Understanding RAID concepts. Dell OpenManage Server Administrator Version 8.3

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Understanding RAID concepts

Storage Management uses the Redundant Array of Independent Disks (RAID) technology to provide Storage Management capability.

Understanding Storage Management requires an understanding of RAID concepts, as well as some familiarity with how the RAID controllers and operating system view disk space on your system.

Related concepts

What is RAID?

Organizing Data Storage For Availability And Performance

Choosing RAID Levels And Concatenation

Comparing RAID Level And Concatenation Performance

Topics:

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What is RAID?

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Organizing Data Storage For Availability And Performance

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Choosing RAID Levels And Concatenation

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No-RAID

What is RAID?

RAID is a technology for managing the storage of data on the physical disks that reside or are attached to the system. A key aspect of

RAID is the ability to span physical disks so that the combined storage capacity of multiple physical disks can be treated as a single, extended disk space. Another key aspect of RAID is the ability to maintain redundant data which can be used to restore data in the event of a disk failure. RAID uses different techniques, such as striping, mirroring, and parity, to store and reconstruct data. There are different

RAID levels that use different methods for storing and reconstructing data. The RAID levels have different characteristics in terms of read/write performance, data protection, and storage capacity. Not all RAID levels maintain redundant data, which means for some RAID levels lost data cannot be restored. The RAID level you choose depends on whether your priority is performance, protection, or storage capacity.

NOTE: The RAID Advisory Board (RAB) defines the specifications used to implement RAID. Although RAB defines the

RAID levels, commercial implementation of RAID levels by different vendors may vary from the actual RAID specifications. An implementation of a particular vendor may affect the read and write performance and the degree of data redundancy.

Hardware and software RAID

RAID can be implemented with either hardware or software. A system using hardware RAID has a RAID controller that implements the

RAID levels and processes data reads and writes to the physical disks. When using software RAID provided by the operating system, the operating system implements the RAID levels. For this reason, using software RAID by itself can slow the system performance. You can, however, use software RAID along with hardware RAID volumes to provide better performance and variety in the configuration of RAID volumes. For example, you can mirror a pair of hardware RAID 5 volumes across two RAID controllers to provide RAID controller redundancy.

RAID concepts

RAID uses particular techniques for writing data to disks. These techniques enable RAID to provide data redundancy or better performance. These techniques include:

• Mirroring — Duplicating data from one physical disk to another physical disk. Mirroring provides data redundancy by maintaining two copies of the same data on different physical disks. If one of the disks in the mirror fails, the system can continue to operate using the

18 Understanding RAID concepts

unaffected disk. Both sides of the mirror contain the same data always. Either side of the mirror can act as the operational side. A mirrored RAID disk group is comparable in performance to a RAID 5 disk group in read operations but faster in write operations.

• Striping — Disk striping writes data across all physical disks in a virtual disk. Each stripe consists of consecutive virtual disk data addresses that are mapped in fixed-size units to each physical disk in the virtual disk using a sequential pattern. For example, if the virtual disk includes five physical disks, the stripe writes data to physical disks one through five without repeating any of the physical disks. The amount of space consumed by a stripe is the same on each physical disk. The portion of a stripe that resides on a physical disk is a stripe element. Striping by itself does not provide data redundancy. Striping in combination with parity does provide data redundancy.

• Stripe size — The total disk space consumed by a stripe not including a parity disk. For example, consider a stripe that contains 64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe size is 64KB and the stripe element size is

16KB.

• Stripe element — A stripe element is the portion of a stripe that resides on a single physical disk.

• Stripe element size — The amount of disk space consumed by a stripe element. For example, consider a stripe that contains 64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe element size is 16KB and the stripe size is

64KB.

• Parity — Parity refers to redundant data that is maintained using an algorithm in combination with striping. When one of the striped disks fails, the data can be reconstructed from the parity information using the algorithm.

• Span — A span is a RAID technique used to combine storage space from groups of physical disks into a RAID 10, 50, or 60 virtual disk.

RAID Levels

Each RAID level uses some combination of mirroring, striping, and parity to provide data redundancy or improved read and write performance. For specific information on each RAID level, see

Choosing RAID Levels And Concatenation .

Organizing Data Storage For Availability And

Performance

RAID provides different methods or RAID levels for organizing the disk storage. Some RAID levels maintain redundant data so that you can restore data after a disk failure. Different RAID levels also entail an increase or decrease in the I/O (read and write) performance of a system.

Maintaining redundant data requires the use of additional physical disks. The possibility of a disk failure increases with an increase in the number of disks. Since the differences in I/O performance and redundancy, one RAID level may be more appropriate than another based on the applications in the operating environment and the nature of the data being stored.

When choosing concatenation or a RAID level, the following performance and cost considerations apply:

• Availability or fault-tolerance — Availability or fault-tolerance refers to the ability of a system to maintain operations and provide access to data even when one of its components has failed. In RAID volumes, availability or fault-tolerance is achieved by maintaining redundant data. Redundant data includes mirrors (duplicate data) and parity information (reconstructing data using an algorithm).

• Performance — Read and write performance can be increased or decreased depending on the RAID level you choose. Some RAID levels may be more appropriate for particular applications.

• Cost efficiency — Maintaining the redundant data or parity information associated with RAID volumes requires additional disk space. In situations where the data is temporary, easily reproduced, or non-essential, the increased cost of data redundancy may not be justified.

• Mean Time Between Failure (MTBF) — Using additional disks to maintain data redundancy also increases the chance of disk failure at any given moment. Although this option cannot be avoided in situations where redundant data is a requirement, it does have implications on the workload of the system support staff within your organization.

• Volume — Volume refers to a single disk non-RAID virtual disk. You can create volumes using external utilities like the O-ROM <Ctrl>

<r>. Storage Management does not support the creation of volumes. However, you can view volumes and use drives from these volumes for creation of new virtual disks or Online Capacity Expansion (OCE) of existing virtual disks, provided free space is available.

Storage Management allows Rename and Delete operations on such volumes.

Choosing RAID Levels And Concatenation

You can use RAID or concatenation to control data storage on multiple disks. Each RAID level or concatenation has different performance and data protection characteristics.

The following topics provide specific information on how each RAID level or concatenation store data as well as their performance and protection characteristics:

Understanding RAID concepts 19

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Concatenation

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RAID Level 0 (Striping)

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RAID Level 1 (Mirroring)

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RAID Level 5 (Striping With Distributed Parity)

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RAID Level 6 (Striping With Additional Distributed Parity)

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RAID Level 50 (Striping Over RAID 5 Sets)

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RAID Level 60 (Striping Over RAID 6 Sets)

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RAID Level 10 (Striping Over Mirror Sets)

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RAID Level 1-Concatenated (Concatenated Mirror)

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No-RAID

Related concepts

Starting And Target RAID Levels For Virtual Disk Reconfiguration And Capacity Expansion

Concatenation

In Storage Management, concatenation refers to storing data on either one physical disk or on disk space that spans multiple physical disks. When spanning more than one disk, concatenation enables the operating system to view multiple physical disks as a single disk. Data stored on a single disk can be considered a simple volume. This disk could also be defined as a virtual disk that comprises only a single physical disk.

Data that spans more than one physical disk can be considered a spanned volume. Multiple concatenated disks can also be defined as a virtual disk that comprises more than one physical disk.

A dynamic volume that spans to separate areas of the same disk is also considered concatenated.

When a physical disk in a concatenated or spanned volume fails, the entire volume becomes unavailable. Because the data is not redundant, it cannot be restored by rebuilding from a mirrored disk or parity information. Restoring from a backup is the only option.

Because concatenated volumes do not use disk space to maintain redundant data, they are more cost-efficient than volumes that use mirrors or parity information. A concatenated volume may be a good choice for data that is temporary, easily reproduced, or that does not justify the cost of data redundancy. In addition, a concatenated volume can easily be expanded by adding an additional physical disk.

• Concatenates n disks as one large virtual disk with a capacity of n disks.

• Data fills up the first disk before it is written to the second disk.

• No redundant data is stored. When a disk fails, the large virtual disk fails.

• No performance gain.

• No redundancy.


RAID level 0 (striping)

RAID 0 uses data striping, which is writing data in equal-sized segments across the physical disks. RAID 0 does not provide data redundancy.

RAID 0 characteristics:

• Groups n disks as one large virtual disk with a capacity of (smallest disk size) * n disks.

• Data is stored to the disks alternately.

• No redundant data is stored. When a disk fails, the large virtual disk fails with no means of rebuilding the data.

• Better read and write performance.

RAID level 1 (mirroring)

RAID 1 is the simplest form of maintaining redundant data. In RAID 1, data is mirrored or duplicated on one or more physical disks. If a physical disk fails, data can be rebuilt using the data from the other side of the mirror.


• Groups n + n disks as one virtual disk with the capacity of n disks. The controllers currently supported by Storage Management allow the selection of two disks when creating a RAID 1. Because these disks are mirrored, the total storage capacity is equal to one disk.


• Data is replicated on both the disks.

• When a disk fails, the virtual disk still works. The data is read from the mirror of the failed disk.

• Better read performance, but slightly slower write performance.

• Redundancy for protection of data.

• RAID 1 is more expensive in terms of disk space since twice the number of disks are used than required to store the data without redundancy.

RAID level 5 (striping with distributed parity)

RAID 5 provides data redundancy by using data striping in combination with parity information. Rather than dedicating a physical disk to parity, the parity information is striped across all physical disks in the disk group.


• Groups n disks as one large virtual disk with a capacity of ( n -1) disks.

• Redundant information (parity) is alternately stored on all disks.

• When a disk fails, the virtual disk still works, but it is operating in a degraded state. The data is reconstructed from the surviving disks.

• Better read performance, but slower write performance.


RAID level 6 (striping with additional distributed parity)

RAID 6 provides data redundancy by using data striping in combination with parity information. Similar to RAID 5, the parity is distributed within each stripe. RAID 6, however, uses an additional physical disk to maintain parity, such that each stripe in the disk group maintains two disk blocks with parity information. The additional parity provides data protection in the event of two disk failures. In the following image, the two sets of parity information are identified as P and Q .



• Groups n disks as one large virtual disk with a capacity of ( n -2) disks.

• Redundant information (parity) is alternately stored on all disks.

• The virtual disk remains functional with up to two disk failures. The data is reconstructed from the surviving disks.


• Increased redundancy for protection of data.

• Two disks per span are required for parity. RAID 6 is more expensive in terms of disk space.

RAID level 50 (striping over RAID 5 sets)

RAID 50 is striping over more than one span of physical disks. For example, a RAID 5 disk group that is implemented with three physical disks and then continues on with a disk group of three more physical disks would be a RAID 50.

It is possible to implement RAID 50 even when the hardware does not directly support it. In this case, you can implement more than one

RAID 5 virtual disks and then convert the RAID 5 disks to dynamic disks. You can then create a dynamic volume that is spanned across all

RAID 5 virtual disks.



• Groups n * s disks as one large virtual disk with a capacity of s *( n -1) disks, where s is the number of spans and n is the number of disks within each span.

• Redundant information (parity) is alternately stored on all disks of each RAID 5 span.


• Requires as much parity information as standard RAID 5.

• Data is striped across all spans. RAID 50 is more expensive in terms of disk space.

RAID level 60 (striping over RAID 6 sets)

RAID 60 is striping over more than one span of physical disks that are configured as a RAID 6. For example, a RAID 6 disk group that is implemented with four physical disks and then continues on with a disk group of four more physical disks would be a RAID 60.



• Groups n * s disks as one large virtual disk with a capacity of s *( n -2) disks, where s is the number of spans and n is the number of disks within each span.

• Redundant information (parity) is alternately stored on all disks of each RAID 6 span.


• Increased redundancy provides greater data protection than a RAID 50.

• Requires proportionally as much parity information as RAID 6.

• Two disks per span are required for parity. RAID 60 is more expensive in terms of disk space.

RAID level 10 (striped-mirrors)

The RAB considers RAID level 10 to be an implementation of RAID level 1. RAID 10 combines mirrored physical disks (RAID 1) with data striping (RAID 0). With RAID 10, data is striped across multiple physical disks. The striped disk group is then mirrored onto another set of physical disks. RAID 10 can be considered a mirror of stripes .



• Groups n disks as one large virtual disk with a capacity of ( n /2) disks, where n is an even integer.

• Mirror images of the data are striped across sets of physical disks. This level provides redundancy through mirroring.

• When a disk fails, the virtual disk still works. The data is read from the surviving mirrored disk.

• Improved read performance and write performance.


RAID Level 1-Concatenated (Concatenated Mirror)

RAID 1-concatenated is a RAID 1 disk group that spans across more than a single pair of physical disks. This configuration combines the advantages of concatenation with the redundancy of RAID 1. No striping is involved in this RAID type.

NOTE: You cannot create a RAID 1-concatenated virtual disk or reconfigure to RAID 1-concatenated with Storage

Management. You can only monitor a RAID 1- concatenated virtual disk with Storage Management.


Comparing RAID Level And Concatenation

Performance

The following table compares the performance characteristics associated with the more common RAID levels. This table provides general guidelines for choosing a RAID level. Evaluate your specific environment requirements before choosing a RAID level.

NOTE: The following table does not show all supported RAID levels in Storage Management. For information on all

supported RAID levels in Storage Management, see Choosing RAID Levels And Concatenation .

Table 1. RAID Level and Concatenation Performance Comparison

RAID Level

Concatenation

Data Availability Read

No gain

Performance

No gain

Write

Performance

No gain

Rebuild

Performance

N/A

RAID 0

RAID 1

RAID 5

RAID 10

RAID 50

None

Excellent

Good

Excellent

Good

Very Good

Very Good

Sequential reads: good.

Transactional reads: Very good

Fair, unless using writeback cache

Fair

Very Good

Very Good

Very Good

Good

Fair

Fair

N/A

Good

Good

Fair

Minimum Disks

Required

1 or 2 depending on the controller

N + 2 (N = at least 4)

Suggested

Uses

More cost efficient than redundant RAID levels. Use for noncritical data.

Noncritical data.

N

2N (N = 1)

N + 1 (N = at least two disks)

Small databases, database logs, and critical information.

Databases and other read intensive transactional uses.

2N x X Data intensive environments

(large records).

Medium sized transactional or data intensive uses.


RAID Level

RAID 6

RAID 60

Data Availability Read

Performance

Excellent Sequential reads: good.

Transactional reads: Very good

Write

Performance

Fair, unless using writeback cache

Rebuild

Performance

Poor

Excellent Very Good Fair Poor

Minimum Disks

Required

N + 2 (N = at least two disks)

X x (N + 2) (N = at least 2)

Suggested

Uses

Critical information.

Databases and other read intensive transactional uses.

Critical information.

Medium sized transactional or data intensive uses.

N = Number of physical disks

X = Number of RAID sets

No-RAID

In Storage Management, a virtual disk of unknown metadata is considered a No-RAID volume. Storage Management does not support this type of virtual disks. These must either be deleted or the physical disk must be removed. Storage Management allows Delete and

Rename operation on No-RAID volumes.


Understanding RAID concepts. Dell OpenManage Server Administrator Version 8.3

Understanding RAID concepts

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Understanding RAID concepts. Dell OpenManage Server Administrator Version 8.3