DM Multipath - DM Multipath Configuration and Administration

DM Multipath - DM Multipath Configuration and Administration
Red Hat Enterprise Linux 6
DM Multipath
DM Multipath Configuration and Administration
DM Multipath
Red Hat Enterprise Linux 6 DM Multipath
DM Multipath Configuration and Administration
Edition 1
Copyright © 2010 Red Hat, Inc.
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This book provides information on using the Device-Mapper Multipath feature of Red Hat Enterprise
Linux 6
Preface
v
1. Audience ........................................................................................................................ v
2. Related Documentation ................................................................................................... v
3. We Need Feedback! ....................................................................................................... v
4. Document Conventions ................................................................................................... vi
4.1. Typographic Conventions ..................................................................................... vi
4.2. Pull-quote Conventions ........................................................................................ vii
4.3. Notes and Warnings ........................................................................................... viii
1. Device Mapper Multipathing
1.1. New and Changed Features .........................................................................................
1.2. Overview of DM-Multipath .............................................................................................
1.3. Storage Array Support ..................................................................................................
1.4. DM-Multipath Components ............................................................................................
1.5. DM-Multipath Setup Overview .......................................................................................
1
1
2
5
5
6
2. Multipath Devices
2.1. Multipath Device Identifiers ...........................................................................................
2.2. Consistent Multipath Device Names in a Cluster ............................................................
2.3. Multipath Device Attributes ...........................................................................................
2.4. Multipath Devices in Logical Volumes ............................................................................
7
7
7
8
8
3. Setting Up DM-Multipath
3.1. Setting Up DM-Multipath .............................................................................................
3.2. Ignoring Local Disks when Generating Multipath Devices ..............................................
3.3. Configuring Storage Devices .......................................................................................
11
11
12
14
4. The DM-Multipath Configuration File
4.1. Configuration File Overview ........................................................................................
4.2. Configuration File Blacklist ..........................................................................................
4.2.1. Blacklisting by WWID .......................................................................................
4.2.2. Blacklisting By Device Name ............................................................................
4.2.3. Blacklisting By Device Type .............................................................................
4.2.4. Blacklist Exceptions .........................................................................................
4.3. Configuration File Defaults ..........................................................................................
4.4. Multipaths Device Configuration Attributes ...................................................................
4.5. Configuration File Devices ..........................................................................................
15
15
16
17
17
17
18
18
22
24
5. DM-Multipath Administration and Troubleshooting
5.1. Resizing an Online Multipath Device ...........................................................................
5.2. The Multipath Daemon ...............................................................................................
5.3. Issues with Large Number of LUNs .............................................................................
5.4. Issues with queue_if_no_path feature ..........................................................................
5.5. Multipath Command Output ........................................................................................
5.6. Multipath Queries with multipath Command .................................................................
5.7. Multipath Command Options .......................................................................................
5.8. Determining Device Mapper Entries with the dmsetup Command ...................................
5.9. Troubleshooting with the multipathd Interactive Console ................................................
29
29
30
30
30
30
31
32
32
33
A. Revision History
35
Index
37
iii
iv
Preface
This book describes the Device Mapper Multipath (DM-Multipath) feature of Red Hat Enterprise Linux
for the Red Hat Enterprise Linux 6 release.
1. Audience
This book is intended to be used by system administrators managing systems running the Linux
operating system. It requires familiarity with Red Hat Enterprise Linux.
2. Related Documentation
For more information about using Red Hat Enterprise Linux, refer to the following resources:
• Installation Guide — Documents relevant information regarding the installation of Red Hat
Enterprise Linux 6.
• Deployment Guide — Documents relevant information regarding the deployment, configuration and
administration of Red Hat Enterprise Linux 6.
• Storage Administration Guide — Provides instructions on how to effectively manage storage devices
and file systems on Red Hat Enterprise Linux 6.
For more information about Red Hat Cluster Suite for Red Hat Enterprise Linux 6, refer to the following
resources:
• Red Hat Cluster Suite Overview — Provides a high level overview of the Red Hat Cluster Suite.
• Cluster Administration — Provides information about installing, configuring and managing Red Hat
Cluster components.
• Logical Volume Manager Administration — Provides a description of the Logical Volume Manager
(LVM), including information on running LVM in a clustered environment.
• Global File System 2: Configuration and Administration — Provides information about installing,
configuring, and maintaining Red Hat GFS2 (Red Hat Global File System 2).
• Linux Virtual Server Administration — Provides information on configuring high-performance
systems and services with the Linux Virtual Server (LVS).
• Release Notes — Provides information about the current release of Red Hat products.
Red Hat Cluster Suite documentation and other Red Hat documents are available in HTML,
PDF, and RPM versions on the Red Hat Enterprise Linux Documentation CD and online at http://
www.redhat.com/docs/.
3. We Need Feedback!
If you find a typographical error in this manual, or if you have thought of a way to make this manual
better, we would love to hear from you! Please submit a report in Bugzilla: http://bugzilla.redhat.com/
against the product Red Hat Enterprise Linux 6 and the component doc-DM_Multipath. When
submitting a bug report, be sure to mention the manual's identifier: rh-DM_Multipath(EN)-6
(2010-10-14T15:15). 1.
v
Preface
If you have a suggestion for improving the documentation, try to be as specific as possible when
describing it. If you have found an error, please include the section number and some of the
surrounding text so we can find it easily.
4. Document Conventions
This manual uses several conventions to highlight certain words and phrases and draw attention to
specific pieces of information.
1
In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts set. The
Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not,
alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes
the Liberation Fonts set by default.
4.1. Typographic Conventions
Four typographic conventions are used to call attention to specific words and phrases. These
conventions, and the circumstances they apply to, are as follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight
keycaps and key combinations. For example:
To see the contents of the file my_next_bestselling_novel in your current
working directory, enter the cat my_next_bestselling_novel command at the
shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a keycap, all presented in mono-spaced bold
and all distinguishable thanks to context.
Key combinations can be distinguished from keycaps by the hyphen connecting each part of a key
combination. For example:
Press Enter to execute the command.
Press Ctrl+Alt+F2 to switch to the first virtual terminal. Press Ctrl+Alt+F1 to
return to your X-Windows session.
The first paragraph highlights the particular keycap to press. The second highlights two key
combinations (each a set of three keycaps with each set pressed simultaneously).
If source code is discussed, class names, methods, functions, variable names and returned values
mentioned within a paragraph will be presented as above, in mono-spaced bold. For example:
File-related classes include filesystem for file systems, file for files, and dir for
directories. Each class has its own associated set of permissions.
Proportional Bold
This denotes words or phrases encountered on a system, including application names; dialog box text;
labeled buttons; check-box and radio button labels; menu titles and sub-menu titles. For example:
Choose System → Preferences → Mouse from the main menu bar to launch Mouse
Preferences. In the Buttons tab, click the Left-handed mouse check box and click
1
https://fedorahosted.org/liberation-fonts/
vi
Pull-quote Conventions
Close to switch the primary mouse button from the left to the right (making the mouse
suitable for use in the left hand).
To insert a special character into a gedit file, choose Applications → Accessories
→ Character Map from the main menu bar. Next, choose Search → Find… from the
Character Map menu bar, type the name of the character in the Search field and click
Next. The character you sought will be highlighted in the Character Table. Doubleclick this highlighted character to place it in the Text to copy field and then click the
Copy button. Now switch back to your document and choose Edit → Paste from the
gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific
menu names; and buttons and text found within a GUI interface, all presented in proportional bold and
all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or
variable text. Italics denotes text you do not input literally or displayed text that changes depending on
circumstance. For example:
To connect to a remote machine using ssh, type ssh username@domain.name at
a shell prompt. If the remote machine is example.com and your username on that
machine is john, type ssh john@example.com.
The mount -o remount file-system command remounts the named file
system. For example, to remount the /home file system, the command is mount -o
remount /home.
To see the version of a currently installed package, use the rpm -q package
command. It will return a result as follows: package-version-release.
Note the words in bold italics above — username, domain.name, file-system, package, version and
release. Each word is a placeholder, either for text you enter when issuing a command or for text
displayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and
important term. For example:
Publican is a DocBook publishing system.
4.2. Pull-quote Conventions
Terminal output and source code listings are set off visually from the surrounding text.
Output sent to a terminal is set in mono-spaced roman and presented thus:
books
books_tests
Desktop
Desktop1
documentation
downloads
drafts
images
mss
notes
photos
scripts
stuff
svgs
svn
Source-code listings are also set in mono-spaced roman but add syntax highlighting as follows:
package org.jboss.book.jca.ex1;
import javax.naming.InitialContext;
vii
Preface
public class ExClient
{
public static void main(String args[])
throws Exception
{
InitialContext iniCtx = new InitialContext();
Object
ref
= iniCtx.lookup("EchoBean");
EchoHome
home
= (EchoHome) ref;
Echo
echo
= home.create();
System.out.println("Created Echo");
System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
}
}
4.3. Notes and Warnings
Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.
Note
Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note should
have no negative consequences, but you might miss out on a trick that makes your life easier.
Important
Important boxes detail things that are easily missed: configuration changes that only apply to
the current session, or services that need restarting before an update will apply. Ignoring a box
labeled 'Important' will not cause data loss but may cause irritation and frustration.
Warning
Warnings should not be ignored. Ignoring warnings will most likely cause data loss.
viii
Chapter 1.
Device Mapper Multipathing
Device mapper multipathing (DM-Multipath) allows you to configure multiple I/O paths between server
nodes and storage arrays into a single device. These I/O paths are physical SAN connections that can
include separate cables, switches, and controllers. Multipathing aggregates the I/O paths, creating a
new device that consists of the aggregated paths.
This chapter provides a summary of the features of DM-Multipath that are new for the initial release
of Red Hat Enterprise Linux 6. Following that, this chapter provides a high-level overview of DM
Multipath and its components, as well as an overview of DM-Multipath setup.
1.1. New and Changed Features
This section lists new and changed features of DM-Multipath that are included with the initial release of
Red Hat Enterprise Linux 6.
• For the Red Hat Enterprise Linux 6 release, the initial DM-Multipath setup procedure for a basic
failover configuration has changed. You can now create the DM-Multipath configuration file and
enable DM-Multipath with the mpathconf configuration utility, which can also load the devicemapper-multipath module, start the multipathd daemon, and set chkconfig to start the
daemon automatically on reboot.
For information on the new setup procedure, see Section 3.1, “Setting Up DM-Multipath”. For more
information on the mpathconf command, see the mpathconf(5) man page.
• The Red Hat Enterprise Linux 6 release provides a new mode for setting up multipath devices,
which you set with the find_multipaths configuration file parameter. In previous releases of Red
Hat Enterprise Linux, multipath always tried to create a multipath device for every path that was not
explicitly blacklisted. In Red Hat Enterprise Linux 6, however, if the find_multipath configuration
parameter is set to yes, then multipath will create a device only if one of three conditions are met:
• There are at least two non-blacklisted paths with the same WWID.
• The user manually forces the device creation, by specifying a device with the multipath
command.
• A path has the same WWID as a multipath device that was previously created (even if that
multipath device does not currently exist). For instructions on the procedure to follow if you have
previously created multipath devices when the find_multipaths parameter was not set, see
Section 4.2, “Configuration File Blacklist”.
This feature should allow most users to have multipath automatically choose the correct paths to
make into multipath devices, without having to edit the blacklist.
For information on the find_multipaths configuration parameter, see Section 4.3, “Configuration
File Defaults”.
• The Red Hat Enterprise Linux 6 release provides two new path selector algorithms which determine
which path to use for the next I/O operation: queue-length and service-time. The queuelength algorithm looks at the amount of outstanding I/O to the paths to determine which path to
use next. The service-time algorithm looks at the amount of outstanding I/O and the relative
throughput of the paths to determine which path to use next. For more information on the path
selector parameters in the configuration file, see Chapter 4, The DM-Multipath Configuration File.
• In the Red Hat Enterprise Linux 6 release, priority functions are no longer callout programs. Instead
they are dynamic shared objects like the path checker functions. The prio_callout parameter
1
Chapter 1. Device Mapper Multipathing
has been replaced by the prio parameter. For descriptions of the supported prio functions, see
Chapter 4, The DM-Multipath Configuration File.
• In Red Hat Enterprise Linux 6, the multipath command output has changed format. For
information on the multipath command output, see Section 5.5, “Multipath Command Output”.
• In the Red Hat Enterprise Linux 6 release, the location of the multipath bindings file is /etc/
multipath/bindings.
• The Red Hat Enterprise Linux 6 release provides three new defaults parameters in the
multipath.conf file: checker_timeout, fast_io_fail_tmo, and dev_loss_tmo. For
information on these parameters, see Chapter 4, The DM-Multipath Configuration File.
• When the user_friendly_names option in the multipath configuration file is set to yes, the
name of a multipath device is of the form mpathn. For the Red Hat Enterprise Linux 6 release, n
is an alphabetic character, so that the name of a multipath device might be mpatha or mpathb. In
previous releases, n was an integer.
1.2. Overview of DM-Multipath
DM-Multipath can be used to provide:
• Redundancy
DM-Multipath can provide failover in an active/passive configuration. In an active/passive
configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the
cable, switch, or controller) fails, DM-Multipath switches to an alternate path.
• Improved Performance
DM-Multipath can be configured in active/active mode, where I/O is spread over the paths in a
round-robin fashion. In some configurations, DM-Multipath can detect loading on the I/O paths and
dynamically re-balance the load.
Figure 1.1, “Active/Passive Multipath Configuration with One RAID Device” shows an active/passive
configuration with two I/O paths from the server to a RAID device. There are 2 HBAs on the server, 2
SAN switches, and 2 RAID controllers.
2
Overview of DM-Multipath
Figure 1.1. Active/Passive Multipath Configuration with One RAID Device
In this configuration, there is one I/O path that goes through hba1, SAN1, and controller 1 and a
second I/O path that goes through hba2, SAN2, and controller2. There are many points of possible
failure in this configuration:
• HBA failure
• FC cable failure
• SAN switch failure
• Array controller port failure
With DM-Multipath configured, a failure at any of these points will cause DM-Multipath to switch to the
alternate I/O path.
Figure 1.2, “Active/Passive Multipath Configuration with Two RAID Devices” shows a more complex
active/passive configuration with 2 HBAs on the server, 2 SAN switches, and 2 RAID devices with 2
RAID controllers each.
3
Chapter 1. Device Mapper Multipathing
Figure 1.2. Active/Passive Multipath Configuration with Two RAID Devices
In the example shown in Figure 1.2, “Active/Passive Multipath Configuration with Two RAID Devices”,
there are two I/O paths to each RAID device (just as there are in the example shown in Figure 1.1,
“Active/Passive Multipath Configuration with One RAID Device”). With DM-Multipath configured, a
failure at any of the points of the I/O path to either of the RAID devices will cause DM-Multipath to
switch to the alternate I/O path for that device.
Figure 1.3, “Active/Active Multipath Configuration with One RAID Device” shows an active/active
configuration with 2 HBAs on the server, 1 SAN switch, and 2 RAID controllers. There are four I/O
paths from the server to a storage device:
• hba1 to controller1
• hba1 to controller2
• hba2 to controller1
• hba2 to controller2
In this configuration, I/O can be spread among those four paths.
4
Storage Array Support
Figure 1.3. Active/Active Multipath Configuration with One RAID Device
1.3. Storage Array Support
By default, DM-Multipath includes support for the most common storage arrays that support DMMultipath. The supported devices can be found in the multipath.conf.defaults file. If your
storage array supports DM-Multipath and is not configured by default in this file, you may need to add
them to the DM-Multipath configuration file, multipath.conf. For information on the DM-Multipath
configuration file, see Chapter 4, The DM-Multipath Configuration File.
Some storage arrays require special handling of I/O errors and path switching. These require separate
hardware handler kernel modules.
1.4. DM-Multipath Components
Table 1.1, “DM-Multipath Components”. describes the components of DM-Multipath.
Table 1.1. DM-Multipath Components
Component
Description
dm_multipath kernel module
Reroutes I/O and supports failover for paths and path groups.
mpathconf utility
Configures and enables device mapper multipathing.
multipath command
Lists and configures multipath devices. Normally started up
with /etc/rc.sysinit, it can also be started up by a udev
program whenever a block device is added or it can be run by the
initramfs file system.
multipathd daemon
Monitors paths; as paths fail and come back, it may initiate path
group switches. Provides for interactive changes to multipath
devices. This must be restarted for any changes to the /etc/
multipath.conf file.
kpartx command
Creates device mapper devices for the partitions on a device
It is necessary to use this command for DOS-based partitions
5
Chapter 1. Device Mapper Multipathing
Component
Description
with DM-MP. The kpartx is provided in its own package, but the
device-mapper-multipath package depends on it.
1.5. DM-Multipath Setup Overview
DM-Multipath includes compiled-in default settings that are suitable for common multipath
configurations. Setting up DM-multipath is often a simple procedure.
The basic procedure for configuring your system with DM-Multipath is as follows:
1. Install the device-mapper-multipath rpm.
2. Create the configuration file and enable multipathing with the mpathconf command. You can also
start the multipath daemon with this command if you do not need to edit the configuration file.
3. If necessary, edit the multipath.conf configuration file to modify default values and save the
updated file.
4. Start the multipath daemon.
For detailed setup instructions for multipath configuration see Chapter 3, Setting Up DM-Multipath.
6
Chapter 2.
Multipath Devices
Without DM-Multipath, each path from a server node to a storage controller is treated by the system
as a separate device, even when the I/O path connects the same server node to the same storage
controller. DM-Multipath provides a way of organizing the I/O paths logically, by creating a single
multipath device on top of the underlying devices.
2.1. Multipath Device Identifiers
Each multipath device has a World Wide Identifier (WWID), which is guaranteed to be globally unique
and unchanging. By default, the name of a multipath device is set to its WWID. Alternately, you can set
the user_friendly_names option in the multipath configuration file, which sets the alias to a nodeunique name of the form mpathn.
For example, a node with two HBAs attached to a storage controller with two ports via a single
unzoned FC switch sees four devices: /dev/sda, /dev/sdb, dev/sdc, and /dev/sdd. DMMultipath creates a single device with a unique WWID that reroutes I/O to those four underlying
devices according to the multipath configuration. When the user_friendly_names configuration
option is set to yes, the name of the multipath device is set to mpathn.
When new devices are brought under the control of DM-Multipath, the new devices may be seen in
three different places under the /dev directory: /dev/mapper/mpathn, /dev/mpath/mpathn, and
/dev/dm-n.
• The devices in /dev/mapper are created early in the boot process. Use these devices to access
the multipathed devices, for example when creating logical volumes.
• The devices in /dev/mpath are provided as a convenience so that all multipathed devices can
be seen in one directory. These devices are created by the udev device manager and may not be
available on startup when the system needs to access them. Do not use these devices for creating
logical volumes or filesystems.
• Any devices of the form /dev/dm-n are for internal use only and should never be used.
For information on the multipath configuration defaults, including the user_friendly_names
configuration option, see Section 4.3, “Configuration File Defaults”.
You can also set the name of a multipath device to a name of your choosing by using the alias
option in the multipaths section of the multipath configuration file. For information on the
multipaths section of the multipath configuration file, see Section 4.4, “Multipaths Device
Configuration Attributes”.
2.2. Consistent Multipath Device Names in a Cluster
When the user_friendly_names configuration option is set to yes, the name of the multipath
device is unique to a node, but it is not guaranteed to be the same on all nodes using the multipath
device. Similarly, if you set the alias option for a device in the multipaths section of the
multipath.conf configuration file, the name is not automatically consistent across all nodes in
the cluster. This should not cause any difficulties if you use LVM to create logical devices from the
multipath device, but if you require that your multipath device names be consistent in every node it is
recommended that you not set the user_friendly_names option to yes and that you not configure
aliases for the devices. By default, if you do not set user_friendly_names to yes or configure an
alias for a device, a device name will be the WWID for the device, which is always the same.
7
Chapter 2. Multipath Devices
If you want the system-defined user-friendly names to be consistent across all nodes in the cluster,
however, you can follow this procedure:
1. Set up all of the multipath devices on one machine.
2. Disable all of your multipath devices on your other machines.
3. Copy the /etc/multipath/bindings file from the first machine to all the other machines in the
cluster.
If you add a new device, you will need to repeat this process.
Similarly, if you configure an alias for a device that you would like to be consistent across the nodes in
the cluster, you should ensure that the /etc/multipath.conf file is the same for each node in the
cluster by following the same procedure:
1. Configure the aliases for the multipath devices in the in the multipath.conf file on one
machine.
2. Disable all of your multipath devices on your other machines.
3. Copy the /etc/multipath.conf file from the first machine to all the other machines in the
cluster.
When you add a new device you will need to repeat this process.
2.3. Multipath Device Attributes
In addition to the user_friendly_names and alias options, a multipath device has numerous
attributes. You can modify these attributes for a specific multipath device by creating an entry for
that device in the multipaths section of the multipath configuration file. For information on the
multipaths section of the multipath configuration file, see see Section 4.4, “Multipaths Device
Configuration Attributes”.
2.4. Multipath Devices in Logical Volumes
After creating multipath devices, you can use the multipath device names just as you would use a
physical device name when creating an LVM physical volume. For example, if /dev/mapper/mpatha
is the name of a multipath device, the following command will mark /dev/mapper/mpatha as a
physical volume.
pvcreate /dev/mapper/mpatha
You can use the resulting LVM physical device when you create an LVM volume group just as you
would use any other LVM physical device.
8
Multipath Devices in Logical Volumes
Note
If you attempt to create an LVM physical volume on a whole device on which you have configured
partitions, the pvcreate command will fail. Note that the Anaconda and Kickstart installation
programs create empty partition tables if you do not specify otherwise for every block device. If
you wish to use the whole device rather than a partition, you must remove the existing partitions
from the device. You can remove existing partitions with the kpartx -d and the fdisk
commands. If your system has block devices that are greater that 2Tb, you can use the parted
command to remove partitions.
When you create an LVM logical volume that uses active/passive multipath arrays as the underlying
physical devices, you should include filters in the lvm.conf to exclude the disks that underlie the
multipath devices. This is because if the array automatically changes the active path to the passive
path when it receives I/O, multipath will failover and failback whenever LVM scans the passive path if
these devices are not filtered. For active/passive arrays that require a command to make the passive
path active, LVM prints a warning message when this occurs.
To filter all SCSI devices in the LVM configuration file (lvm.conf), include the following filter in the
devices section of the file.
filter = [ "r/disk/", "r/sd.*/", "a/.*/" ]
9
10
Chapter 3.
Setting Up DM-Multipath
This chapter provides step-by-step example procedures for configuring DM-Multipath. It includes the
following procedures:
• Basic DM-Multipath setup
• Ignoring local disks
• Adding more devices to the configuration file
3.1. Setting Up DM-Multipath
Before setting up DM-Multipath on your system, ensure that your system has been updated and
includes the device-mapper-multipath package.
You set up multipath with the mpathconf utility, which creates the multipath configuration file /etc/
multipath.conf.
• If the /etc/multipath.conf file already exists, the mpathconf utlity will edit it.
• If the /etc/multipath.conf file does not exist, the mpathconf utility will use the /usr/share/
doc/device-mapper-multipath-0.4.9/multipath.conf file as the starting file.
• If the /usr/share/doc/device-mapper-multipath-0.4.9/multipath.conf file does not
exist the mpathconf utility will create the /etc/multipath.conf file from scratch.
If you do not need to edit the /etc/multipath.conf file, you can set up DM-Multipath for a basic
failover configuration by running the following command. This command enables the multipath
configuration file and starts the multipathd daemon.
# mpathconf --enable --with_multipathd y
If you need to edit the /etc/multipath.conf file before starting the multipathd daemon. use the
following procedure to set up DM-Multipath for a basic failover configuration.
1. Run the mpathconf command with the --enable option specified:
# mpathconf --enable
For information on additional options to the mpathconf command you may require, see the
mpathconf man page or run the mpathconf command with the --help option specified.
# mpathconf --help
usage: /sbin/mpathconf <command>
Commands:
Enable: --enable
Disable: --disable
Set user_friendly_names (Default n): --user_friendly_names <y|n>
Set find_multipaths (Default n): --find_multipaths <y|n>
Load the dm-multipath modules on enable (Default y): --with_module
start/stop/reload multipathd (Default n): --with_multipathd <y|n>
chkconfig on/off multipathd (Default y): --with_chkconfig <y|n>
<y|n>
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Chapter 3. Setting Up DM-Multipath
2. Edit the /etc/multipath.conf file if necessary. The default settings for DM-Multipath are
compiled in to the system and do not need to be explicitly set in the /etc/multipath.conf file.
The default value of path_grouping_policy is set to failover, so in this example you do
not need to edit the /etc/multipath.conf file. For information on changing the values in
the configuration file to something other than the defaults, see Chapter 4, The DM-Multipath
Configuration File.
The initial defaults section of the configuration file configures your system so that the names of the
multipath devices are of the form mpathn; without this setting, the names of the multipath devices
would be aliased to the WWID of the device.
3. Save the configuration file and exit the editor, if necessary.
4. Execute the following command:
# service multipathd start
Since the value of user_friendly_name is set to yes in the configuration file, the multipath devices
will be created as /dev/mapper/mpathn. For information on setting the name of the device to an
alias of your choosing, see Chapter 4, The DM-Multipath Configuration File.
If you do not want to use user friendly names, you can run the following command:
# mpathconf --enable --user_friendly_names n
Note
If you find that you need to edit the multipath configuration file after you have started the multipath
daemon, you must execute the service multipath reload command for the changes to
take effect.
3.2. Ignoring Local Disks when Generating Multipath
Devices
Some machines have local SCSI cards for their internal disks. DM-Multipath is not recommended for
these devices. If you set the find_multipaths configuration parameter to yes, you should not have
to blacklist these devices. For information on the find_multipaths configuration parameter, see
Section 4.3, “Configuration File Defaults”.
If you do not set the find_multipaths configuration parameter to yes, can use the following
procedure to modify the multipath configuration file to ignore the local disks when configuring
multipath.
1. Determine which disks are the internal disks and mark them as the ones to blacklist.
In this example, /dev/sda is the internal disk. Note that as originally configured in the default
multipath configuration file, executing the multipath -v2 shows the local disk, /dev/sda, in
the multipath map.
For further information on the multipath command output, see Section 5.5, “Multipath
Command Output”.
12
Ignoring Local Disks when Generating Multipath Devices
[root@rh4cluster1 ~]# multipath -v2
create: SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1 undef WINSYS,SF2372
size=33 GB features="0" hwhandler="0" wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 0:0:0:0 sda 8:0 [--------device-mapper ioctl cmd 9 failed: Invalid argument
device-mapper ioctl cmd 14 failed: No such device or address
create: 3600a0b80001327d80000006d43621677 undef WINSYS,SF2372
size=12G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 2:0:0:0 sdb 8:16 undef ready running
`- 3:0:0:0 sdf 8:80 undef ready running
create: 3600a0b80001327510000009a436215ec undef WINSYS,SF2372
size=12G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 2:0:0:1 sdc 8:32 undef ready running
`- 3:0:0:1 sdg 8:96 undef ready running
create: 3600a0b80001327d800000070436216b3 undef WINSYS,SF2372
size=12G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 2:0:0:2 sdd 8:48 undef ready running
`- 3:0:0:2 sdg 8:112 undef ready running
create: 3600a0b80001327510000009b4362163e undef WINSYS,SF2372
size=12G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 2:0:0:3 sdd 8:64 undef ready running
`- 3:0:0:3 sdg 8:128 undef ready running
2. In order to prevent the device mapper from mapping /dev/sda in its multipath maps, edit the
blacklist section of the /etc/multipath.conf file to include this device. Although you could
blacklist the sda device using a devnode type, that would not be safe procedure since /dev/sda
is not guaranteed to be the same on reboot. To blacklist individual devices, you can blacklist using
the WWID of that device.
Note that in the output to the multipath -v2 command, the WWID of the /dev/sda device is
SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1. To blacklist this device, include the
following in the /etc/multipath.conf file.
blacklist {
wwid SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1
}
3. After you have updated the /etc/multipath.conf file, you must manually tell the
multipathd daemon to reload the file. The following command reloads the updated /etc/
multipath.conf file.
[root@rh4cluster1 ~]# service multipathd reload
4. Run the following command to remove the multipath device:
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Chapter 3. Setting Up DM-Multipath
[root@rh4cluster1 ~]# multipath -f SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1
5. To check whether the device removal worked, you can run the multipath -ll command to
display the current multipath configuration. For information on the multipath -ll command,
see Section 5.6, “Multipath Queries with multipath Command”.
To check that the blacklisted device was not added back, you can run the multipath command,
as in the following example. The multipath command defaults to a verbosity level of v2 if you
do not specify specify a -v option.
[root@rh4cluster1 ~]# multipath
create: 3600a0b80001327d80000006d43621677 undef WINSYS,SF2372
size=12G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 2:0:0:0 sdb 8:16 undef ready running
`- 3:0:0:0 sdf 8:80 undef ready running
create: 3600a0b80001327510000009a436215ec undef WINSYS,SF2372
size=12G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 2:0:0:1 sdc 8:32 undef ready running
`- 3:0:0:1 sdg 8:96 undef ready running
create: 3600a0b80001327d800000070436216b3 undef WINSYS,SF2372
size=12G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 2:0:0:2 sdd 8:48 undef ready running
`- 3:0:0:2 sdg 8:112 undef ready running
create: 3600a0b80001327510000009b4362163e undef WINSYS,SF2372
size=12G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
|- 2:0:0:3 sdd 8:64 undef ready running
`- 3:0:0:3 sdg 8:128 undef ready running
3.3. Configuring Storage Devices
By default, DM-Multipath includes support for the most common storage arrays that support DMMultipath. The default configuration values, including supported devices, can be found in the
multipath.conf.defaults file.
If you need to add a storage device that is not supported by default as a known multipath device, edit
the /etc/multipath.conf file and insert the appropriate device information.
For example, to add information about the HP Open-V series the entry looks like this, where %n is the
device name:
devices {
device {
vendor "HP"
product "OPEN-V."
getuid_callout "/lib/udev/scsi_id --whitelisted --device=/dev/%n"
}
}
For more information on the devices section of the configuration file, see Section 4.5, “Configuration
File Devices”.
14
Chapter 4.
The DM-Multipath Configuration File
By default, DM-Multipath provides configuration values for the most common uses of multipathing.
In addition, DM-Multipath includes support for the most common storage arrays that support DMMultipath. The default configuration values and the supported devices can be found in the /usr/
share/doc/device-mapper-multipath-0.4.9/multipath.conf.defaults file.
You can override the default configuration values for DM-Multipath by editing the /etc/
multipath.conf configuration file. If necessary, you can also add a storage array that is not
supported by default to the configuration file. This chapter provides information on parsing and
modifying the multipath.conf file. It contains sections on the following topics:
• Configuration file overview
• Configuration file blacklist
• Configuration file defaults
• Configuration file multipaths
• Configuration file devices
In the multipath configuration file, you need to specify only the sections that you need
for your configuration, or that you wish to change from the default values specified in the
multipath.conf.defaults file. If there are sections of the file that are not relevant to your
environment or for which you do not need to override the default values, you can leave them
commented out, as they are in the initial file.
The configuration file allows regular expression description syntax.
An annotated version of the configuration file can be found in /usr/share/doc/device-mappermultipath-0.4.9/multipath.conf.annotated.
4.1. Configuration File Overview
The multipath configuration file is divided into the following sections:
blacklist
Listing of specific devices that will not be considered for multipath.
blacklist_exceptions
Listing of multipath candidates that would otherwise be blacklisted according to the parameters of
the blacklist section.
defaults
General default settings for DM-Multipath.
multipaths
Settings for the characteristics of individual multipath devices. These values overwrite what is
specified in the defaults and devices sections of the configuration file.
devices
Settings for the individual storage controllers. These values overwrite what is specified in the
defaults section of the configuration file. If you are using a storage array that is not supported
by default, you may need to create a devices subsection for your array.
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Chapter 4. The DM-Multipath Configuration File
When the system determines the attributes of a multipath device, first it checks the multipath settings,
then the per devices settings, then the multipath system defaults.
4.2. Configuration File Blacklist
The blacklist section of the multipath configuration file specifies the devices that will not be used
when the system configures multipath devices. Devices that are blacklisted will not be grouped into a
multipath device.
In older releases of Red Hat Enterprise Linux, multipath always tried to create a multipath device
for every path that was not explicitly blacklisted. In Red Hat Enterprise Linux 6, however, if the
find_multipath configuration parameter is set to yes, then multipath will create a device only if
one of three conditions are met:
• There are at least two non-blacklisted paths with the same WWID.
• The user manually forces the creation of the device by specifying a device with the multipath
command.
• A path has the same WWID as a multipath device that was previously created (even if that multipath
device does not currently exist). Whenever a multipath device is created, multipath remembers the
WWID of the device so that it will automatically create the device again as soon as it sees a path
with that WWID. This allows you to have multipath automatically choose the correct paths to make
into multipath devices, without have to edit the multipath blacklist.
If you have previously created a multipath device without using the find_multipaths parameter
and then you later set the parameter to yes, you may need to remove the WWIDs of any device you
do not want created as a multipath device from the /etc/multipath/wwids file. The following
shows a sample /etc/multipath/wwids file. The WWIDs are enclose by slashes (/):
# Multipath wwids, Version : 1.0
# NOTE: This file is automatically maintained by multipath and multipathd.
# You should not need to edit this file in normal circumstances.
#
# Valid WWIDs:
/3600d0230000000000e13955cc3757802/
/3600d0230000000000e13955cc3757801/
/3600d0230000000000e13955cc3757800/
/3600d02300069c9ce09d41c31f29d4c00/
/SWINSYS SF2372
0E13955CC3757802/
/3600d0230000000000e13955cc3757803/
With the find_multipaths parameter set to yes, you need to blacklist only the devices with
multiple paths that you do not want to be multipathed. Because of this, it will generally not be
necessary to blacklist devices.
If you do need to blacklist devices, you can do so according to the following criteria:
• By WWID, as described in Section 4.2.1, “Blacklisting by WWID”
• By device name, as described in Section 4.2.2, “Blacklisting By Device Name”
• By device type, as described in Section 4.2.3, “Blacklisting By Device Type”
By default, a variety of device types are blacklisted, even after you comment out the initial blacklist
section of the configuration file. For information, see Section 4.2.2, “Blacklisting By Device Name”.
16
Blacklisting by WWID
4.2.1. Blacklisting by WWID
You can specify individual devices to blacklist by their World-Wide IDentification with a wwid entry in
the blacklist section of the configuration file.
The following example shows the lines in the configuration file that would blacklist a device with a
WWID of 26353900f02796769.
blacklist {
wwid 26353900f02796769
}
4.2.2. Blacklisting By Device Name
You can blacklist device types by device name so that they will not be grouped into a multipath device
by specifying a devnode entry in the blacklist section of the configuration file.
The following example shows the lines in the configuration file that would blacklist all SCSI devices,
since it blacklists are sd* devices.
blacklist {
devnode "^sd[a-z]"
}
You can use a devnode entry in the blacklist section of the configuration file to specify individual
devices to blacklist rather than all devices of a specific type. This is not recommended, however, since
unless it is statically mapped by udev rules, there is no guarantee that a specific device will have the
same name on reboot. For example, a device name could change from /dev/sda to /dev/sdb on
reboot.
By default, the following devnode entries are compiled in the default blacklist; the devices that these
entries blacklist do not generally support DM-Multipath. To enable multipathing on any of these
devices, you would need to specify them in the blacklist_exceptions section of the configuration
file, as described in Section 4.2.4, “Blacklist Exceptions”.
blacklist {
devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
devnode "^hd[a-z]"
}
4.2.3. Blacklisting By Device Type
You can specify specific device types in the blacklist section of the configuration file with a device
section. The following example blacklists all IBM DS4200 device and all HP devices.
blacklist {
device {
vendor
product
}
device {
vendor
product
"IBM"
"3S42"
#DS4200 Product 10
"HP"
"*"
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Chapter 4. The DM-Multipath Configuration File
}
}
4.2.4. Blacklist Exceptions
You can use the blacklist_exceptions section of the configuration file to enable multipathing on
devices that have been blacklisted by default.
For example, if you have a large number of devices and want to multipath only one of them (with the
WWID of 3600d0230000000000e13955cc3757803), instead of individually blacklisting each of the
devices except the one you want, you could instead blacklist all of them, and then allow only the one
you want by adding the following lines to the /etc/multipath.conf file.
blacklist {
wwid "*"
}
blacklist_exceptions {
wwid "3600d0230000000000e13955cc3757803"
}
When specifying devices in the blacklist_exceptions section of the configuration file, you must
specify the exceptions in the same way they were specified in the blacklist. For example, a WWID
exception will not apply to devices specified by a devnode blacklist entry, even if the blacklisted
device is associated with that WWID. Similarly, devnode exceptions apply only to devnode entries,
and device exceptions apply only to device entries.
4.3. Configuration File Defaults
The /etc/multipath.conf configuration file includes a defaults section that sets the
user_friendly_names parameter to yes, as follows.
defaults {
user_friendly_names yes
}
This overwrites the default value of the user_friendly_names parameter.
The configuration file includes a template of configuration defaults. This section is commented out, as
follows.
#defaults {
#
udev_dir
#
polling_interval
#
selector
#
path_grouping_policy
#
getuid_callout
# prio
const
# path_checker directio
# rr_min_io 1000
# rr_weight uniform
# failback manual
# no_path_retry fail
# user_friendly_names no
#}
18
/dev
5
"round-robin 0"
failover
"/lib/dev/scsi_id --whitelisted --device=/dev/%n"
Configuration File Defaults
To overwrite the default value for any of the configuration parameters, you can copy the relevant
line from this template into the defaults section and uncomment it. For example, to overwrite
the path_grouping_policy parameter so that it is multibus rather than the default value
of failover, copy the appropriate line from the template to the initial defaults section of the
configuration file, and uncomment it, as follows.
defaults {
user_friendly_names
path_grouping_policy
}
yes
multibus
Table 4.1, “Multipath Configuration Defaults” describes the attributes that are set in the defaults
section of the multipath.conf configuration file. These values are used by DM-Multipath unless
they are overwritten by the attributes specified in the devices and multipaths sections of the
multipath.conf file.
Table 4.1. Multipath Configuration Defaults
Attribute
Description
polling_interval
Specifies the interval between two path checks in seconds.
For properly functioning paths, the interval between checks will
gradually increase to (4 * polling_interval). The default value
is 5.
udev_dir
The directory where udev device nodes are created. The default
value is /dev.
multipath_dir
The directory where the dynamic shared objects are stored. The
default value is system dependent, commonly /lib/multipath.
find_multipaths
Defines the mode for setting up multipath devices. If this
parameter is set to yes, then multipath will not try to create a
device for every non-blacklisted path. Instead multipath will create
a device only if one of three conditions are met:
- There are at least two non-blacklisted paths with the same
WWID.
- The user manually forces the creation of the device by specifying
a device with the multipath command.
- A path has the same WWID as a multipath device that was
previously created. Whenever a multipath device is created with
find_multipaths set, multipath remembers the WWID of the
device so that it will automatically create the device again as
soon as it sees a path with that WWID. This allows you to have
multipath automatically choose the correct paths to make into
multipath devices, without having to edit the multipath blacklist.
For instructions on the procedure to follow if you have previously
created multipath devices when the find_multipaths
parameter was not set, see Section 4.2, “Configuration File
Blacklist”.
The default value is no.
verbosity
The default verbosity. Higher values increase the verbosity level.
Valid levels are between 0 and 6. The default value is 2.
path_selector
Specifies the default algorithm to use in determining what path to
use for the next I/O operation. Possible values include:
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Chapter 4. The DM-Multipath Configuration File
Attribute
Description
round-robin 0: Loop through every path in the path group,
sending the same amount of I/O to each.
queue-length 0: Send the next bunch of I/O down the path with
the least number of outstanding I/O requests.
service-time 0: Send the next bunch of I/O down the path
with the shortest estimated service time, which is determined by
dividing the total size of the outstanding I/O to each path by its
relative throughput.
The default value is round-robin 0.
path_grouping_policy
Specifies the default path grouping policy to apply to unspecified
multipaths. Possible values include:
failover: 1 path per priority group.
multibus: all valid paths in 1 priority group.
group_by_serial: 1 priority group per detected serial number.
group_by_prio: 1 priority group per path priority value. Priorities
are determined by callout programs specified as global, percontroller, or per-multipath options.
group_by_node_name: 1 priority group per target node
name. Target node names are fetched in /sys/class/
fc_transport/target*/node_name.
The default value is multibus.
getuid_callout
Specifies the default program and arguments to call out to obtain a
unique path identifier. An absolute path is required.
The default value is /lib/udev/scsi_id --whitelisted -device=/dev/%n.
prio
Specifies the default function to call to obtain a path priority value.
For example, the ALUA bits in SPC-3 provide an exploitable prio
value. Possible values include:
const: Set a priority of 1 to all paths.
emc: Generate the path priority for EMC arrays.
alua: Generate the path priority based on the SCSI-3 ALUA
settings.
tpg_pref: Generate the path priority based on the SCSI-3 ALUA
settings, using the preferred port bit.
ontap: Generate the path priority for NetApp arrays.
rdac: Generate the path priority for LSI/Engenio RDAC controller.
hp_sw: Generate the path priority for Compaq/HP controller in
active/standby mode.
hds: Generate the path priority for Hitachi HDS Modular storage
arrays.
The default value is const.
features
The default extra features of multipath devices. The only existing
feature is queue_if_no_path, which is the same as setting
no_path_retry to queue. For information on issues that may
arise when using this feature, see Section 5.4, “Issues with
queue_if_no_path feature”.
path_checker
Specifies the default method used to determine the state of the
paths. Possible values include:
readsector0: Read the first sector of the device.
tur: Issue a TEST UNIT READY to the device.
20
Configuration File Defaults
Attribute
Description
emc_clariion: Query the EMC Clariion specific EVPD page
0xC0 to determine the path.
hp_sw: Check the path state for HP storage arrays with Active/
Standby firmware.
rdac: Check the path stat for LSI/Engenio RDAC storage
controller.
directio: Read the first sector with direct I/O.
The default value is readsector0.
failback
Manages path group failback.
A value of immediate specifies immediate failback to the highest
priority path group that contains active paths.
A value of manual specifies that there should not be immediate
failback but that failback can happen only with operator
intervention.
A value of followover specifies that automatic failback should
be performed when the first path of a path group becomes active.
This keeps a node from automatically failing back when another
node requested the failover.
A numeric value greater than zero specifies deferred failback,
expressed in seconds.
The default value is manual.
rr_min_io
Specifies the number of I/O requests to route to a path before
switching to the next path in the current path group. The default
value is 1000.
rr_weight
If set to priorities, then instead of sending rr_min_io
requests to a path before calling path_selector to choose
the next path, the number of requests to send is determined by
rr_min_io times the path's priority, as determined by the prio
function. If set to uniform, all path weights are equal. The default
value is uniform.
no_path_retry
A numeric value for this attribute specifies the number of times
the system should attempt to use a failed path before disabling
queueing.
A value of fail indicates immediate failure, without queuing.
A value of queue indicates that queuing should not stop until the
path is fixed.
The default value is 0.
user_friendly_names
If set to yes, specifies that the system should use the /etc/
multipath/bindings file to assign a persistent and unique
alias to the multipath, in the form of mpathn. If set to no, specifies
that the system should use use the WWID as the alias for the
multipath. In either case, what is specified here will be overriden
by any device-specific aliases you specify in the multipaths
section of the configuration file. The default value is no.
queue_without_daemon
If set to no, the multipathd daemon will disable queueing for all
devices when it is shut down. The default value is yes.
flush_on_last_del
If set to yes, the multipathd daemon will disable queueing
when the last path to a device has been deleted. The default value
is no.
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Chapter 4. The DM-Multipath Configuration File
Attribute
Description
max_fds
Sets the maximum number of open file descriptors that can be
opened by multipath and the multipathd daemon. This is
equivalent to the ulimit -n command. A value of max will set
this to the system limit from /proc/sys/fs/nr_open. If this
is not set, the maximum number of open file descriptors is taken
from the calling process; it is usually 1024. To be safe, this should
be set to the maximum number of paths plus 32, if that number is
greater than 1024.
checker_timer
The timeout to use for path checkers that issue scsi commands
with an explicit timeout, in seconds. The default value is taken
from sys/block/sdx/device/timeout.
fast_io_fail_tmo
The number of seconds the scsi layer will wait after a problem has
been detected on an FC remote port before failing I/O to devices
on that remote port. This value should be smaller than the value of
dev_loss_tmo. Setting this to off will disable the timeout. The
default value is determined by the OS.
dev_loss_tmo
The number of seconds the scsi layer will wait after a problem has
been detected on an FC remote port before removing it from the
system. The default value is determined by the OS.
mode
The mode to use for the multipath device nodes, in octal. The
default value is determined by the process.
uid
The user ID to use for the multipath device nodes. You may
use either the numeric or symbolic user ID. The default value is
determined by the process.
gid
The group ID to use for the multipath device nodes. You may
use either the numeric or symbolic group ID. The default value is
determined by the process.
4.4. Multipaths Device Configuration Attributes
Table 4.2, “Multipath Attributes” shows the attributes that you can set in the multipaths section of
the multipath.conf configuration file for each specific multipath device. These attributes apply only
to the one specified multipath. These defaults are used by DM-Multipath and override attributes set in
the defaults and devices sections of the multipath.conf file.
Table 4.2. Multipath Attributes
Attribute
Description
wwid
Specifies the WWID of the multipath device to which the
multipath attributes apply. This parameter is mandatory for this
section of the multipath.conf file.
alias
Specifies the symbolic name for the multipath device to
which the multipath attributes apply. If you are using
user_friendly_names, do not set this value to mpathn; this
may conflict with an automatically assigned user friendly name
and give you incorrect device node names.
path_grouping_policy
Specifies the default path grouping policy to apply to unspecified
multipaths. Possible values include:
failover = 1 path per priority group
22
Multipaths Device Configuration Attributes
Attribute
Description
multibus = all valid paths in 1 priority group
group_by_serial = 1 priority group per detected serial number
group_by_prio = 1 priority group per path priority value
group_by_node_name = 1 priority group per target node name
path_selector
Specifies the default algorithm to use in determining what path to
use for the next I/O operation. Possible values include:
round-robin 0: Loop through every path in the path group,
sending the same amount of I/O to each.
queue-length 0: Send the next bunch of I/O down the path with
the least number of outstanding I/O requests.
service-time 0: Send the next bunch of I/O down the path
with the shortest estimated service time, which is determined by
dividing the total size of the outstanding I/O to each path by its
relative throughput.
The default value is round-robin 0.
failback
Manages path group failback.
A value of immediate specifies immediate failback to the highest
priority path group that contains active paths.
A value of manual specifies that there should not be immediate
failback but that failback can happen only with operator
intervention.
A value of followover specifies that automatic failback should
be performed when the first path of a path group becomes active.
This keeps a node from automatically failing back when another
node requested the failover.
A numeric value greater than zero specifies deferred failback,
expressed in seconds.
The default value is manual.
prio
Specifies the default function to call to obtain a path priority value.
For example, the ALUA bits in SPC-3 provide an exploitable prio
value. Possible values include:
const: Set a priority of 1 to all paths.
emc: Generate the path priority for EMC arrays.
alua: Generate the path priority based on the SCSI-3 ALUA
settings.
tpg_pref: Generate the path priority based on the SCSI-3 ALUA
settings, using the preferred port bit.
ontap: Generate the path priority for NetApp arrays.
rdac: Generate the path priority for LSI/Engenio RDAC controller.
hp_sw: Generate the path priority for Compaq/HP controller in
active/standby mode.
hds: Generate the path priority for Hitachi HDS Modular storage
arrays.
no_path_retry
A numeric value for this attribute specifies the number of times
the system should attempt to use a failed path before disabling
queueing.
A value of fail indicates immediate failure, without queueing.
A value of queue indicates that queuing should not stop until the
path is fixed.
23
Chapter 4. The DM-Multipath Configuration File
Attribute
Description
rr_min_io
Specifies the number of I/O requests to route to a path before
switching to the next path in the current path group. The default
value is 1000.
rr_weight
If set to priorities, then instead of sending rr_min_io
requests to a path before calling path_selector to choose
the next path, the number of requests to send is determined by
rr_min_io times the path's priority, as determined by the prio
function. If set to uniform, all path weights are equal.
flush_on_last_del
If set to yes, then multipath will disable queueing when the last
path to a device has been deleted. The default value is no.
mode
The mode to use for the multipath device nodes, in octal.
uid
The user ID to use for the multipath device nodes. You may use
the either the symbolic or numeric user ID. The default value is
determined by the process.
gid
The group ID to use for the multipath device nodes. You may use
either the symbolic or the numeric group ID. The default value is
determined by the process.
The following example shows multipath attributes specified in the configuration file for two specific
multipath devices. The first device has a WWID of 3600508b4000156d70001200000b0000 and a
symbolic name of yellow.
The second multipath device in the example has a WWID of 1DEC_____321816758474 and a
symbolic name of red. In this example, the rr_weight attributes is set to priorities.
multipaths {
multipath {
wwid
alias
path_grouping_policy
path_checker
path_selector
failback
rr_weight
no_path_retry
}
multipath {
wwid
alias
rr_weight
}
}
3600508b4000156d70001200000b0000
yellow
multibus
readsector0
"round-robin 0"
manual
priorities
5
1DEC_____321816758474
red
priorities
4.5. Configuration File Devices
Table 4.3, “Device Attributes” shows the attributes that you can set for each individual storage device
in the devices section of the multipath.conf configuration file. These attributes are used by DMMultipath unless they are overwritten by the attributes specified in the multipaths section of the
multipath.conf file for paths that contain the device. These attributes override the attributes set in
the defaults section of the multipath.conf file.
Many devices that support multipathing are included by default in a multipath configuration. The values
for the devices that are supported by default are listed in the multipath.conf.defaults file.
24
Configuration File Devices
You probably will not need to modify the values for these devices, but if you do you can overwrite the
default values by including an entry in the configuration file for the device that overwrites those values.
You can copy the device configuration defaults from the multipath.conf.defaults file for the
device and override the values that you want to change.
To add a device to this section of the configuration file that is not configured automatically by default,
you need to set the vendor and product parameters. You can find these values by looking at /sys/
block/device_name/device/vendor and /sys/block/device_name/device/model where
device_name is the device to be multipathed, as in the following example:
[root@cypher-06 ~]# cat /sys/block/sda/device/vendor
WINSYS
[root@cypher-06 ~]# cat /sys/block/sda/device/model
SF2372
The additional parameters to specify depend on your specific device. If the device is active/active, you
will usually not need to set additional parameters. You may want to set path_grouping_policy
to multibus. Other parameters you may need to set are no_path_retry and rr_min_io, as
described in Table 4.3, “Device Attributes”.
If the device is active/passive, but it automatically switches paths with I/O to the passive path,
you need to change the checker function to one that does not send I/O to the path to test if it is
working (otherwise, your device will keep failing over). This almost always means that you set the
path_checker to tur; this works for all SCSI devices that support the Test Unit Ready command,
which most do.
If the device needs a special command to switch paths, then configuring this device for multipath
requires a hardware handler kernel module. The current available hardware handler is emc. If this is
not sufficient for your device, you may not be able to configure the device for multipath.
Table 4.3. Device Attributes
Attribute
Description
vendor
Specifies the vendor name of the storage device to which the
device attributes apply, for example COMPAQ.
product
Specifies the product name of the storage device to which the
device attributes apply, for example HSV110 (C)COMPAQ.
revision
Specifies the product revision identifier of the storage device.
product_blacklist
Specifies a regular expression used to blacklist devices by
product.
hardware_handler
Specifies a module that will be used to perform hardware specific
actions when switching path groups or handling I/O errors.
Possible values include:
1 emc: hardware handler for EMC storage arrays.
1 alua: hardware handler for SCSI-3 ALUA arrays.
1 hp_sw: hardware handler for Compaq/HP controllers.
1 rdac: hardware handler for the LSI/Engenio RDAC controllers.
path_grouping_policy
Specifies the default path grouping policy to apply to unspecified
multipaths. Possible values include:
failover = 1 path per priority group
multibus = all valid paths in 1 priority group
group_by_serial = 1 priority group per detected serial number
group_by_prio = 1 priority group per path priority value
25
Chapter 4. The DM-Multipath Configuration File
Attribute
Description
group_by_node_name = 1 priority group per target node name
getuid_callout
Specifies the default program and arguments to call out to obtain a
unique path identifier. An absolute path is required.
path_selector
Specifies the default algorithm to use in determining what path to
use for the next I/O operation. Possible values include:
round-robin 0: Loop through every path in the path group,
sending the same amount of I/O to each.
queue-length 0: Send the next bunch of I/O down the path with
the least number of outstanding I/O requests.
service-time 0: Send the next bunch of I/O down the path
with the shortest estimated service time, which is determined by
dividing the total size of the outstanding I/O to each path by its
relative throughput.
The default value is round-robin 0.
path_checker
Specifies the default method used to determine the state of the
paths. Possible values include:
readsector0: Read the first sector of the device.
tur: Issue a TEST UNIT READY to the device.
emc_clariion: Query the EMC Clariion specific EVPD page
0xC0 to determine the path.
hp_sw: Check the path state for HP storage arrays with Active/
Standby firmware.
rdac: Check the path stat for LSI/Engenio RDAC storage
controller.
directio: Read the first sector with direct I/O.
The default value is readsector0.
features
The extra features of multipath devices. The only existing
feature is queue_if_no_path, which is the same as setting
no_path_retry to queue. For information on issues that may
arise when using this feature, see Section 5.4, “Issues with
queue_if_no_path feature”.
prio
Specifies the default function to call to obtain a path priority value.
For example, the ALUA bits in SPC-3 provide an exploitable prio
value. Possible values include:
const: Set a priority of 1 to all paths.
emc: Generate the path priority for EMC arrays.
alua: Generate the path priority based on the SCSI-3 ALUA
settings.
tpg_pref: Generate the path priority based on the SCSI-3 ALUA
settings, using the preferred port bit.
ontap: Generate the path priority for NetApp arrays.
rdac: Generate the path priority for LSI/Engenio RDAC controller.
hp_sw: Generate the path priority for Compaq/HP controller in
active/standby mode.
hds: Generate the path priority for Hitachi HDS Modular storage
arrays.
The default value is const.
failback
Manages path group failback.
A value of immediate specifies immediate failback to the highest
priority path group that contains active paths.
26
Configuration File Devices
Attribute
Description
A value of manual specifies that there should not be immediate
failback but that failback can happen only with operator
intervention.
A value of followover specifies that automatic failback should
be performed when the first path of a path group becomes active.
This keeps a node from automatically failing back when another
node requested the failover.
A numeric value greater than zero specifies deferred failback,
expressed in seconds.
The default value is manual.
rr_weight
If set to priorities, then instead of sending rr_min_io
requests to a path before calling path_selector to choose
the next path, the number of requests to send is determined by
rr_min_io times the path's priority, as determined by the prio
function. If set to uniform, all path weights are equal. The default
value is uniform.
no_path_retry
A numeric value for this attribute specifies the number of times
the system should attempt to use a failed path before disabling
queueing.
A value of fail indicates immediate failure, without queuing.
A value of queue indicates that queuing should not stop until the
path is fixed.
The default value is 0.
rr_min_io
The number of I/O requests to route to a path before switching to
the next path in the current path group. The default value is 1000.
fast_io_fail_tmo
The number of seconds the scsi layer will wait after a problem has
been detected on an FC remote port before failing I/O to devices
on that remote port. This value should be smaller than the value of
dev_loss_tmo. Setting this to off will disable the timeout.
dev_loss_tmo
The number of seconds the scsi layer will wait after a problem has
been detected on an FC remote port before removing it from the
system.
flush_on_last_del
If set to yes, the multipathd daemon will disable queueing
when the last path to a device has been deleted. The default value
is no.
The following example shows a device entry in the multipath configuration file.
# }
# device {
# vendor
"COMPAQ "
# product
"MSA1000
"
# path_grouping_policy multibus
# path_checker tur
# rr_weight priorities
# }
#}
27
28
Chapter 5.
DM-Multipath Administration and
Troubleshooting
This chapter will provide information on administering DM-Multipath on a running system. It includes
sections on the following topics:
• Resizing an Online Multipath Device
• The Multipath Daemon
• Issues with Large Number of LUNs
• Issues with queue_if_no_path feature
• Multipath Command Output
• Multipath Queries with multipath Command
• Multipath Command Options
• Multipath Queries with dmsetup Command
• Troubleshooting with the multipathd Interactive Console
5.1. Resizing an Online Multipath Device
If you need to resize an online multipath device, use the following procedure.
1. Resize your physical device.
2. Use the following command to find the paths to the LUN:
# multipath -l
3. Resize your paths. For SCSI devices, writing a 1 to the rescan file for the device causes the
SCSI driver to rescan, as in the following command:
# echo 1 > /sys/block/device_name/device/rescan
4. Resize your multipath device by running the multipathd resize command:
# multipathd -k'resize map mpatha'
5. Resize the filesystem (assuming no LVM or DOS partitions are used):
# resize2fs /dev/mapper/mpatha
29
Chapter 5. DM-Multipath Administration and Troubleshooting
5.2. The Multipath Daemon
If you find you have trouble implementing a multipath configuration, you should ensure that the
multipath daemon is running, as described in Chapter 3, Setting Up DM-Multipath. The multipathd
daemon must be running in order to use multipathed devices.
5.3. Issues with Large Number of LUNs
When a large number of LUNs are added to a node, using multipathed devices can significantly
increase the time it takes for the udev device manager to create device nodes for them. If
you experience this problem, you can correct it by deleting the following line in /etc/udev/
rules.d/40-multipath.rules:
KERNEL!="dm-[0-9]*", ACTION=="add", PROGRAM=="/bin/bash -c '/sbin/lsmod | /bin/grep
^dm_multipath'", RUN+="/sbin/multipath -v0 %M:%m"
This line causes the udev device manager to run multipath every time a block device is added
to the node. Even with this line removed, the multipathd daemon will still automatically create
multipathed devices, and multipath will still be called during the boot process for nodes with
multipathed root file systems. The only change is that multipathed devices will not be automatically
created when the multipathd daemon is not running, which should not be a problem for the vast
majority of multipath users.
5.4. Issues with queue_if_no_path feature
If features "1 queue_if_no_path" is specified in the /etc/multipath.conf file, then
any process that issues I/O will hang until one or more paths are restored. To avoid this, set the
no_path_retry N parameter in the /etc/multipath.conf file (where N is the number of times
the system should retry a path).
When you set the no_path_retry parameter, remove the features "1 queue_if_no_path"
option from the /etc/multipath.conf file as well. If, however, you are using a multipathed device
for which the features "1 queue_if_no_path" option is set as a compiled-in default, as it is for
many SAN devices, you must explicitly add features "0" to override this default. You can do this
by copying the existing devices section for your device from /usr/share/doc/device-mappermultipath-0.4.9/multipath.conf.defaults into /etc/multipath.conf and editing it to
suit your needs.
If you need to use the features "1 queue_if_no_path" option and you experience the issue
noted here, use the dmsetup command to edit the policy at runtime for a particular LUN (that is, for
which all the paths are unavailable). For example, if you want to change the policy on the multipath
device mpathc from "queue_if_no_path" to "fail_if_no_path", execute the following
command.
dmsetup message mpathc 0 "fail_if_no_path"
Note that you must specify the mpathn alias rather than the path.
5.5. Multipath Command Output
When you create, modify, or list a multipath device, you get a printout of the current device setup. The
format is as follows.
30
Multipath Queries with multipath Command
For each multipath device:
action_if_any: alias (wwid_if_different_from_alias) dm_device_name_if_known vendor,product
size=size features='features' hwhandler='hardware_handler' wp=write_permission_if_known
For each path group:
-+- policy='scheduling_policy' prio=prio_if_known status=path_group_status_if_known
For each path:
`- host:channel:id:lun devnode major:minor path_status dm_status_if_known
For example, the output of a multipath command might appear as follows:
3600d0230000000000e13955cc3757800 dm-1 WINSYS,SF2372
size=269G features='0' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=1 status=active
| `- 6:0:0:0 sdb 8:16 active ready running
`-+- policy='round-robin 0' prio=1 status=enabled
`- 7:0:0:0 sdf 8:80 active ready running
If the path is up and ready for I/O, the status of the path is ready or active. If the path is down, the
status is faulty or failed. The path status is updated periodically by the multipathd daemon
based on the polling interval defined in the /etc/multipath.conf file.
The dm status is similar to the path status, but from the kernel's point of view. The dm status has
two states: failed, which is analogous to faulty, and active which covers all other path states.
Occasionally, the path state and the dm state of a device will temporarily not agree.
Note
When a multipath device is being created or modified, the path group status, the dm device
name, the write permissions, and the dm status are not known. Also, the features are not always
correct.
5.6. Multipath Queries with multipath Command
You can use the -l and -ll options of the multipath command to display the current multipath
configuration. The -l option displays multipath topology gathered from information in sysfs and
the device mapper. The -ll option displays the information the -l displays in addition to all other
available components of the system.
When displaying the multipath configuration, there are three verbosity levels you can specify with the
-v option of the multipath command. Specifying -v0 yields no output. Specifying -v1 outputs the
created or updated multipath names only, which you can then feed to other tools such as kpartx.
Specifying -v2 prints all detected paths, multipaths, and device maps.
The following example shows the output of a multipath -l command.
31
Chapter 5. DM-Multipath Administration and Troubleshooting
# multipath -l
3600d0230000000000e13955cc3757800 dm-1 WINSYS,SF2372
size=269G features='0' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=1 status=active
| `- 6:0:0:0 sdb 8:16 active ready running
`-+- policy='round-robin 0' prio=1 status=enabled
`- 7:0:0:0 sdf 8:80 active ready running
The following example shows the output of a multipath -ll command.
# multipath -ll
3600d0230000000000e13955cc3757801 dm-10 WINSYS,SF2372
size=269G features='0' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=1 status=enabled
| `- 19:0:0:1 sdc 8:32 active ready running
`-+- policy='round-robin 0' prio=1 status=enabled
`- 18:0:0:1 sdh 8:112 active ready running
3600d0230000000000e13955cc3757803 dm-2 WINSYS,SF2372
size=125G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 19:0:0:3 sde 8:64 active ready running
`- 18:0:0:3 sdj 8:144 active ready running
5.7. Multipath Command Options
Table 5.1, “Useful multipath Command Options” describes some options of the multipath
command that you may find useful.
Table 5.1. Useful multipath Command Options
Option
Description
-l
Display the current multipath configuration gathered from sysfs and
the device mapper.
-ll
Display the current multipath configuration gathered from sysfs, the
device mapper, and all other available components on the system.
-f device
Remove the named multipath device.
-F
Remove all multipath devices.
5.8. Determining Device Mapper Entries with the dmsetup
Command
You can use the dmsetup command to find out which device mapper entries match the multipathed
devices.
The following command displays all the device mapper devices and their major and minor numbers.
The minor numbers determine the name of the dm device. For example, a minor number of 3
corresponds to the multipathed device /dev/dm-3.
# dmsetup ls
mpathd (253, 4)
mpathep1
(253, 12)
mpathfp1
(253, 11)
mpathb (253, 3)
mpathgp1
(253, 14)
32
Troubleshooting with the multipathd Interactive Console
mpathhp1
(253, 13)
mpatha (253, 2)
mpathh (253, 9)
mpathg (253, 8)
VolGroup00-LogVol01
(253, 1)
mpathf (253, 7)
VolGroup00-LogVol00
(253, 0)
mpathe (253, 6)
mpathbp1
(253, 10)
mpathd (253, 5)
5.9. Troubleshooting with the multipathd Interactive
Console
The multipathd -k command is an interactive interface to the multipathd daemon. Entering
this command brings up an interactive multipath console. After entering this command, you can enter
help to get a list of available commands, you can enter a interactive command, or you can enter
CTRL-D to quit.
The multipathd interactive console can be used to troubleshoot problems you may be having with
your system. For example, the following command sequence displays the multipath configuration,
including the defaults, before exiting the console.
# multipathd -k
> > show config
> > CTRL-D
The following command sequence ensures that multipath has picked up any changes to the
multipath.conf,
# multipathd -k
> > reconfigure
> > CTRL-D
Use the following command sequence to ensure that the path checker is working properly.
# multipathd -k
> > show paths
> > CTRL-D
33
34
Appendix A. Revision History
Revision 6.0-1 Wed Nov 10 2010
Steven Levine slevine@redhat.com
First version for the Red Hat Enterprise Linux 6 release
35
36
Index
Symbols
/etc/multipath.conf package, 11
A
active/active configuration
definition, 2
illustration, 4
active/passive configuration
definition, 2
illustration, 2
alias parameter , 22
configuration file, 7
B
blacklist
configuration file, 16
default devices, 17
device name, 17
device type, 17
WWID, 17
blacklist_exceptions section
multipath.conf file, 18
C
checker_timer parameter, 19
configuration file
alias parameter, 22
blacklist, 16
checker_timer parameter, 19
dev_loss_tmo parameter, 19, 25
failback parameter, 19, 22, 25
fast_io_fail_tmo parameter, 19, 25
features parameter, 19, 25
flush_on_last_del parameter, 19, 22, 25
getuid_callout parameter, 19, 25
gid parameter, 19, 22
hardware_handler parameter, 25
max_fds parameter, 19
mode parameter, 19, 22
no_path_retry parameter, 19, 22, 25
overview, 15
path_checker parameter, 19, 25
path_grouping_policy parameter, 19, 22, 25
path_selector parameter, 19, 22, 25
polling-interval parameter, 19
prio parameter, 19, 25
product parameter, 25
product_blacklist parameter, 25
queue_without_daemon parameter, 19
revision parameter, 25
rr_min_io parameter, 19, 22
rr_weight parameter, 19, 22, 25
udev_dir parameter, 19
uid parameter, 19, 22
user_friendly_names parameter, 18, 19
vendor parameter, 25
verbosity parameter, 19
wwid parameter, 22
configuring
DM-Multipath, 11
D
defaults section
multipath.conf file, 18
dev/mapper directory, 7
dev/mpath directory, 7
device name, 7
device-mapper-multipath package, 11
devices
adding, 14, 24
devices section
multipath.conf file, 24
dev_loss_tmo parameter, 19, 25
DM-Multipath
and LVM, 8, 8
components, 5
configuration file, 15
configuring, 11
definition, 1
device name, 7
devices, 7
failover, 2
overview, 2
redundancy, 2
setup, 11
setup, overview, 6
dm-n devices, 7
dmsetup command, determining device mapper
entries, 32
dm_multipath kernel module , 5
F
failback parameter, 19, 22, 25
failover, 2
fast_io_fail_tmo parameter, 19, 25
features parameter, 19, 25
features, new and changed, 1
feedback
contact information for this manual, v
flush_on_last_del parameter, 19, 22, 25
G
getuid_callout parameter, 19, 25
37
Index
gid parameter, 19, 22
H
hardware_handler parameter, 25
K
kpartx command , 5
L
local disks, ignoring, 12
LVM physical volumes
multipath devices, 8
lvm.conf file , 8
M
max_fds parameter, 19
mode parameter, 19, 22
mpathconf command , 5
multipath command , 5
options, 32
output, 30
queries, 31
multipath daemon (multipathd), 30
multipath devices, 7
logical volumes, 8
LVM physical volumes, 8
multipath.conf file, 5, 15
blacklist_exceptions section, 18
defaults section, 18
devices section, 24
multipaths section, 22
multipath.conf.annotated file, 15
multipath.conf.defaults file, 5, 15
multipathd
command, 33
interactive console, 33
multipathd daemon , 5
multipathd start command, 11
multipaths section
multipath.conf file, 22
N
no_path_retry parameter, 19, 22, 25
O
overview
features, new and changed, 1
P
path_checker parameter, 19, 25
path_grouping_policy parameter, 19, 22, 25
path_selector parameter, 19, 22, 25
polling_interval parameter, 19
38
prio parameter, 19, 25
product parameter, 25
product_blacklist parameter, 25
Q
queue_without_daemon parameter, 19
R
resizing a multipath device, 29
revision parameter, 25
rr_min_io parameter, 19, 22
rr_weight parameter, 19, 22, 25
S
setup
DM-Multipath, 11
storage array support, 5
storage arrays
adding, 14, 24
U
udev_dir parameter, 19
uid parameter, 19, 22
user_friendly_names parameter , 7, 18, 19
V
vendor parameter, 25
verbosity parameter, 19
W
World Wide Identifier (WWID), 7
wwid parameter, 22
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