Red Hat Enterprise Linux 5 DM Multipath
DM Multipath Configuration and Administration

DM Multipath

Red Hat Enterprise Linux 5 DM Multipath DM Multipath Configuration and Administration Edition 3
Copyright 2010 Red Hat, Inc. The text of and illustrations in this document are licensed by Red Hat under a Creative Commons AttributionShare Alike 3.0 Unported license ("CC-BY-SA"). An explanation of CC-BY-SA is available at http://creativecommons.org/licenses/by-sa/3.0/. In accordance with CC-BY-SA, if you distribute this document or an adaptation of it, you must provide the URL for the original version. Red Hat, as the licensor of this document, waives the right to enforce, and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent permitted by applicable law. Red Hat, Red Hat Enterprise Linux, the Shadowman logo, JBoss, MetaMatrix, Fedora, the Infinity Logo, and RHCE are trademarks of Red Hat, Inc., registered in the United States and other countries. Linux is the registered trademark of Linus Torvalds in the United States and other countries. Java is a registered trademark of Oracle and/or its affiliates. XFS is a trademark of Silicon Graphics International Corp. or its subsidiaries in the United States and/or other countries. MySQL is a registered trademark of MySQL AB in the United States, the European Union and other countries. All other trademarks are the property of their respective owners. 1801 Varsity Drive Raleigh, NC 27606-2072 USA Phone: +Phone: 4281 Fax: +754 3701
This book provides information on using the Device-Mapper Multipath feature of Red Hat Enterprise Linux 5
Preface v 1. Audience.... v 2. Related Documentation.... v 3. 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. Overview of DM-Multipath... 1.2. Storage Array Support... 1.3. DM-Multipath Components.... 1.4. DM-Multipath Setup Overview.... 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... 6 6
3. Setting Up DM-Multipath 7 3.1. Setting Up DM-Multipath.... 7 3.2. Ignoring Local Disks when Generating Multipath Devices.. 8 3.3. Adding Devices to the Multipathing Database... 10 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... 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.. A. Revision History Index 29 31

Preface

This book describes the Device Mapper Multipath (DM-Multipath) feature of Red Hat Enterprise Linux for the Red Hat Enterprise Linux 5 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: Red Hat Enterprise Linux Installation Guide Provides information regarding installation of Red Hat Enterprise Linux 5. Red Hat Enterprise Linux Deployment Guide Provides information regarding the deployment, configuration and administration of Red Hat Enterprise Linux 5. For more information about Red Hat Cluster Suite for Red Hat Enterprise Linux 5, refer to the following resources: Red Hat Cluster Suite Overview Provides a high level overview of the Red Hat Cluster Suite. Configuring and Managing a Red Hat Cluster 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: Configuration and Administration Provides information about installing, configuring, and maintaining Red Hat GFS (Red Hat Global File System). Global File System 2: Configuration and Administration Provides information about installing, configuring, and maintaining Red Hat GFS2 (Red Hat Global File System 2). Using GNBD with Global File System Provides an overview on using Global Network Block Device (GNBD) with Red Hat GFS. Linux Virtual Server Administration Provides information on configuring high-performance systems and services with the Linux Virtual Server (LVS). Red Hat Cluster Suite Release Notes Provides information about the current release of Red Hat Cluster Suite. 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/.

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: vii

books books_tests

Desktop Desktop1

documentation downloads

drafts images

mss notes

photos scripts

stuff svgs

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; 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.
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.

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.
1.1. 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.

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 1
Chapter 1. Device Mapper Multipathing 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.
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.

Storage Array Support

Figure 1.3. Active/Active Multipath Configuration with One RAID Device
1.2. 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.
1.3. DM-Multipath Components
Table 1.1, DM-Multipath Components. describes the components of DM-Multipath. Table 1.1. DM-Multipath Components Component dm-multipath kernel module multipath command Description Reroutes I/O and supports failover for paths and path groups. 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. 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. Creates device mapper devices for the partitions on a device It is necessary to use this command for DOS-based partitions with DM-MP. The kpartx is provided in its own package, but the device-mapper-multipath package depends on it.

chkconfig multipathd on

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.
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. The following procedure shows how 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.
[root@rh4cluster1 ~]# multipath -v2 create: SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1 [size=33 GB][features="0"][hwhandler="0"] \_ round-robin 0 \_ 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 [size=12 GB][features="0"][hwhandler="0"] \_ round-robin 0 \_ 2:0:0:0 sdb 8:16 \_ 3:0:0:0 sdf 8:80 create: 3600a0b80001327510000009a436215ec [size=12 GB][features="0"][hwhandler="0"] \_ round-robin 0 \_ 2:0:0:1 sdc 8:32 \_ 3:0:0:1 sdg 8:96 create: 3600a0b80001327d800000070436216b3 [size=12 GB][features="0"][hwhandler="0"] \_ round-robin 0 \_ 2:0:0:2 sdd 8:48 \_ 3:0:0:2 sdh 8:112
Ignoring Local Disks when Generating Multipath Devices
create: 3600a0b80001327510000009b4362163e [size=12 GB][features="0"][hwhandler="0"] \_ round-robin 0 \_ 2:0:0:3 sde 8:64 \_ 3:0:0:3 sdi 8:128

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.
service multipathd reload
4. Run the following commands:
multipath -F multipath -v2
The local disk or disks should no longer be listed in the new multipath maps, as shown in the following example.
[root@rh4cluster1 ~]# multipath -F [root@rh4cluster1 ~]# multipath -v2 create: 3600a0b80001327d80000006d43621677 [size=12 GB][features="0"][hwhandler="0"] \_ round-robin 0 \_ 2:0:0:0 sdb 8:16 \_ 3:0:0:0 sdf 8:80 create: 3600a0b80001327510000009a436215ec [size=12 GB][features="0"][hwhandler="0"] \_ round-robin 0 \_ 2:0:0:1 sdc 8:32 \_ 3:0:0:1 sdg 8:96 create: 3600a0b80001327d800000070436216b3 [size=12 GB][features="0"][hwhandler="0"] \_ round-robin 0 \_ 2:0:0:2 sdd 8:48 \_ 3:0:0:2 sdh 8:112
Chapter 3. Setting Up DM-Multipath
3.3. Adding Devices to the Multipathing Database
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:
devices { device { vendor "HP" product "OPEN-V." getuid_callout "/sbin/scsi_id -g -u -p0x80 -s /block/%n" } }
For more information on the devices section of the configuration file, see Section 4.5, Configuration File Devices.

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-0.4.7/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-mappermultipathd-0.4.7/multipath.conf.annotated.

path_grouping_policy

getuid_callout

prio_callout

path_checker

features

rr_min_io

max_fds

rr_weight

failback

Chapter 4. The DM-Multipath Configuration File Attribute Description A value of 0 or immediate specifies that as soon as there is a path group with a higher priority than the current path group the system switches to that path group. A numeric value greater than zero specifies deferred failback, expressed in seconds. A value of manual specifies that failback can happen only with operator intervention. The default value is manual. 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 (null). (Red Hat Enterprise Linux 5.3 and later) 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. (Red Hat Enterprise Linux 5.3 and later) If set to no, the multipathd daemon will disable queueing for all devices when it is shut down. The default value is yes. If set to yes, specifies that the system should using the bindings file to assign a persistent and unique alias to the multipath, in the form of mpathn. The default location of the bindings file is / var/lib/multipath/bindings, but this can be changed with the bindings_file option. 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 overridden by any devicespecific aliases you specify in the multipaths section of the configuration file. The default value is no. (Red Hat Enterprise Linux 5.2 and later) The location of the bindings file that is used with the user_friend_names option. The default value is /var/lib/multipath/bindings. (Red Hat Enterprise Linux 5.3 and later) The mode to use for the multipath device nodes, in octal. The default value is determined by the process. (Red Hat Enterprise Linux 5.3 and later) The user ID to use for the multipath device nodes. You must use the numeric user ID. The default value is determined by the process. (Red Hat Enterprise Linux 5.3 and later) The group ID to use for the multipath device nodes. You must use the numeric group ID. The default value is determined by the process. (Red Hat Enterprise Linux 5.5 and later) The timeout value 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. (Red Hat Enterprise Linux 5.6 and later) If set to avg, sets the priority of path groups to the average of the priorities of the included paths. If set to sum, sets the priority of path groups to

flush_on_last_del

queue_without_daemon

user_friendly_names

bindings_file

checker_timeout

pg_prio_calc
Multipaths Device Configuration Attributes Attribute Description the sum of the priorities of the included paths. The default value is sum. log_checker_err (Red Hat Enterprise Linux 5.6 and later) If set to once, a path checker error is logged once at logging level 2 and any later errors are logged at level 3 until the device is restored. If set to always, every path checker error is logged at logging level 2. The default value is always.
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 wwid alias path_grouping_policy Description Specifies the WWID of the multipath device to which the multipath attributes apply. Specifies the symbolic name for the multipath device to which the multipath attributes apply. 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 group_by_node_name = 1 priority group per target node name Specifies the default program and arguments to call out to obtain a path weight. Weights are summed for each path group to determine the next path group to use in case of failure. "none" is a valid value. Specifies the default algorithm to use in determining what path to use for the next I/O operation. Specifies path group failback. A value of 0 or immediate specifies that as soon as there is a path group with a higher priority than the current path group the system switches to that path group. A numeric value greater than zero specifies deferred failback, expressed in seconds. A value of manual specifies that failback can happen only with operator intervention. If set to priorities, then instead of sending rr_min_io requests to a path before calling 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_callout program. Currently, there are priority callouts only for devices that use the group_by_prio path grouping policy, 17

path_selector failback

Chapter 4. The DM-Multipath Configuration File Attribute Description which means that all the paths in a path group will always have the same priority. If set to uniform, all path weights are equal. 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. (Red Hat Enterprise Linux 5.3 and later) 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. Specifies the number of I/O requests to route to a path before switching to the next path in the current path group. (Red Hat Enterprise Linux 5.3 and later) The mode to use for the multipath device nodes, in octal. The default value is determined by the process. (Red Hat Enterprise Linux 5.3 and later) The user ID to use for the multipath device nodes. You must use the numeric user ID. The default value is determined by the process. (Red Hat Enterprise Linux 5.3 and later) The group ID to use for the multipath device nodes. You must use the numeric group ID. The default value is determined by the process.

getuid_callout prio_callout
Chapter 4. The DM-Multipath Configuration File Attribute Description determine the next path group to use in case of failure. "none" is a valid value. path_checker Specifies the default method used to determine the state of the paths. Possible values include readsector0, rdac, tur, cciss_tur, hp_tur, emc_clariion, hp_sw, and directio. Specifies the default algorithm to use in determining what path to use for the next I/O operation. 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. Specifies a module that will be used to perform hardware specific actions when switching path groups or handling I/O errors. Possible values include 0, 1 emc, and 1 rdac. The default value is 0. If set to priorities, then instead of sending rr_min_io requests to a path before calling 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_callout program. Currently, there are priority callouts only for devices that use the group_by_prio path grouping policy, which means that all the paths in a path group will always have the same priority. If set to uniform, all path weights are equal. The default value is uniform. 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 (null). Specifies path group failback. A value of 0 or immediate specifies that as soon as there is a path group with a higher priority than the current path group the system switches to that path group. A numeric value greater than zero specifies deferred failback, expressed in seconds. A value of manual specifies that failback can happen only with operator intervention. 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. (Red Hat Enterprise Linux 5.3 and later) 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.

path_selector features

hardware_handler

no_path_retry

Configuration File Devices Attribute product_blacklist Description Specifies a regular expression used to blacklist devices by product.
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 # } #}

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 mpath0'
5. Resize the filesystem (assuming no LVM or DOS partitions are used):
# resize2fs /dev/mapper/mpath0
For further information on resizing an online LUN, see the Online Storage Reconfiguration Guide.
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-0.4.7/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 mpath2 from "queue_if_no_path" to "fail_if_no_path", execute the following command.
dmsetup message mpath"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. 24
Multipath Queries with multipath Command For each multipath device:
action_if_any: alias (wwid_if_different_from_alias) [size][features][hardware_handler]

For each path group:

\_ scheduling_policy [path_group_priority_if_known] [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:

mpath1 (3600d0230003228bc000339414edb8101) [size=10 GB][features="0"][hwhandler="0"] \_ round-robin 0 [prio=1][active] \_ 2:0:0:6 sdb 8:16 [active][ready] \_ round-robin 0 [prio=1][enabled] \_ 3:0:0:6 sdc 8:64 [active][ready]
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.
When a multipath device is being created or modified, the path group status and the dm status are not known. Also, the features are not always correct. When a multipath device is being listed, the path group priority is not known.
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.
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 -l -ll -f device -F Description Display the current multipath configuration gathered from sysfs and the device mapper. Display the current multipath configuration gathered from sysfs, the device mapper, and all other available components on the system. Remove the named multipath device. 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.

bindings_file parameter, 14 blacklist configuration file, 12 default devices, 13 device name, 12 device type, 13 in configuration file, 7 WWID, 12 blacklist_exceptions section multipath.conf file, 13
defaults section multipath.conf file, 14 dev/mapper directory, 5 dev/mpath directory, 5 device name, 5 device-mapper-multipath package, 7 devices adding, 10, 18 devices section multipath.conf file, 18 DM-Multipath and LVM, 6, 6 components, 3 configuration file, 11 configuring, 7 definition, 1 device name, 5 devices, 5 failover, 1 overview, 1 redundancy, 1 setup, 7 setup, overview, 4 dm-multipath kernel module , 3 dm-n devices, 5 dmsetup command, determining device mapper entries, 26
checker_timeout parameter, 14 chkconfig command, 8 configuration file alias parameter, 17 bindings_file parameter, 14 blacklist, 7, 12 checker_timeout parameter, 14 failback parameter, 14, 17, 19 features parameter, 14, 19 flush_on_last_del parameter, 14, 17, 19 getuid_callout parameter, 14, 19 gid parameter, 14, 17 hardware_handler parameter, 19 log_checker_err parameter, 14 max_fds parameter, 14 mode parameter, 14, 17 no_path_retry parameter, 14, 17, 19 overview, 11 path_checker parameter, 14, 19 path_grouping_policy parameter, 14, 17, 19 path_selector parameter, 17, 19 pg_prio_calc parameter, 14 polling-interval parameter, 14 prio_callout parameter, 14, 17, 19
failback parameter, 14, 17, 19 failover, 1 features parameter, 14, 19 feedback, v, v flush_on_last_del parameter, 14, 17, 19
getuid_callout parameter, 14, 19 31
Index gid parameter, 14, 17 product_blacklist parameter, 19
hardware_handler parameter, 19
queue_without_daemon parameter, 14

kpartx command , 3

resizing a multipath device, 23 rr_min_io parameter, 14, 17 rr_weight parameter, 14, 17, 19
local disks, ignoring, 8 log_checke_err parameter, 14 LVM physical volumes multipath devices, 6 lvm.conf file , 6
selector parameter, 14 setup DM-Multipath, 7 storage array support, 3 storage arrays adding, 10, 18
max_fds parameter, 14 mode parameter, 14, 17 modprobe command, 7 multipath command , 3, 7 options, 26 output, 24 queries, 25 multipath daemon (multipathd), 24 multipath devices, 5 logical volumes, 6 LVM physical volumes, 6 multipath.conf file, 3, 11 blacklist_exceptions section, 13 defaults section, 14 devices section, 18 multipaths section, 17 multipath.conf.annotated file, 11 multipath.conf.defaults file, 3, 11 multipathd command, 26 interactive console, 26 multipathd daemon , 3 multipathd start command, 7 multipaths section multipath.conf file, 17

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 2011 Red Hat, Inc. and others. The text of and illustrations in this document are licensed by Red Hat under a Creative Commons AttributionShare Alike 3.0 Unported license ("CC-BY-SA"). An explanation of CC-BY-SA is available at http://creativecommons.org/licenses/by-sa/3.0/. In accordance with CC-BY-SA, if you distribute this document or an adaptation of it, you must provide the URL for the original version. Red Hat, as the licensor of this document, waives the right to enforce, and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent permitted by applicable law. Red Hat, Red Hat Enterprise Linux, the Shadowman logo, JBoss, MetaMatrix, Fedora, the Infinity Logo, and RHCE are trademarks of Red Hat, Inc., registered in the United States and other countries. Linux is the registered trademark of Linus Torvalds in the United States and other countries. Java is a registered trademark of Oracle and/or its affiliates. XFS is a trademark of Silicon Graphics International Corp. or its subsidiaries in the United States and/or other countries. MySQL is a registered trademark of MySQL AB in the United States, the European Union and other countries. All other trademarks are the property of their respective owners. 1801 Varsity Drive Raleigh, NC 27606-2072 USA Phone: +Phone: 4281 Fax: +754 3701
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.... 1.1.1. New and Changed Features for Red Hat Enterprise Linux 6.0.. 1.1.2. New and Changed Features for Red Hat Enterprise Linux 6.1.. 1.2. Overview of DM-Multipath... 1.3. Storage Array Support... 1.4. DM-Multipath Components.... 1.5. DM-Multipath Setup Overview.... 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... 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.... 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... 5. DM-Multipath Administration and Troubleshooting 5.1. Resizing an Online Multipath Device... 5.2. Moving root File Systems from a Single Path Device to a Multipath Device. 5.3. Moving swap File Systems from a Single Path Device to a Multipath Device. 5.4. The Multipath Daemon.... 5.5. Issues with Large Number of LUNs.... 5.6. Issues with queue_if_no_path feature... 5.7. Multipath Command Output... 5.8. Multipath Queries with multipath Command... 5.9. Multipath Command Options.... 5.10. Determining Device Mapper Entries with the dmsetup Command.. 5.11. Troubleshooting with the multipathd Interactive Console... A. Revision History Index 8

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. 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

Chapter 1. Device Mapper Multipathing 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 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 5.7, 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.
1.1.2. New and Changed Features for Red Hat Enterprise Linux 6.1
Red Hat Enterprise Linux 6.1 includes the following documentation and feature updates and changes. This document now contains a new chapter, Section 5.2, Moving root File Systems from a Single Path Device to a Multipath Device. This document now contains a new chapter, Section 5.3, Moving swap File Systems from a Single Path Device to a Multipath Device.
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.

2.2. 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. 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. 7
Chapter 2. Multipath Devices 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.
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 8
Multipath Devices in Logical Volumes 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/.*/" ]

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 utility 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>

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
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 5.7, Multipath Command Output. 12

Ignoring Local Disks when Generating Multipath Devices
# 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.
# service multipathd reload
4. Run the following command to remove the multipath device:
Chapter 3. Setting Up DM-Multipath
# 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 5.8, 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.

# 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.

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.

rr_min_io

rr_weight

no_path_retry

user_friendly_names
queue_without_daemon flush_on_last_del
Chapter 4. The DM-Multipath Configuration File Attribute max_fds Description 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. 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. 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. 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. The mode to use for the multipath device nodes, in octal. The default value is determined by the process. 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. 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.

checker_timer

fast_io_fail_tmo

dev_loss_tmo

mode uid
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 wwid Description 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. 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. Specifies the default path grouping policy to apply to unspecified multipaths. Possible values include: failover = 1 path per priority group

flush_on_last_del mode uid gid
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. 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. 24
Configuration File Devices 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:

4. Resize your multipath device by running the multipathd resize command:
# multipathd -k'resize map mpatha'
5. Resize the file system (assuming no LVM or DOS partitions are used):
# resize2fs /dev/mapper/mpatha
Chapter 5. DM-Multipath Administration and Troubleshooting
5.2. Moving root File Systems from a Single Path Device to a Multipath Device
If you have installed your system on a single-path device and later add another path to the root file system, you will need to move your root file system to a multipathed device. This section documents the procedure for moving from a single-path to a multipathed device. After ensuring that you have installed the device-mapper-multipath package, perform the following procedure: 1. Execute the following command to create the /etc/multipath.conf configuration file, load the multipath module, and set chkconfig for the multipathd to on:
For further information on using the mpathconf command to set up multipathing, see Section 3.1, Setting Up DM-Multipath. 2. Edit the blacklist and blacklist_exceptions sections of the /etc/multipath.conf file, as described in Section 4.2, Configuration File Blacklist. 3. To confirm that your configuration file is set up correctly, you can run the /sbin/multipath command with the -v3 option to check whether the multipath daemon tried to create a multipath device over your root device. The command will fail since the root the device is in use, but the output from the command should show the root device in the paths list. You should look in the command output for a line of the following format:
H:B:T:L devname MAJOR:MINOR
For example, if your root file system is set up on sda or one of its partitions, you would see a line in the output such as the following:
===== paths list =====. 1ATA WDC WD800JD-75MSA3.
WD-WMAM9F 1:0:0:0 sda 8:0
Later in the output, you will see an indication that the command failed with a line of the following format, in which the wwid parameter matches the wwid for the root device path in the paths list:
time | wwid: domap (0) failure for create/reload map
In the example noted above, you would see the following line in the command output:
Feb 24 03:58:15 | 1ATA WDC WD800JD-75MSA3 WD-WMAM9FU71154: domap (0) failure for create/reload map
4. To rebuild the initramfs file system with multipath, execute the dracut with the following options: 30
Moving swap File Systems from a Single Path Device to a Multipath Device
# dracut --force -H -add multipath --include /etc/multipath /etc/multipath
Using the -H option creates the initramfs file system specifically for the system it was built on and makes the file system vastly smaller, which saves space and cuts down boot time. 5. If your root device is not an LVM volume and it is mounted by device name, you may need to edit the fstab file to switch to the appropriate multipath device name. If your root device is an LVM device or is mounted by UUID or something else, this step is not necessary. a. Use the procedure described in Step 3 of running the /sbin/multipath command with the -v3 to determine the WWID of the root device. b. Set up an alias for the root device in the /etc/multipath.conf file:

multipaths { multipath { wwid WWID_of_root_device alias rootdev } }
Edit the /etc/fstab and replace the old device path to the root device with the multipath device. For example, if you had the following entry in the /etc/fstab file:

/dev/sda1 /

defaults
You would change the entry to the following:

/dev/mapper/rootdev /

6. Shut the machine down. 7. Configure the FC switch so that other paths are visible to the machine. 8. Boot the machine. 9. Check whether the root file system ('/') is on the multipathed device.
5.3. Moving swap File Systems from a Single Path Device to a Multipath Device
By default, swap devices are set up as logical volumes. This does not require any special configuration for configuring them as multipath devices as long as you set up multipathing on the physical volumes that constitute the logical volume group. If your swap device is not an LVM volume, however, and it is mounted by device name, you may need to edit the fstab file to switch to the appropriate multipath device name.
Chapter 5. DM-Multipath Administration and Troubleshooting 1. Determine the WWID number of the swap device by running the /sbin/multipath command with the -v3 option. The output from the command should show the swap device in the paths list. You should look in the command output for a line of the following format, showing the swap device:
For example, if your swap file system is set up on sda or one of its partitions, you would see a line in the output such as the following:
2. Set up an alias for the swap device in the /etc/multipath.conf file:
multipaths { multipath { wwid WWID_of_swap_device alias swapdev } }
3. Edit the /etc/fstab and replace the old device path to the root device with the multiapth device. For example, if you had the following entry in the /etc/fstab file:

/dev/sda2 swap

/dev/mapper/swapdev swap
5.4. 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.

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
getuid_callout parameter, 19, 25 gid parameter, 19, 22 39
hardware_handler parameter, 25

kpartx command , 5

polling_interval parameter, 19 prio parameter, 19, 25 product parameter, 25 product_blacklist parameter, 25
queue_without_daemon parameter, 19
local disks, ignoring, 12 LVM physical volumes multipath devices, 8 lvm.conf file , 8
resizing a multipath device, 29 revision parameter, 25 root file system, 30 rr_min_io parameter, 19, 22 rr_weight parameter, 19, 22, 25
max_fds parameter, 19 mode parameter, 19, 22 mpathconf command , 5 multipath command , 5 options, 35 output, 33 queries, 34 multipath daemon (multipathd), 32 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, 36 interactive console, 36 multipathd daemon , 5 multipathd start command, 11 multipathed root file system, 30 multipathed swap file system, 31 multipaths section multipath.conf file, 22
setup DM-Multipath, 11 storage array support, 5 storage arrays adding, 14, 24 swap file system, 31
udev_dir parameter, 19 uid parameter, 19, 22 user_friendly_names parameter , 7, 18, 19
vendor parameter, 25 verbosity parameter, 19
World Wide Identifier (WWID), 7 wwid parameter, 22
no_path_retry parameter, 19, 22, 25
overview features, new and changed, 1
path_checker parameter, 19, 25 path_grouping_policy parameter, 19, 22, 25 path_selector parameter, 19, 22, 25 40