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Chapter 5. Modifying 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 themselves support DM Multipath. You can override the default configuration values for DM Multipath by editing the /etc/multipath.conf configuration file. If necessary, you can also add an unsupported by default storage array to the configuration file.

For information on the default configuration values, including supported devices, run either of the following commands: 

# multipathd show config
# multipath -t
Note

If you run multipath from the initramfs file system and you make any changes to the multipath configuration files, you must rebuild the initramfs file system for the changes to take effect

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. 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.

5.1. 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 devices that would otherwise be ignored 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 overrides, devices, and defaults 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.
overrides
Settings that are applied to all devices. These values overwrite what is specified in the devices and defaults sections of the configuration file.

When the system determines the attributes of a multipath device, it checks the settings of the separate sections from the multipath.conf file in the following order:

  1. multipaths section
  2. overrides section
  3. devices section
  4. defaults section

5.2. DM Multipath overrides of the device timeout

The recovery_tmo sysfs option controls the timeout for a particular iSCSI device. The following options globally override the recovery_tmo values:

  • The replacement_timeout configuration option globally overrides the recovery_tmo value for all iSCSI devices.

  • For all iSCSI devices that are managed by DM Multipath, the fast_io_fail_tmo option in DM Multipath globally overrides the recovery_tmo value.

    The fast_io_fail_tmo option in DM Multipath also overrides the fast_io_fail_tmo option in Fibre Channel devices.

The DM Multipath fast_io_fail_tmo option takes precedence over replacement_timeout. Red Hat does not recommend using replacement_timeout to override recovery_tmo in devices managed by DM Multipath because DM Multipath always resets recovery_tmo, when the multipathd service reloads.

5.3. Modifying multipath 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 default values that are set in the defaults section on the multipath.conf file, are used by DM Multipath unless they are overwritten by the attributes specified in the devices, multipath, or overrides sections of the multipath.conf file.

Procedure

  1. View the /etc/multipath.conf configuration file, which includes a template of configuration defaults: 

    #defaults {
#       polling_interval        10
#       path_selector           "round-robin 0"
#       path_grouping_policy    multibus
#       uid_attribute           ID_SERIAL
#       prio                    alua
#       path_checker            readsector0
#       rr_min_io               100
#       max_fds                 8192
#       rr_weight               priorities
#       failback                immediate
#       no_path_retry           fail
#       user_friendly_names     yes
#}

  2. 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 to multibus instead of 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
}

  3. Validate the /etc/multipath.conf file after modifying the multipath configuration file by running one of the following commands:

    • To display any configuration errors, run: 

      # multipath -t > /dev/null

    • To display the new configuration with the changes added, run: 

      # multipath -t

  4. Reload the /etc/multipath.conf file and reconfigure the multipathd daemon for changes to take effect: 

    # service multipathd reload

Additional resources

  • multipath.conf(5) and multipathd(8) man pages

5.4. Modifying multipath settings for specific devices

In the multipaths section of the multipath.conf configuration file, you can add configurations that are specific to an individual multipath device, referenced by the mandatory WWID parameter.

These defaults are used by DM Multipath and override attributes set in the overrides, defaults, and devices sections of the multipath.conf file. There can be any number of multipath subsections in the multipaths section.

Procedure

  1. Modify the multipaths section for specific multipath device. 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 attribute is set to priorities. 

    multipaths {
       multipath {
              wwid                  3600508b4000156d70001200000b0000
              alias                 yellow
              path_grouping_policy  multibus
              path_selector         "round-robin 0"
              failback              manual
              rr_weight             priorities
              no_path_retry         5
       }
       multipath {
              wwid                  1DEC_321816758474
              alias                 red
              rr_weight             priorities
        }
}

  2. Validate the /etc/multipath.conf file after modifying the multipath configuration file by running one of the following commands:

    • To display any configuration errors, run: 

      # multipath -t > /dev/null

    • To display the new configuration with the changes added, run: 

      # multipath -t

  3. Reload the /etc/multipath.conf file and reconfigure the multipathd daemon for changes to take effect: 

    # service multipathd reload

Additional resources

  • multipath.conf(5) man page

5.5. Modifying multipath settings for storage controllers

The devices section of the multipath.conf configuration file sets attributes for individual storage devices. These attributes are used by DM Multipath unless they are overwritten by the attributes specified in the multipaths or overrides sections 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.

Procedure

  1. View the information on the default configuration value, including supported devices: 

    # multipathd show config
# multipath -t

    Many devices that support multipathing are included by default in a multipath configuration.

  2. Optional: If you need to modify the default configuration values, 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 for the device that the multipathd show config command displays and override the values that you want to change.

  3. Add a device that is not configured automatically by default to the devices section of the configuration file by setting the vendor and product parameters. Find these values by opening the /sys/block/device_name/device/vendor and /sys/block/device_name/device/model files where device_name is the device to be multipathed, as mentioned in the following example: 

    # cat /sys/block/sda/device/vendor
WINSYS
# cat /sys/block/sda/device/model
SF2372

  4. Optional: Specify the additional parameters depending on your specific device:

    active/active device
    Usually there is no need to set additional parameters in this case. If required, you might set path_grouping_policy to multibus. Other parameters you may need to set are no_path_retry and rr_min_io.
    active/passive device
    If 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 means that you have set the path_checker to tur, which 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 might not be able to configure the device for multipath.

    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
#	}
#}

  5. Validate the /etc/multipath.conf file after modifying the multipath configuration file by running one of the following commands:

    • To display any configuration errors, run: 

      # multipath -t > /dev/null

    • To display the new configuration with the changes added, run: 

      # multipath -t

  6. Reload the /etc/multipath.conf file and reconfigure the multipathd daemon for changes to take effect: 

    # service multipathd reload

Additional resources

  • multipath.conf(5) and multipathd(8) man pages

5.6. Setting multipath values for all devices

Using the overrides section of the multipath.conf configuration file, you can set a configuration value for all of your devices. This section supports all attributes that are supported by both the devices and defaults section of the multipath.conf configuration file, which is all of the devices section attributes except vendor, product, and revision.

These attributes are used by DM Multipath for all devices 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 devices and defaults sections of the multipath.conf file.

Procedure

  1. Override device specific settings. For example, you might want all devices to set no_path_retry to fail. Use the following command to turn off queueing, when all paths have failed. This overrides any device specific settings. 

    overrides {
        no_path_retry fail
}

  2. Validate the /etc/multipath.conf file after modifying the multipath configuration file by running one of the following commands:

    • To display any configuration errors, run: 

      # multipath -t > /dev/null

    • To display the new configuration with the changes added, run: 

      # multipath -t

  3. Reload the /etc/multipath.conf file and reconfigure the multipathd daemon for changes to take effect: 

    # service multipathd reload

Additional resources

  • multipath.conf(5) man page