It also plays an important role in preventing split-brain
conditions should a network or fabric partition between arrays occur. The tiebreaker should be located at a
site physically independent of arrays participating in a Metro Cluster. This may be in a customer-owned data
center or public cloud. Although tiebreaker availability is not required for Live Volume I/O during normal
operating conditions, the tiebreaker service is required for automatic failover and network latency to the
tiebreaker from each array must not exceed 200 ms RTT.
8.9
Common automatic failover scenarios
The following sections outline failure scenarios and the resulting Live Volume automatic failover behavior.
These are scenarios commonly asked about by customers, not necessarily scenarios that are likely to
commonly occur.
8.9.1
Primary Live Volume array failure
If an unplanned event impacts an SC Series array that is hosting a primary Live Volume, Live Volume
availability is automatically failed over to the surviving array.
Primary Live Volume failure
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8.9.2
Secondary Live Volume array failure
If an unplanned event impacts an SC Series array hosting a secondary Live Volume, the primary Live Volume
remains available on the surviving array.
Secondary Live Volume failure
8.9.3
Replication network partition
If an unplanned event impacts the replication link between SC Series arrays, the primary Live Volume will
continue to remain available serving I/O and no automatic failover will occur. The secondary Live Volume will
be unable to proxy I/O requests to the primary Live Volume while the replication link is down.
Replication link failure
8.9.4
SC Series back-end outage
If an unplanned event impacts primary Live Volume back-end SC Series components (such as the loss of
several drives or a drive shelf), the primary Live Volume will automatically fail over to the surviving array. Both
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arrays will continue to service I/O requests from the primary Live Volume locally and remotely through the
replication link.
Primary back-end failure
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8.9.5
Tiebreaker failure
If an unplanned event impacts the availability of the tiebreaker service on the DSM/EM server, both SC Series
arrays will continue to service I/O requests but no automatic failover can occur during this time.
Tiebreaker failure
8.9.6
Tiebreaker service link failure
Similar to the previous example, if either SC Series array loses network connectivity to the tiebreaker service
on the DSM/EM server, both arrays will continue to service I/O requests but no automatic failover can occur
during this time.
Tiebreaker link failure
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8.10
Detailed failure scenarios
The following table outlines tested design and component failure scenarios with Live Volume automatic
failover enabled with vSphere HA.
Event scenario
Live Volume behavior
vSphere HA behavior
Uniform: Complete site outage
takes down Primary Live
Volumes
Primary Live Volumes
automatically recovered at remote
site
vSphere HA restarts impacted VMs
at remote site
Uniform: Complete site outage
takes down Secondary Live
Volumes
Primary Live Volumes remain
available at remote site
vSphere HA restarts impacted VMs
at remote site
Non-uniform: Complete site
outage takes down Primary Live
Volumes
Primary Live Volumes
automatically recovered at remote
site
vSphere HA restarts impacted VMs
at remote site
Non-uniform: Complete site
outage takes down Secondary
Live Volumes
Primary Live Volumes remain
available at remote site
vSphere HA restarts impacted VMs
at remote site
Uniform: SC Series front-end
outage takes down Primary Live
Volumes
Impacted Primary Live Volumes
automatically recovered at remote
site
None – VMs remain running
Non-impacted Primary Live
Volumes at remote site continue to
remain available
Uniform: SC Series front-end
outage takes down Secondary
Live Volumes
Primary Live Volumes remain
available at remote site
None – VMs remain running
Non-uniform: SC Series frontend outage takes down Primary
Live Volumes
Impacted Primary Live Volumes
automatically recovered at remote
site
VMs which were running on Primary
Live Volumes: vSphere HA restarts
VMs at remote site
Non-impacted Primary Live
VMs which were running on
Volumes at remote site continue to Secondary Live Volumes: None –
remain available
VMs remain running
Non-uniform: SC Series frontend outage takes down
Secondary Live Volumes
Primary Live Volumes remain
available at remote site
VMs which were running on
Secondary Live Volumes: vSphere
HA restarts VMs at remote site
VMs which were running on Primary
Live Volumes: None – VMs remain
running
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Event scenario
Live Volume behavior
vSphere HA behavior
Uniform: SC Series back-end
outage takes down Primary Live
Volumes
Primary Live Volumes
automatically recovered at remote
site
None – VMs remain running
SC Series continues to provide
Live Volume access from both
arrays
Uniform: SC Series back-end
outage takes down Secondary
Live Volumes
Primary Live Volumes
automatically recovered at remote
site
None – VMs remain running
SC Series continues to provide
Live Volume access from both
arrays.
Non-uniform: SC Series-back
end outage takes down Primary
Live Volumes
Primary Live Volumes
automatically recovered at remote
site
None – VMs remain running
Non-uniform: SC Series backend outage takes down
Secondary Live Volumes
Primary Live Volumes remain
available at remote site
None – VMs remain running
Uniform: Replication link failure
between sites
No auto failover
None – VMs remain running
Primary Live Volumes remain
available
Secondary Live Volumes go offline
Non-uniform: Replication link
failure between sites
No auto failover
VMs which were running on Primary
Live Volumes: None – VMs remain
running
Primary Live Volumes remain
available
VMs which were running on
Secondary Live Volumes: vSphere
HA restarts VMs at remote site
Secondary Live Volumes go offline
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Event scenario
Live Volume behavior
vSphere HA behavior
Tiebreaker service failure
No auto failover
None – VMs remain running
Primary Live Volumes remain
available
Secondary Live Volumes remain
available
Tiebreaker service network
No auto failover
isolated from either or both array
sites
None – VMs remain running
Primary Live Volumes remain
available
Secondary Live Volumes remain
available
8.11
Live Volume Restore Automatically
Live Volumes configured for automatic failover are also be configured for automatic restore by default (Figure
49). If Live Volume automatic failover has occurred and the impacted array and data center infrastructure is
subsequently recovered and brought back online, the Restore Automatically feature will repair Live Volumes
by replicating changed data back to the impacted site (leveraging the built-in minimal recopy feature) and then
bring the failed Live Volume back online as a secondary. The Live Volume Restore Automatically feature is
configured on a per-Live-Volume basis through Enterprise Manager and it is recommended to leave this
enabled for automation, consistency, efficiency, and high availability of consistent data purposes.
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Live Volume configured for automatic restore
8.12
VMware DRS/HA and Live Volume
VMware Distributed Resource Scheduler (DRS) is a cluster-centric configuration that uses vMotion to
automatically move virtual machine compute resources to other nodes in a cluster. However, this is performed
without local, metro, or stretched site awareness. In addition, vSphere is unaware of storage virtualization that
occurs in Live Volume. When Live Volume is configured with vSphere stretched cluster, it is a best practice to
keep virtual machines running at the same site as their primary Live Volumes. Additionally, it is best to keep
virtual machines that share a common Live Volume datastore together at the same site. This ensures that
compute and storage resources remain local to the vSphere host running the virtual machine(s). If DRS is
activated on a vSphere cluster with nodes in each site, DRS could automatically move some of the virtual
machines running on a Live Volume datastore to a host that resides in the other data center. In vSphere 4.1
and later, DRS host groups and VM groups can be used in a few ways to benefit a vSphere Metro Storage
Cluster environment. Virtual machines that share a common Live Volume datastore can be placed into VM
groups. Movement of virtual machines and management of their respective Live Volume datastore can then
be performed at a containerized group level rather than at an individual virtual machine level. Hosts that share
a common site can be placed into host groups. Once the host groups are configured, they can represent
locality for the primary Live Volume. At this point, VM groups can be assigned to host groups using the DRS
Groups Manager. This will ensure all virtual machines that share a common Live Volume datastore are
consistently running from the same datastore. The virtual machines can be vMotioned as a group from one
site to another. After a polling threshold is met, SC Series storage can perform an automatic role swap of the
primary Live Volume datastore to the site where the VMs were migrated. The infrastructure can also be
designed in such a way where separate DRS enabled clusters exist at both sites keeping automatic migration
of virtual machines within the respective site where the primary Live Volume resides. In the event of a Live
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Volume role swap, all virtual machines associated with the Live Volume can be vMotioned from the site A
cluster to the site B cluster.
vSphere High Availability (HA) is also a cluster-centric configuration. In the event of a host or storage failure,
HA will attempt to restart virtual machines on a host candidate within the same cluster that may be local or
stretched in a vMSC deployment. In a non-uniform storage configuration, if an outage impacts primary Live
Volume availability and Live Volume automatic failover occurs, vSphere HA can be configured to restart
impacted virtual machines on the surviving array.
Live Volume data access in a non-uniform vSphere environment
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The following vSphere advanced tuning should also be configured for non-uniform stretched cluster
configurations. This tuning allows HA to power off and migrate virtual machines during storage-related
availability events after the primary Live Volume becomes available again using the Preserve Live Volume or
Live Volume Failover Automatically feature.
Note: The Live Volume Failover Automatically feature is only supported with vSphere 5.5 and newer.
Set one of the following, depending on the vSphere version:


vSphere 5.0u1+/5.1: Disk.terminateVMOnPDLDefault = True (/etc/vmware/settings file on each host)
or
vSphere 5.5+: VMkernel.Boot.terminateVMOnPDL = yes (advanced setting on each host, reboot
required)
And set one of the following, depending on the vSphere version:


vSphere 5.5: Disk.AutoremoveOnPDL = 0 (VMware recommended non-default advanced setting on
each host)
or
vSphere 6.0+: Disk.AutoremoveOnPDL = 1 (VMware recommended default advanced setting on
each host)
And set the following, regardless of the vSphere version:

vSphere 5.0u1+: das.maskCleanShutdownEnabled = True (Cluster advanced options)
Configuring HA to restart VMs impacted by a non-uniform storage outage was improved in vSphere 6. In
addition to supporting HA restart for Permanent Device Loss (PDL) events, HA restart can also react to All
Paths Down (APD) events. The tuning becomes easier and more intuitive with pull-down menus in VM
Component Protection (VMCP).
1. Enable Protect against Storage Connectivity Loss.
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2. Configure VMCP for PDL and APD events. For PDL events, select Power off and restart VMs. For
APD events, VMware recommends selecting Power off and restart VMs (conservative). For the
remaining APD timers, refer to VMware documentation or the VMware vSphere Metro Storage
Cluster Recommended Practices available on the VMware Documentation site.
The Disk.AutoremoveOnPDL advanced setting is not configurable in VMCP and should remain at its default
value of 1 for each vSphere 6 host in the cluster. For more information on the Disk.AutoremoveOnPDL
feature, refer to VMware KB article 2059622, PDL AutoRemove feature in vSphere 5.5 and vSphere 6.0.
In a non-uniform configuration, if an outage impacts primary Live Volume availability and Live Volume
automatic failover cannot occur, vSphere HA will not be able to immediately restart impacted virtual
machines.
If the VMware virtual infrastructure is version 4.0 or earlier, vSphere Metro Storage Cluster is not supported
and other steps should be taken to prevent virtual machines from unexpectedly running from the secondary
Live Volume. An individual VM or group of VMs may be associated with a DRS rule that keeps them together
but this does not guarantee they will stay on the same host or group of hosts over a period of time where the
primary Live Volume is located. As a last resort, DRS can be configured for manual mode or disabled when
using Live Volume in a multi-site configuration that will prevent the automatic migration of VMs to Secondary
Live Volume hosts in the same cluster.
VMware vSphere monitors SCSI sense codes sent by an array to determine if a device is in a PDL state.
These SCSI sense codes are outlined in VMware KB article 2004684, Permanent Device Loss (PDL) and AllPaths-Down (APD) in vSphere 5.x and 6.x. Dell SC Series storage supports two SCSI sense codes (see
Table 1) to which will be sent to vSphere hosts when a PDL condition is met.
SCSI sense codes
62
SCSI sense code
Description
H:0x0 D:0x2 P:0x0 Valid sense data: 0x4 0x3e 0x1
LOGICAL UNIT FAILURE
H:0x0 D:0x2 P:0x0 Valid sense data: 0x5 0x25 0x0
LOGICAL UNIT NOT SUPPORTED
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8.13
vMSC and Live Volume Requirements
A vSphere Metro Storage Cluster design with Live Volume requires the following:













8.14
Dell SCOS 6.7 or newer with both SC Series arrays having the same firmware version
Dell Storage Manager/Enterprise Manager 2015 R2 or newer with tiebreaker service located at a site
physically independent of SC Series arrays
VMware vSphere 5.5 or newer
Dell SC Series best practices with VMware vSphere are followed and configured
Uniform or non-uniform storage presentation of volumes to vSphere hosts
Fixed or Round Robin path selection policy (PSP)
Live Volume with auto failover is supported with VMFS datastores or virtual mode raw device
mappings (RDMs), with support for physical mode RDMs added with SCOS 7.1
Live Volumes are enabled to failover automatically
Fibre Channel or iSCSI synchronous high availability replication between SC Series arrays
Maximum supported latency for synchronous high availability replication of 10 ms RTT
Maximum supported latency of vSphere management network of 10 ms RTT
Maximum supported latency from array to tiebreaker of 200 ms RTT
Redundant vMotion network supporting a minimum throughput of 250 Mbps
VMware and Live Volume Managed Replication
Live Volume fulfills the needs of high availability use cases between two SC Series arrays in a local or
stretched configuration. A third array can be added to the design to host a Live Volume Managed Replication.
A Managed Replication is a replica of the primary Live Volume and may be either synchronous or
asynchronous depending on the type of replication used between the Live Volume pair. When Live Volume is
configured to provide high availability intra site, the Managed Replication provides an additional level of data
protection and recovery in the event of an unplanned outage impacting both Live Volume arrays. Live Volume
Managed Replication and automatic failover are supported together. If an automatic failover occurs, Managed
Replication will follow the primary Live Volume which was automatically failed over.
Synchronous Live Volume with asynchronous Live Volume Managed Replication
Although Managed Replications are controlled by Live Volume, they fundamentally work the same way as
standard synchronous or asynchronous volume replication. This includes the recovery options available with
view volumes and DR activation.
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Asynchronous Live Volume with synchronous Live Volume Managed Replication
One differentiator between standard replication and Live Volume Managed Replication is that the source
volume of a Managed Replication is dynamic and changes as Live Volume role swaps occur. The easiest way
to think of this is to understand that the Managed Replication source is always the primary Live Volume.
Because the primary Live Volume role can shift manually or automatically between arrays, the flow of
replicated data will also seamlessly and automatically follow this pattern. The reason for this is that the
primary Live Volume is the volume where write I/O is first committed. Therefore, and especially in an
asynchronous Live Volume configuration, in the event of an unplanned outage or disaster that disables both
Live Volumes, data should be recovered from the most transaction consistent volume to minimize loss of
data. For synchronous Live Volumes, transaction inconsistency is less of an issue because both primary and
secondary Live Volumes should have a sync status of current and not out of date unless the synchronous
replication was paused or became out of date due to excess latency in high availability mode. Disaster
recovery at a remote site is a good use case for the Live Volume Managed Replication feature. However, Live
Volume Managed Replications are not explicitly supported with vSphere Site Recovery Manager. LUN
presentation and data recovery on a Managed Replication can be handled automatically by DSM/EM, but
before virtual machines can be powered on, they must be registered into vSphere inventory, which may be a
manual DR documentation or scripted process.
Non-uniform Live Volume pair with Managed Replication at a third site
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9
Live Volume support for Microsoft Windows/Hyper-V
In this section, aspects of the Live Volume feature set that are specific to Microsoft environments will be
reviewed in greater detail, such as best practices for MPIO settings and Live Volume automatic failover (LVAFO).
9.1
MPIO
Microsoft Windows/Hyper-V servers running 2008/R2 and newer on SC Series arrays can use the in-box
Microsoft MPIO device specific module (DSM). The DSM comes with a number of different MPIO policy
options. Similar to VMware, there are two policies that are supported with Live Volume:


Round Robin
Failover Only
Note: For more information about Windows Server and MPIO policies, settings and best practices, review the
Dell Storage Center: Microsoft Multipath I/O Best Practices guide on Dell TechCenter.
9.2
Round Robin
Round Robin simply spreads the read and write I/O evenly over all available data paths to a data volume
without any consideration for latency or bandwidth for individual data paths. Typically, this unintelligent yet
simple approach is an ideal configuration when:



Using non-uniform server mappings (where a host is presented with multiple data paths only to the
SC Series array that is local to it)
This SC Series array hosts the primary Live Volume(s) with the workload.
Bandwidth and latency performance is the same for all the available data paths.
Round Robin will also work well with uniform server mappings in cases where:


Both the primary and secondary Live Volumes are accessible by high-bandwidth low-latency primary
and secondary data paths.
The slight latency penalty caused by proxied data over secondary data paths does not negatively
impact the workload.
In many cases, using non-uniform server mappings with Round Robin will provide the best overall design
while minimizing complexity, and it provides a good design baseline to start with.
9.3
Round Robin with Subset
Round Robin with Subset is not supported with Live Volume.
9.4
Failover Only
With this MPIO policy, the primary data path is configured as active/optimized, and all other paths are set to
standby. This policy is a good choice when using uniform server mappings when secondary data paths to the
secondary Live Volume have limited bandwidth or higher latency that would negatively affect the workload.
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Failover Only MPIO policy properties
9.5
Uniform server mappings with Live Volume and Round Robin
In Figure 55, a Round Robin Live Volume MPIO configuration is depicted. In this scenario, the server has two
adapters and is mapped to both SC Series arrays that host the Primary and Secondary Live Volume pair. This
configuration is referred to as uniform server mapping since the server is mapped to both SC Series arrays.
Since all four paths are included in an MPIO Round Robin policy, about half of the storage traffic has to
traverse the proxy link between the two SC Series arrays. In cases where the two SC Series arrays are well
connected (for example, if they reside within the same data center), the slight latency penalty incurred for
proxied data may be negligible for the workload, and this design would perform well. However, this
configuration would be sub-optimal if there were significant latency or bandwidth limitations for the secondary
paths, in which case, Failover Only would be a better choice.
Round Robin with uniform server mappings also prevents a Live Volume from automatically swapping roles
(when this feature is enabled on a Live Volume) because about 50 percent of the traffic will always be going
through each array and therefore the thresholds that trigger a role swap won’t be exceeded.
Whether to use uniform or non-uniform server mappings, in conjunction with MPIO Round Robin or Failover
Only is a function of environmental variables that are unique to each customer. Because there are so many
different configuration possibilities, administrators have great flexibility to tailor a solution that is best for their
environment.
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Uniform server mapping with MPIO
9.6
Hyper-V and Live Volume
The full Live Volume feature set works well with Microsoft Hyper-V in both clustered and non-clustered
environments. These Live Volume features include support for synchronous and asynchronous replication,
load balancing, disaster avoidance, managed replication, pre-defined DR plans, etc., as covered elsewhere in
this document.
Note: The Live Volume Automatic Failover feature (LV-AFO) supports host and guest clusters running
Windows Server/Hyper-V 2012 or newer. More on LV-AFO is covered below under Section 9.9.
9.6.1
VM Live Migration and Live Volume Automatic Role Swap
With clustered Hyper-V servers running Windows Server 2008 R2 and newer, live migration of virtual
machines to another node can trigger a Live Volume automatic role swap. In order for automatic role swap to
work (using a two-node Hyper-V cluster as an example), each node is mapped only to the SC Series array
that is local to it. Therefore, when a VM workload is live migrated to the second node, the I/O will follow, and
for a brief time, the I/O be proxied over secondary data paths until the auto role swap thresholds are
exceeded. As noted above, the Live Volume Automatic Role Swap feature will not work when using MPIO
Round Robin because the minimum I/O thresholds that trigger a role swap will not be exceeded.
9.6.2
Single site configuration
In a single-site configuration, multiple Hyper-V servers can be mapped to both SC Series arrays for uniform
server mappings. If latency and bandwidth are not a concern over secondary data paths, then Round Robin is
typically the best (and simplest) MPIO policy choice. If latency and bandwidth are a concern, then Failover
Only may be used to limit I/O to optimized data paths. If administrators don’t want to be burdened with
configuring individual MPIO paths, then using non-uniform mappings with MPIO Round Robin may be a better
choice.
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9.6.3
Multi-site configuration
In a multi-site stretch-cluster configuration, typically the individual Hyper-V hosts are mapped only to the SC
Series array at that particular site, for a non-uniform mapping configuration. In this scenario, the best MPIO
policy would typically be Round Robin for each Hyper-V host. If Live Volume automatic role swap is enabled,
then VM placement and workload determines which array will host the Primary Live Volume, as the Live
Volume will follow the workload, based on the auto swap thresholds. The scenario in Figure 56 depicts a
virtual machine that is live-migrated from Host A to Host B. The secondary SC Series array will proxy I/O to
the primary array for short time and then automatically swap the primary Live Volume role from the primary
array to the secondary array.
Non-uniform server access and Live Volume role swap
9.7
SCVMM/SCOM and Performance and Resource Optimization
(PRO)
Microsoft System Center Virtual Machine Manager with System Center Configuration Manager is capable of
providing intelligent placement as well as automatic migrations of virtual machines from highly utilized nodes
to lower utilized nodes, depending on the action setting of Automatic or Manual. If using Live Volume in a
multi-site Hyper-V cluster with PRO where there are latency and bandwidth limitations for secondary data
paths, use the Manual action for virtual machine placement.
In a multi-site Live Volume Hyper-V cluster, the virtual machines will typically be kept running on nodes within
the same site as their respective primary Live Volume(s). If PRO is activated on a Hyper-V cluster with nodes
in each site, it could automatically migrate some of the virtual machines running on Cluster Shared Volumes
(CSV) that are also Live Volumes to a node that resides in the other data center, thereby splitting the I/O
between data centers.
9.8
Live Volume and Cluster Shared Volumes
Hyper-V has a feature called Cluster Shared Volume (CSV) that allows administrators to place multiple virtual
machines on the volume in a way that VMs on difference nodes can all have read and write access to the
same volume concurrently. CSVs also have a resiliency feature called Network Redirection that by design
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makes Hyper-V cluster data access a little more fault tolerant. If the CSV is owned by a node in the cluster
that has access to the volume, it can redirect data access to that volume through the network to another node
that has lost access to that volume.
One of the best practices with CSVs is utilizing the ability to control which node is the CSV owner. Set the
CSV to be owned by a cluster node that is in the primary site and mapped directly to the SC Series array. In
this way, if the CSV goes into Network Redirected mode, the CSV owner is in the same site and downtime
can be eliminated or reduced.
Figure 57 depicts a multi-site Hyper-V cluster with Live Volume. In this figure, the SC Series storage at Site B
is offline. CSV Network Redirection can take over and proxy all the data traffic through the CSV owner on
Node A.
CSV resiliency with Live Volume and Network Redirection
If a failure happens that takes down SC Series B, Hyper-V can redirect access to the volume over the network
using CSV Network Redirected access.
9.9
Live Volume Automatic Failover for Microsoft
Live Volume Automatic Failover (LV-AFO) is a recent feature enhancement that first became available with
SCOS 6.7 and was supported initially in VMware environments only. See Section 8 for more information on
VMware support for LV-AFO.
With the release of SCOS 7.1 and Dell Storage Manager 2016 R2, support for LV-AFO was extended to
Microsoft Window Server cluster and Hyper-V cluster environments in addition to VMware. LV-AFO functions
essentially the same given Microsoft or VMware hosts as far as triggers that cause a Live Volume to
automatically fail over given a DR situation. The main difference is that VMware offers some additional
resiliencies with VM and workload recovery given a disaster.
To learn more about how LV-AFO works with Microsoft, including a deep dive into LV-AFO functionality and a
live lab demo, visit Dell TechCenter and view the demo video series on LV-AFO for Microsoft, Parts 1, 2, and
3.
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9.9.1
SC Series requirements for LV-AFO for Microsoft
The SC Series storage requirements to support LV-AFO for Microsoft are as follows:

A pair of SC Series arrays that support Live Volume (SCv2000 series is not supported)
-


Replication and Live Volume feature licenses applied to both SC Series arrays
SCOS 7.1 or newer on both SC Series arrays
-



Both SC Series arrays should be running matching SCOS versions
At least 1 Gbps of available bandwidth between the SC Series arrays for replication and proxied data
(iSCSI or FC)
No more than 5 to 10 ms of latency between the SC Series arrays and the host servers (higher
latencies may be tolerable for some workloads but generally anything over 10 ms will start to be
noticeable)
An instance of the Dell Storage Manager (DSM) at a third site to act as a tiebreaker/quorum witness
for LV-AFO
-
-
9.9.2
Similarly configured SC Series arrays are recommended to ensure uniform performance
Dissimilar SC Series arrays are supported but performance will be no greater that the lower
performing of the two SC Series arrays
DSM version 2016 R2 or newer is required to support LV-AFO for Microsoft
DSM can be configured as a physical host or as a VM
DSM supports running from a cloud-based service if the customer does not have third site
This DSM instance should be dedicated to a tiebreaker/quorum witness role (run other DSM
instances at the primary and/or secondary site for day-to-day management, monitoring, reporting,
etc.)
No more than 200 ms of round-trip latency between the DSM tiebreaker and each SC Series
array
Microsoft requirements for LV-AFO
The Microsoft requirements to support LV-AFO for Microsoft are as follows:


Physical server clusters running Windows Server 2012 and newer
Physical Hyper-V clusters running Windows Hyper-V 2012 or newer
-

Guest VM clusters running Windows Server 2012 and newer, that use the following methods for
guest VM clustering:
-
70
Uniform or non-uniform host mappings to SC Series storage mappings are supported
Fibre Channel or iSCSI transports are supported
It is a best practice to avoid presenting Fibre Channel and iSCSI paths concurrently to the same
volume (mixed transports) as it can result in unpredicatable behavior with Microsoft clustering in
some LV-AFO failure scenarios
Shared virtual hard disks with VHDX format (host servers require Hyper-V 2012 R2 in this case)
In-guest iSCSI (guest VMs running on either Hyper-V or VMware hosts are supported)
Physical raw device mappings (pRDMs) on VMware 5.5 or 6.0 hosts
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
Quorum witness: With Windows/Hyper-V clusters (physical or virtual), administrators can choose
between a SAN-based quorum disk witness or a file share witness to serve as a tiebreaker. This is
particularly important when there are equal numbers of nodes at each site, given a stretch cluster with
LV-AFO. Configuring a quorum witness with Server 2012 R2 Hyper-V is recommended, regardless of
the number of cluster nodes.
-
-
-
The optimal quorum configuration for LV-AFO is a file share witness that is located at a third site
to ensure that the surviving node(s) at either site A or site B (given a complete site failure at either
location) always have continuous, uninterrupted access to the quorum. This is true for both
physical node clusters and guest VM clusters.
If a third site is not available for a file share witness, then the next best configuration would be a
SAN-based quorum disk that is also a Live Volume, configured for LV-AFO, so it can
automatically fail over given a DR situation. However, this may not prevent loss of quorum given
a complete site faiulure, if the site that goes down (1) hosts the quorum disk (as a primary LV),
and (2) it hosts the node that owns the quorum disk. Given a complete site failure, if the quorum
disk is configured for LV-AFO, it will fail over to the other site and become available as a primary
LV to a surving node. Howerver, with a complete site failure, the combination of a surviving node
needing to take ownship of the quorum disk, concurrent with the short pause in I/O to the quorum
disk (20 to 30 seconds) while LV-AFO completes, may result in a failure of a surviving node to
take ownership of the quorum disk. If this occurs, workloads dependent on cluster resources at
the surviving site may go off line if the surviving nodes require a tiebreaker to achieve quorum, as
would be the case if there are an equal number of cluster nodes at each site. To avoid this
scenario, configure a file share witness at a third location.
Configuring a file share witness that is local to either site A or site B given a stretch cluster
configuration with LV-AFO should be avoided. If the site that hosts the file share witness suffers
a site outage, access to the file share witness by the surving nodes at the surviving site will fail. If
the file share witness is unavailable to the surviving nodes as a tiebreadker, and its vote is
needed to maintain quorum, then cluster resources will go off line.
Note: Each customer will need to make an informed choice about how they configure the quorum for a
clustered environment that utilizes LV-AFO. The risk of an outage due to loss of quorum due to a less
resilient design might be permissible in some cases, such as for test or development environments.
Note: The LV-AFO feature requires Server 2012 or newer.
Note: There are two additional methods of guest VM clustering that have not been tested with LV-AFO:
Windows Server guest VM clusters on Hyper-V that use pass-through disks or virtual Fibre Channel disks as
cluster disks. While they may work, they are not supported configuraitons with LV-AFO.
9.9.3
Enabling LV-AFO for a Live Volume
To enable LV-AFO, edit the settings for a Live Volume. Under Replication Attributes, the Type should be set
to Synchronous and Sync Mode should be set to High Availability. Once the correct sync mode is set, then
the Failover Automatically and Restore Automatically options become visible at the bottom of the
configuration screen. Check the box to enable Failover Automatically, and this will by default enable Restore
Automatically as well. The recommendation is to leave the Restore Automatically option enabled. This will
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minimize the amount of time a Live Volume is vulnerable to failure after an automatic failover occurs and the
primary site comes back on line. This will ensure that the Live Volume achieves a protected state as quickly
as possible (fully synchronized) allowing automatic failback to occur, given another DR event that affects the
secondary location.
Enable Failover Automatically and Restore Automatically for a Live Volume
9.9.4
DR failure scenarios
Please refer to Section 8 for a list of DR events and how LV-AFO will function to protect the environment. The
DR events that will cause a Live Volume to automatically fail over are platform agnostic and so LV-AFO itself
works exactly the same given VMware or Microsoft Server/Hyper-V hosts/clusters. However, the steps
necessary to recover VMs and workloads given a DR situation will be different based on:




The nature, scope, and duration of the disaster
The platform (VMware or Microsoft)
The type of server mappings (uniform or non-uniform)
Configuration of the workload.
In general, VMware (when configured) may offer more HA resiliency with being able to automatically move
and recover VMs and workloads at another location given a disaster.
9.9.5
Best practices for DSM/EM tiebreaker placement with LV-AFO
One of the most commonly asked questions is about the placement of the Dell Storage Manager (DSM) or
Enterprise Manager (EM) tiebreaker role. Does it really need to be installed at third site? The simple answer
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is, yes. From a best practices standpoint, particularly for production workloads, it is critical to place the
DSM/EM tiebreaker at a third location to prevent site bias, as will be explained below.
While nothing will prevent a customer from co-locating the DSM/EM tiebreaker role at the same site as the
primary or secondary Live Volume, cutting corners with this design puts the customer at an elevated risk of an
unintended outage due to loss of LV-AFO quorum. To avoid confusion, the reference to LV-AFO quorum is
separate, and in addition to the Windows/Hyper-V quorum, as discussed in section 9.9.2.
For example, if the DSM/EM tiebreaker is co-located at the same site as the secondary Live Volume, and this
site suffers an outage so that the primary Live Volume loses connectivity to both the secondary Live Volume
and DSM/EM tiebreaker, the primary Live Volume will also go off line due to loss of quorum. The result is that
there will be a complete outage for any workloads that use this Live Volume. This is by design. When the
primary Live Volume loses connectivity to both the DSM/EM tiebreaker and the secondary Live Volume
concurrently (assuming the Live Volume pair is in sync at the time), the primary Live Volume has to assume
that by becoming completely isolated (loss of quorum), that the DSM/EM tiebreaker and secondary Live
Volume are still on line, and that the DSM/EM tiebreaker is now promoting the secondary Live Volume to
primary status.
Placing the DSM/EM tiebreaker at the primary site with the primary Live Volume will help prevent an
unintended loss of quorum, but only as long as the primary Live Volume stays at that site. If the primary Live
Volume ever fails over (for any reason – manually or automatically) to the other site, then the customer is
again vulnerable to an unintended loss of quorum and an outage due to site bias. To work around this, the
customer could install another instance of DSM/EM at the secondary site to manually “seize” the local
tiebreaker role for that particular Live Volume if it ever fails over to that site, but this is not a very practical
situation from an administration standpoint.
If the customer does not have a third site of their own for the DSM/EM tiebreaker role, then a cloud service
can be used. DSM/EM can be installed on a physical host or a VM, as long as the OS is supported.
Each customer will need to make an informed choice about DSM/EM tiebreaker placement. The risk of an
outage due to unintended loss of quorum might be permissible in some cases, such as for test or
development environments.
9.10
Live Volume with SQL Server
While SQL Server database files can be stored on Live Volumes, it’s important to understand how a primary
volume failure can impact database availability. If a database cannot be recovered after a primary volume
failure, from either a frozen snapshot or the active snapshot on the secondary volume, a restore from backup
will be required. Recovering from backup can greatly increase the time it takes to recover from a failure of the
primary volume. If Live Volume is used across multiple sites, it’s important to ensure that current database
backups are always available at the secondary site.
Replay Manager cannot be used to protect SQL Server data stored on Live Volumes. Snapshots on a Live
Volume will always be crash consistent. Unfortunately, database recovery from crash consistent snapshots is
not always successful, especially when database files are spread across multiple volumes. While generally
not a best practice, placing all files for a given database on the same volume will increase the odds of a
successful recovery from crash consistent snapshots.
As long as the primary and secondary volumes are synchronized, database recovery should be reliable from
the active snapshot on the secondary volume, if the primary volume fails. Since asynchronous Live Volume
does not keep the primary and secondary volumes synchronized, synchronous Live Volume should be used
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for SQL Server database files. If synchronous Live Volume is used in high availability mode, the secondary
volume may not always be in sync with the primary. A successful recovery from the active snapshot on the
secondary volume may not be possible if it is out of sync. Synchronous Live Volume in high consistency
mode provides the best protection.
Live Volume with automatic failover can be used to create a multi-site failover cluster instance of SQL Server.
As long as the sites are synchronized, SQL Server will automatically bring the instance online at the
secondary site if the primary site were to fail. If Live Volume is setup in a uniform configuration, it is possible
for SQL Server to be running in one site and the primary volume in another. Using Live Volume in a nonuniform configuration, where each node of the cluster only has volume mappings to the local array, with
automatic role swap enabled, will help ensure that SQL Server and the primary volume are running at the
same site. Since Live Volume with automatic failover uses synchronous replication in high availability mode,
the secondary volume is not guaranteed to always be in sync with the primary. If a failure occurs while the
secondary is out of sync, manual intervention will be required to recover the SQL Server instance.
If automatic failover is not needed, regular replication may be a better choice to protect SQL Server data.
Replication alone offers recovery times comparable to Live Volume without losing the protection benefits of
application consistent snapshots provided by Replay Manager, reducing the risk of having to recover from
backup in the event of a failure. The entire recovery can be scripted using PowerShell, reducing the recovery
time and minimizing mistakes.
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10
Live Volume with Linux / UNIX
Synchronous replication is a feature of SC Series that allows two copies of data to be maintained on separate
SC Series arrays using one or multiple replication relationships. These two arrays can be in the same
location, or can be geographically dispersed and connected by a WAN. The integrity of these two copies of
data is guaranteed. Synchronous replication functionality is subdivided into two user-configurable operating
modes, high consistency and high availability; these two modes are discussed in further detail in section 3.1.
Additionally, the synchronous replication mode can be changed on demand depending on the needs of the
customer.
Note: The serial number of the replicated volume on the destination SC Series array is unique, and different
from that of the source volume.
When Live Volume is coupled with synchronous replication, it allows customers to deploy and manage
identical data integrity guaranteed copies of data to a secondary SC Series array at an alternate location. The
use of Live Volume creates value in this scenario, by masking the serial number of the replicated volume on
the destination SC Series array and making the serial number appear identical to that of the source volume.
The ability to maintain an identical serial number volume (or volumes) on the destination SC Series array
simplifies and expedites the ability to recover from this volume (or volumes) into the destination application
stack.
10.1
Live Volume and Synchronous Replication
The use of Live Volume in conjunction with synchronous replication in Red Hat® Enterprise Linux®
deployments offers customers the ability to safely and resiliently manage, back up, and recover their
business-critical data across both single- and multi-site deployments.
The remainder of this section discusses some of these use cases along with certain technical considerations
in these scenarios, respectively.
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10.2
Live Volume Managed Replication
Live Volume Managed Replication (LVMR) is the ability of SC Series arrays to replicate and manage a third
copy of data. It is always attached to the primary SC Series array in a Live Volume scenario. If the SC Series
roles are swapped, the LVMR volume will always follow the array that assumes the primary role. The mode of
replication used for the LVMR volume is described in the table below, which shows there is a maximum of
one synchronous replication pair in any scenario.
Live Volume replication modes
Live Volume at Site A
Live Volume
Replication mode of Live Volume
to site B
Replication mode of LVMR to site C
Synchronous
Asynchronous
Asynchronous
Synchronous
Asynchronous
Asynchronous
An LVMR volume (in asynchronous mode with Active Snapshot/Replay enabled, or in synchronous mode)
can be used to manage and provide an offsite copy of business-critical data for disaster recovery or data
distribution use cases (where locating data closer to the audience could significantly reduce data access
latency). The figure below depicts this scenario.
Live Volume or synchronous replication, and LVMR
10.3
Use cases
This section discusses various use cases of Live Volume and synchronous replication with Linux, and
highlights technical considerations that should be considered. These uses cases are examples and starting
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points for consideration. They are not a complete list of scenarios in which Live Volume and synchronous
replication can be adapted. The scenarios discussed can apply to both single- and multi-site deployments by
varying and either scaling the transport mechanisms (LAN, MAN, WAN) connecting site A to site B, or site A
to site C.
10.3.1
Single-site
In this single-site scenario, the Linux hosts and SC Series arrays are geographically located within the same
site boundaries though different buildings or labs if necessary. The figure below depicts this scenario.
Live Volume or synchronous replication in a single-site use case
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A volume (or multiple volumes) is first created and mapped to a Linux host. The volumes are scanned,
identified and brought into multipath awareness as shown below.
[root@tssrv216 ~]# multipath -ll
vol_02 (36000d31000fba6000000000000000015) dm-4 COMPELNT,Compellent Vol
size=10G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 0:0:0:2 sdd 8:48
active ready running
|- 1:0:0:2 sde 8:64
active ready running
|- 0:0:2:2 sdh 8:112 active ready running
|- 1:0:2:2 sdi 8:128 active ready running
vol_01 (36000d31000fba6000000000000000014) dm-5 COMPELNT,Compellent Vol
size=10G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 0:0:7:1 sdj 8:144 active ready running
|- 1:0:10:1 sdk 8:160 active ready running
|- 0:0:9:1 sdl 8:176 active ready running
|- 1:0:12:1 sdm 8:192 active ready running
vol_00 (36000d31000fba6000000000000000013) dm-3 COMPELNT,Compellent Vol
size=10G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 1:0:0:1 sdc 8:32
active ready running
|- 0:0:0:1 sdb 8:16
active ready running
|- 0:0:2:1 sdf 8:80
active ready running
|- 1:0:2:1 sdg 8:96
active ready running
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These volumes are then converted into Live Volumes and synchronously (either HA or HC modes) replicated
to the alternate array. The volumes on the alternate array are then mapped back to the same Linux host. The
volumes are once again scanned, identified, and brought into multipath awareness as shown below. Each
volume is now represented by four additional paths, these paths are the mappings from the alternate array.
[root@tssrv216 ~]# multipath -ll
vol_02 (36000d31000fba6000000000000000015) dm-4 COMPELNT,Compellent Vol
size=10G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 0:0:0:2 sdd 8:48
active ready running
|- 1:0:0:2 sde 8:64
active ready running
|- 0:0:2:2 sdh 8:112 active ready running
|- 1:0:2:2 sdi 8:128 active ready running
|- 0:0:5:2 sdo 8:224 active ready running
|- 0:0:8:2 sdq 65:0
active ready running
|- 1:0:4:2 sdu 65:64 active ready running
`- 1:0:8:2 sdw 65:96 active ready running
vol_01 (36000d31000fba6000000000000000014) dm-5 COMPELNT,Compellent Vol
size=10G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 0:0:7:1 sdj 8:144 active ready running
|- 1:0:10:1 sdk 8:160 active ready running
|- 0:0:9:1 sdl 8:176 active ready running
|- 1:0:12:1 sdm 8:192 active ready running
|- 0:0:5:1 sdn 8:208 active ready running
|- 0:0:8:1 sdp 8:240 active ready running
|- 1:0:4:1 sdt 65:48 active ready running
`- 1:0:8:1 sdv 65:80 active ready running
vol_00 (36000d31000fba6000000000000000013) dm-3 COMPELNT,Compellent Vol
size=10G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 1:0:0:1 sdc 8:32
active ready running
|- 0:0:0:1 sdb 8:16
active ready running
|- 0:0:2:1 sdf 8:80
active ready running
|- 1:0:2:1 sdg 8:96
active ready running
|- 0:0:11:1 sdr 65:16 active ready running
|- 0:0:13:1 sds 65:32 active ready running
|- 1:0:11:1 sdx 65:112 active ready running
`- 1:0:13:1 sdy 65:128 active ready running
It should be noted that even though these additional paths are shown as active and will be actively used for
I/O requests (Round Robin), any I/O requests sent to these paths will be proxied through the replication link to
the primary array for commits. The use of these paths would thus introduce unintended latency to any
applications that may be latency-adverse; at this time, path priority definitions and grouping (for example, all
paths are prio=1 by default and used in equal fashion) are not configurable between Linux hosts and SC
Series arrays. Therefore, it is not recommended to use this approach for any critical production use cases.
That being said, this scenario can still apply in certain use cases. In situations where a maintenance event
requires SC Series arrays to be powered down, the volumes on the primary array can be Live Volume
replicated to an alternate array in a different building or lab. The roles of the arrays can then be swapped,
making the alternate array adopt the primary array role (and the paths from them). The latent paths (from the
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formerly primary array) can then be removed from multipath for the duration of these maintenance events
while maintaining uptime and zero disruption to any and all functions and applications that may reside on the
Linux hosts. Upon completion of these maintenance events, the volumes and roles of the arrays can then be
swapped back or left in place to the discretion of the business requirements.
10.3.2
Multi-site
In this multi-site scenario, the Linux hosts and SC arrays are geographically dispersed within a metropolitan
(or multi-state) region; sites may be connected with MAN or WAN technologies. The figure below depicts this
scenario. It should be noted that this scenario can also be scaled down and applied towards single-site
deployments as well.
Live Volume or synchronous replication in a multi-site use case
The Live Volumes are synchronously (in either HA or HC modes) replicated across SC Series arrays. In this
scenario, the alternate array volumes are mapped to a secondary Linux host instead. It should be noted that
the volumes mapped to the secondary Linux host are not shared volumes and do not possess shared I/O
management and locking mechanisms. These secondary volumes would need to be remounted to reflect any
data changes that were written to the primary volumes. For this reason, the integrity of data across both
primary and secondary volumes is guaranteed (in either HA or HC modes) as long as the replication link is in
a known good state.
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The secondary Linux host can be used in various ways including, but not limited to, the following use cases:



These volumes can be used to present a consistent read-only copy of this data to a remotely located
site; this can apply not only for data distribution reasons, but to also manage and minimize any data
access latency concerns.
The consistency of these volumes (and the integrity of the data that it guarantees) also lends itself
towards database replication use. Databases (and applications) can be brought online at the
remotely-located site, in either read-only mode or used in a disaster recovery after the roles of the SC
Series arrays are swapped (the secondary array becomes the primary array).
These volumes can also be used to complement virtualization technologies such as Red Hat
Enterprise Virtualization (RHEV) to allow for the replication of virtual machine workgroups from one
site or hypervisor to another site or hypervisor. RHEV hosts its virtual machines on a storage domain
that in turn is correlated through a one-to-many relationship to one or multiple backing SC Series
volumes. These volumes are enabled with Live Volume or are synchronously replicated to an
alternate site or secondary array. At the alternate site, these storage volumes can then be used to
reconstruct or import the storage domain into the alternate data center object, and from which object,
reconstruct the virtual machine workgroup.
The following technical and performance considerations should be taken when exploring these use cases.
/etc/multipath.conf: This keeps the contents of /etc/multipath.conf file consistent across all hosts sharing
these Live Volumes; this file contains the definitions of volume WWIDs and their respective aliases and will
ensure that volumes are identified across the different hosts as the same device, and contain the same data
to maintain application integrity.
/etc/fstab: Use the /etc/fstab file in conjunction with /etc/multipath.conf to ensure that volumes are accurately
mounted to their respective and proper mount points in the filesystem. Use the multipath device aliases (if
defined) or default multipath device naming (mpathX) in /etc/fstab to maintain this consistency.
/etc/ntp.conf: Always use the /etc/ntp.conf file to maintain a certain degree of time-based integrity across all
infrastructure. The use of /etc/ntp.conf file becomes even more critical when attempting to maintain data
integrity across multiple cluster nodes/sites dispersed across larger geographic deployments (MAN, WAN).
Cluster configurations: In addition to the considerations mentioned above, take into account cluster-specific
configuration files as well and the best practices of their respective vendors. The discussion of cluster-specific
configuration remains outside the scope of this paper. The list of enterprise-class clustering technologies
include but is not limited to Red Hat Cluster Suite, IBM® PowerHA®, Oracle® Solaris Cluster, Oracle RAC, HPUX ServiceGuard, and Symantec™ Veritas™ Cluster Server to name a few.
Performance considerations: It should be noted that the dual commit nature of synchronous replication (I/O
writes must be committed to the secondary SC Series volume before it is committed to the primary volume)
may introduce latency to the applications generating these write requests. The use of synchronous replication
guarantees the consistency and integrity of the data at both SC Series sites when the acknowledgment is
received by the requesting application. The use of synchronous replication should be made upon the detailed
and thorough analysis and understanding of the applications and its I/O needs.
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The following demonstration has a small dataset (40 MB) that is written to a primary Live Volume and is
synchronously replicated to an alternate array. The filesystem is formatted as ext4 and mounted (@ /vol_00)
with the discard and sync (the sync option disables filesystem buffer caching) flags.
[tssrv216:/root]# cd /etc; time tar cvf - . | (cd /vol_00; tar xvf -)
[snip]
real
0m24.147s
user
0m0.122s
sys
0m1.421s
In this next demonstration, the synchronously replicated Live Volume at the alternate site is mounted to the
secondary Linux host and writes are applied to the secondary Linux host. This demonstrates additional
latency as all write I/O requests are proxied through the secondary SC Series array to the primary array for
processing.
[tssrv217:/root]# cd /etc; time tar cvf - . | (cd /vol_00; tar xvf -)
[snip]
real
0m30.864s
user
0m0.111s
sys
0m1.422s
In this final demonstration, the replication link is quickly changed from synchronous replication to
asynchronous replication and write I/O requests are applied to the primary Live Volume. Note the reduction in
time required to commit these writes compared to the previous use of synchronous replication.
[tssrv216:/root]# cd /etc; time tar cvf - . | (cd /vol_00; tar xvf -)
[snip]
real
user
sys
82
0m18.266s
0m0.104s
0m1.328s
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10.3.3
Multi-site with LVMR
In this alternate multi-site scenario, the Linux hosts and SC Series arrays are geographically dispersed within
a metro (or multi-state) region. Sites may be connected through MAN or WAN technologies. Additionally, a
third site has been included using the LVMR feature of SC Series. The figure below depicts this scenario. It
should be noted that this scenario can also be scaled down and applied towards single site deployments as
well.
Live Volume or Synchronous Replication in multi-site use case with LVMR
The third site and data in this scenario is replicated asynchronously. One of its uses may include the remotely
located disaster recovery copy of business-critical data. It should be noted that the LVMR copy is always
associated or paired with the primary SC Series array. If the primary and secondary array roles are swapped,
the LVMR copy automatically transfers and associates or pairs itself with the array that assumes the primary
role. Additionally, this asynchronously replicated copy (unlike a synchronously replicated) is current if the
replication link has sufficient bandwidth and the Replicate Active Snapshot/Replay feature is enabled, or
current as of the last captured Replay if the feature is disabled. Consequently, discussions should also be
conducted around acceptable RPO and RTO thresholds and maintaining agreeable SLAs with all business
entities involved.
The technical considerations discussed in the section 10.3.2 should be kept in consideration when setting up
this use case.
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11
Use cases
This section exhibits additional examples of how Live Volume can be used in a variety of environments. Live
Volume is not limited to these use cases.
11.1
Zero-downtime SAN maintenance and data migration
With Live Volume, maintenance activities can be performed without downtime on an SC Series array. This
includes tasks such as taking an array offline to move its location, perform service-affecting enclosure or disk
firmware updates, and migrate the volume to a new SAN.
The requirements for this operation include:




MPIO installed and appropriately configured on the host computers
Server(s) properly zoned into both SC Series arrays
Server(s) configured on both arrays
At least a 1 Gb low-latency replication link between arrays
Summary: In advance of a planned outage, Live Volume can non-disruptively migrate volumes from one SC
Series array to another, enabling continuous operation for all applications — even after one array has
completely powered down.
Operation: In an on-demand, operator-driven process, Live Volume can transparently move volumes from
one SC Series array to another. The applications operate continuously. This enables several options for
improved system operation:



Redefine remote site as primary for all volumes on local site
Shut down local site
Reverse process after planned outage is completed
Maintaining availability of applications and services during scheduled maintenance
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Applications remain continuously available on SC2 while SC1 undergoes maintenance
11.2
Storage migration for virtual machine migration
As VMware, Hyper-V, or XenServer virtual machines are migrated from data center to data center, Live
Volume can automatically migrate the related volumes to optimize performance and minimize I/O network
overhead.
Live Volume continuously monitors for changes in I/O traffic for each volume and non-disruptively moves the
primary storage to the optimal SC Series array for optimum efficiency. This involves the following:


85
Migrate the virtual machine using the server virtualization software
Live Volume will track the changes in I/O traffic mapping and will perform a primary/secondary swap
after a fixed amount of time and data have been transferred
Dell EMC SC Series Storage: Synchronous Replication and Live Volume | CML1064
Use cases
Storage follows the application (server virtualization)
The requirements for this operation include the following:




11.3
Server(s) properly zoned into both SC Series arrays
Server(s) configured on both arrays
Stretched Layer 2 networking between source and destination sites or hypervisors
1 Gbps or better low-latency iSCSI or Fibre Channel connectivity between the arrays to support
asynchronous or synchronous Live Volume replication
Disaster avoidance
In anticipation of an unplanned outage (for instance, an approaching hurricane), Live Volume can migrate
data to remote systems before the local system has an outage. Live Volume used in this manner will prevent
data loss and will enable an extremely rapid restart at the remote site.
Operation: In an on-demand, operator-driven process, Live Volume can transparently move volumes from
one SC Series array to another. The applications operate continuously. This enables several options for
improved system operation:




86
Redefine remote site as primary for all volumes on local site
Shut down applications on local site
Restart applications on remote site
Reverse process after risk of potential outage is gone
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Use cases
Disaster avoidance
11.4
On-demand load distribution
In this use case, Live Volume transparently distributes the workload, balances storage utilization, or balances
I/O traffic between two SC Series arrays.
Configuration: SC Series arrays must be connected using high-bandwidth and low-latency connections,
especially when synchronous replication is used with Live Volume.
Operation: In an on-demand, operator-driven process, Live Volume can transparently move volumes from
one SC Series array to another. The applications operate continuously. This enables several options for
improved system operation:




87
Distribution of I/O workload
Distribution of storage
Distribution of front-end Ioad traffic
Reallocation of workload to match capabilities of heterogeneous systems
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Use cases
On-demand local distribution
11.5
Cloud computing
In this use case, Live Volume transparently distributes the workload, balances storage utilization, or balances
I/O traffic between multiple SC Series arrays within a data center, enabling continuous flexibility to meet
changes in workload and to provide a higher-level system up time.
Configuration: Live Volumes can be created between any two SC Series arrays in a data center. Each array
can have many Live Volumes, each potentially connecting to a different array.
Operation: In an on-demand, operator-driven process, Live Volume can transparently move volumes from
one SC Series array to another. The applications operate continuously. This enables several options for
improved system operation:




88
Distribution of I/O workload
Distribution of storage
Distribution of front-end Ioad traffic
Reallocation of workload to match capabilities of heterogeneous systems
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Use cases
Cloud computing
11.6
Replay Manager and Live Volume
Replay Manager and Live Volume are mutually exclusive features that cannot be used together. If storage
hosts are split between data centers in a stretched cluster configuration using Live Volume, then Replay
Manager cannot be used to create application consistent snapshots.
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Technical support and additional resources
A
Technical support and additional resources
Dell.com/support is focused on meeting customer needs with proven services and support.
Dell TechCenter is an online technical community where IT professionals have access to numerous resources
for Dell software, hardware, and services.
Storage Solutions Technical Documents on Dell TechCenter provide expertise that helps to ensure customer
success on Dell EMC storage platforms.
A.1
Related resources
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Dell Storage Center Best Practices with VMware vSphere 5.x
Dell Storage Center Best Practices with VMware vSphere 6.x
Dell SC Series Best Practices with VMware Site Recovery Manager
High Availability and Disaster Recovery with VMware vSphere Solutions Guide
Hyper-V 2012 R2 Best Practices for Dell Storage Center
Dell Storage Center Disaster Recovery for Microsoft Hyper-V Best Practices
Dell Storage Center Red Hat Enterprise Linux (RHEL) 6.x Best Practices
Red Hat 6 Device Mapper Multipathing
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