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Red Hat Enterprise Linux 6

DM Multipath

DM Multipath Configuration and Administration

Edition 1


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Abstract
This book provides information on using the Device-Mapper Multipath feature of Red Hat Enterprise Linux 6

Preface
1. Audience
2. Related Documentation
3. We Need Feedback!
4. Document Conventions
4.1. Typographic Conventions
4.2. Pull-quote Conventions
4.3. Notes and Warnings
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
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. 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

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.

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.
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 SystemPreferencesMouse from the main menu bar to launch Mouse Preferences. In the Buttons tab, click the Left-handed mouse check box and click 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 ApplicationsAccessoriesCharacter Map from the main menu bar. Next, choose SearchFind… 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. Double-click 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 EditPaste 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 [email protected].
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        Desktop   documentation  drafts  mss    photos   stuff  svn
books_tests  Desktop1  downloads      images  notes  scripts  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.

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.

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 device-mapper-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 queue-length 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 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.
Active/Passive Multipath Configuration with One RAID Device
Active/Passive Multipath Configuration with One RAID Device
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.
Active/Passive Multipath Configuration with Two RAID Devices
Active/Passive Multipath Configuration with Two RAID Devices
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.
Active/Active Multipath Configuration with One RAID Device
Active/Active Multipath Configuration with One RAID Device
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 DM-Multipath. 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 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.

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 node-unique 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. DM-Multipath 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.
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.

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/.*/" ]

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  <y|n>
    start/stop/reload multipathd (Default n): --with_multipathd  <y|n>
    chkconfig on/off multipathd (Default y): --with_chkconfig  <y|n>
    
  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”.
    [[email protected] ~]# 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.
    [[email protected] ~]# service multipathd reload
  4. Run the following command to remove the multipath device:
    [[email protected] ~]# 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.
    [[email protected] ~]# 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 DM-Multipath. 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 DM-Multipath. 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-mapper-multipath-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.
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 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”.

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  "IBM"
               product "3S42"       #DS4200 Product 10
       }
       device {
               vendor  "HP"
               product "*"
       }
}

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                /dev
#       polling_interval        5
#       selector                "round-robin 0"
#       path_grouping_policy    failover
#       getuid_callout          "/lib/dev/scsi_id --whitelisted --device=/dev/%n"
#	prio			const
#	path_checker		directio
#	rr_min_io		1000
#	rr_weight		uniform
#	failback		manual
#	no_path_retry		fail
#	user_friendly_names	no
#}
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     yes
        path_grouping_policy    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:
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, per-controller, 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.
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.
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
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.
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                  3600508b4000156d70001200000b0000
              alias                 yellow
              path_grouping_policy  multibus
              path_checker          readsector0
              path_selector         "round-robin 0"
              failback              manual
              rr_weight             priorities
              no_path_retry         5
       }
       multipath {
              wwid                  1DEC_____321816758474
              alias                 red
              rr_weight             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 DM-Multipath 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.
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:
[[email protected] ~]# cat /sys/block/sda/device/vendor
WINSYS  
[[email protected] ~]# 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
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.
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
#	}
#}

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

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-mapper-multipath-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.
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.
# 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)
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

Revision History

Revision History
Revision 6.0-1Wed Nov 10 2010Steven Levine
First version for the Red Hat Enterprise Linux 6 release

Index

Symbols

/etc/multipath.conf package, Setting Up DM-Multipath

A

active/active configuration
definition, Overview of DM-Multipath
illustration, Overview of DM-Multipath
active/passive configuration
definition, Overview of DM-Multipath
illustration, Overview of DM-Multipath
alias parameter , Multipaths Device Configuration Attributes
configuration file, Multipath Device Identifiers

B

blacklist
configuration file, Configuration File Blacklist
default devices, Blacklisting By Device Name
device name, Blacklisting By Device Name
device type, Blacklisting By Device Type
WWID, Blacklisting by WWID
blacklist_exceptions section
multipath.conf file, Blacklist Exceptions

C

checker_timer parameter, Configuration File Defaults
configuration file
alias parameter, Multipaths Device Configuration Attributes
blacklist, Configuration File Blacklist
checker_timer parameter, Configuration File Defaults
dev_loss_tmo parameter, Configuration File Defaults, Configuration File Devices
failback parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
fast_io_fail_tmo parameter, Configuration File Defaults, Configuration File Devices
features parameter, Configuration File Defaults, Configuration File Devices
flush_on_last_del parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
getuid_callout parameter, Configuration File Defaults, Configuration File Devices
gid parameter, Configuration File Defaults, Multipaths Device Configuration Attributes
hardware_handler parameter, Configuration File Devices
max_fds parameter, Configuration File Defaults
mode parameter, Configuration File Defaults, Multipaths Device Configuration Attributes
no_path_retry parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
overview, Configuration File Overview
path_checker parameter, Configuration File Defaults, Configuration File Devices
path_grouping_policy parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
path_selector parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
polling-interval parameter, Configuration File Defaults
prio parameter, Configuration File Defaults, Configuration File Devices
product parameter, Configuration File Devices
product_blacklist parameter, Configuration File Devices
queue_without_daemon parameter, Configuration File Defaults
revision parameter, Configuration File Devices
rr_min_io parameter, Configuration File Defaults, Multipaths Device Configuration Attributes
rr_weight parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
udev_dir parameter, Configuration File Defaults
uid parameter, Configuration File Defaults, Multipaths Device Configuration Attributes
user_friendly_names parameter, Configuration File Defaults
vendor parameter, Configuration File Devices
verbosity parameter, Configuration File Defaults
wwid parameter, Multipaths Device Configuration Attributes
configuring
DM-Multipath, Setting Up DM-Multipath

D

defaults section
multipath.conf file, Configuration File Defaults
dev/mapper directory, Multipath Device Identifiers
dev/mpath directory, Multipath Device Identifiers
device name, Multipath Device Identifiers
device-mapper-multipath package, Setting Up DM-Multipath
devices
adding, Configuring Storage Devices, Configuration File Devices
devices section
multipath.conf file, Configuration File Devices
dev_loss_tmo parameter, Configuration File Defaults, Configuration File Devices
DM-Multipath
and LVM, Multipath Devices in Logical Volumes
components, DM-Multipath Components
configuration file, The DM-Multipath Configuration File
configuring, Setting Up DM-Multipath
definition, Device Mapper Multipathing
device name, Multipath Device Identifiers
devices, Multipath Devices
failover, Overview of DM-Multipath
overview, Overview of DM-Multipath
redundancy, Overview of DM-Multipath
setup, Setting Up DM-Multipath
setup, overview, DM-Multipath Setup Overview
dm-n devices, Multipath Device Identifiers
dmsetup command, determining device mapper entries, Determining Device Mapper Entries with the dmsetup Command
dm_multipath kernel module , DM-Multipath Components

H

hardware_handler parameter, Configuration File Devices

K

kpartx command , DM-Multipath Components

O

overview
features, new and changed, New and Changed Features

Q

queue_without_daemon parameter, Configuration File Defaults

S

setup
DM-Multipath, Setting Up DM-Multipath
storage array support, Storage Array Support
storage arrays
adding, Configuring Storage Devices, Configuration File Devices

V

vendor parameter, Configuration File Devices
verbosity parameter, Configuration File Defaults

W

World Wide Identifier (WWID), Multipath Device Identifiers
wwid parameter, Multipaths Device Configuration Attributes