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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 a storage array that is not supported by default 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

You can run set up multipathing in the initramfs file system. 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 candidates 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, first it checks the multipath settings, then the devices settings, then the multipath system defaults.

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 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. Preventing devices from multipathing

You can configure DM Multipath to ignore selected devices when it configures multipath devices. DM Multipath does not group these ignored devices into a multipath device.

5.3.1. Conditions when DM Multipath creates a multipath device for a path

DM Multipath has a set of default rules to determine whether to create a multipath device for a path or whether to ignore the path. You can configure the behavior.

If the find_multipaths configuration parameter is set to off, multipath always tries to create a multipath device for every path that is not explicitly disabled. If the find_multipaths configuration parameter is set to on, then multipath will create a device only if one of following conditions is met:

  • There are at least two paths with the same WWID that are not disabled.
  • You manually force 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 disable multipathing on other devices.

    If you have previously created a multipath device without using the find_multipaths parameter and then you later set the parameter to on, 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 enclosed 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/

In addition to on and off, you can also set find_multipaths to the following values:

strict
Multipath never accepts paths that have not previously been multipathed and are therefore not in the /etc/multipath/wwids file.
smart
Multipath always accepts non-disabled devices in udev as soon as they appear but if multipathd does not create the device within a timeout set with the find_multipaths_timeout parameter, it will release its claim on the device.

The built-in default value of find_multipaths is off. The default multipath.conf file created by mpathconf, however, will set the value of find_multipaths to on.

With the find_multipaths parameter set to on, you need to disable multipathing only on the devices with multiple paths that you do not want to be multipathed. Because of this, it will generally not be necessary to disable multipathing on devices.

Additional resources

  • For information on the find_multipaths_timeout parameter or the values that you can set for find_multipaths, see the multipath.conf(5) man page.

5.3.2. Criteria for disabling multipathing on certain devices

You can disable multipathing on devices by any of the following criteria:

  • WWID
  • device name
  • device type
  • property
  • protocol

For every device, DM Multipath evaluates these criteria in the following order:

  1. property
  2. devnode
  3. device
  4. protocol
  5. wwid

If a device turns out to be disabled by any criterion, DM Multipath excludes it from handling by multipathd, and does not evaluate the later criteria. For each criterion, the exception list takes precedence over the list of disabled devices if a device matches both.

Additional resources

5.3.3. Disabling multipathing by WWID

You can disable multipathing on individual devices by their World-Wide Identification (WWID).

Procedure

  • Disable devices in the /etc/multipath.conf configuration file using the wwid entry.

    The following example shows the lines in the DM Multipath configuration file that disable a device with a WWID of 26353900f02796769:

    blacklist {
           wwid 26353900f02796769
    }

5.3.4. Disabling multipathing by device name

You can disable multipathing on device types by device name so that DM Multipath will not group them into a multipath device.

Procedure

  • Disable devices in the /etc/multipath.conf configuration file using the devnode entry.

    The following example shows the lines in the DM Multipath configuration file that disable all SCSI devices, because it disables all sd* devices:

    blacklist {
           devnode "^sd[a-z]"
    }

You can use a devnode entry to disable individual devices rather than all devices of a specific type. This is not recommended, however, because 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, DM Multipath disables all devices that are not SCSI, NVMe, or DASD, using the following devnode entry:

blacklist {
       devnode "!^(sd[a-z]|dasd[a-z]|nvme[0-9])"
}

The devices that this entry disables do not generally support DM Multipath.

Additional resources

5.3.5. Disabling multipathing by device type

You can disable multipathing on devices by their device types.

Procedure

  • Disable devices in the /etc/multipath.conf configuration file using the device section.

    The following example disables multipathing on all IBM DS4200 and HP devices:

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

5.3.6. Disabling multipathing by udev property

You can disable multipathing on devices by their udev environment variable names.

Procedure

  • Disable devices in the /etc/multipath.conf configuration file using the property parameter. The parameter is a regular expression string that matches against the udev environment variable name for the devices.

    The following example disables multipathing on all devices with the udev property ID_ATA:

    blacklist {
            property "ID_ATA"
    }

5.3.7. Disabling multipathing by device protocol

You can disable multipathing on devices by their connection protocols.

Procedure

  1. Optional: View the protocol that a path is using:

    # multipathd show paths format "%d %P"
  2. Disable devices in the /etc/multipath.conf configuration file using the protocol section.

    The following example disables multipathing on all devices with an undefined protocol or an unknown SCSI transport type:

    blacklist {
            protocol "scsi:unspec"
            protocol "undef"
    }

    DM Multipath recognizes the following protocol strings:

    • scsi:fcp
    • scsi:spi
    • scsi:ssa
    • scsi:sbp
    • scsi:srp
    • scsi:iscsi
    • scsi:sas
    • scsi:adt
    • scsi:ata
    • scsi:unspec
    • ccw
    • cciss
    • nvme
    • undef

5.3.8. Adding exceptions for devices with disabled multipathing

You can enable multipathing on devices where multipathing is currently disabled.

Prerequisites

  • Multipathing is disabled on certain devices.

Procedure

  • Enable multipathing on the devices using the blacklist_exceptions section of the /etc/multipath.conf configuration file.

    When specifying devices in the blacklist_exceptions section of the configuration file, you must specify the exceptions using the same criteria as they were specified in the blacklist section. For example, a WWID exception does not apply to devices disabled by a devnode entry, even if the disabled device is associated with that WWID. Similarly, devnode exceptions apply only to devnode entries, and device exceptions apply only to device entries.

    Example 5.1. An exception by WWID

    If you have a large number of devices and want to multipath only one of them (with the WWID of 3600d0230000000000e13955cc3757803), instead of individually disabling each of the devices except the one you want, you could disable all of them, and then enable only the one you want by adding the following lines to the /etc/multipath.conf file:

    blacklist {
            wwid ".*"
    }
    
    blacklist_exceptions {
            wwid "3600d0230000000000e13955cc3757803"
    }

    Alternatively, you could use an exclamation mark (!) to invert the blacklist entry, which disables all devices except the specified WWID:

    blacklist {
            wwid "!3600d0230000000000e13955cc3757803"
    }

    Example 5.2. An exception by udev property

    The property parameter works differently than the other blacklist_exception parameters. If the parameter is set, the device must have a udev variable that matches. Otherwise, the device is disabled. This parameter allows you to disable multipathing on certain SCSI devices, such as USB sticks and local hard drives.

    To enable multipathing only on SCSI devices that could reasonably be multipathed, set this parameter to SCSI_IDENT_|ID_WWN) as in the following example:

    blacklist_exceptions {
            property "(SCSI_IDENT_|ID_WWN)"
    }

5.4. 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 configuration file includes a template of configuration defaults. This section is commented out, as follows.

#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
#}

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
}

For information on the attributes that are set in the defaults section of the multipath.conf configuration file see the multipath.conf(5) man page. These values are used by DM Multipath unless they are overwritten by the attributes specified in the devices, multipaths, or overrides sections of the multipath.conf file.

5.5. Modifying multipath settings for specific devices

The attributes in the multipaths section of the multipath.conf configuration file apply only to the one specified multipath. These defaults are used by DM Multipath and override attributes set in the overrides, defaults, and devices sections of the multipath.conf file.

For information on the attributes that are set in the multipaths section of the multipath.conf configuration file see the multipath.conf(5) man page.

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_selector         "round-robin 0"
              failback              manual
              rr_weight             priorities
              no_path_retry         5
       }
       multipath {
              wwid                  1DEC_321816758474
              alias                 red
              rr_weight             priorities
        }
}

5.6. 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.

For information on the attributes that are set in the devices section of the multipath.conf configuration file see the multipath.conf(5) man page.

Many devices that support multipathing are included by default in a multipath configuration. For information on the default configuration value, including supported devices, run either of the following commands.

# multipathd show config
# multipath -t

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 for the device that the multipathd show config command displays and override the values that you want to change.

To add a device that is not configured automatically by default to this section of the configuration file, 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:

# cat /sys/block/sda/device/vendor
WINSYS
# 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.

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.

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
#	}
#}

5.7. Setting multipath values for all devices

The overrides section of the multipath.conf configuration file allows you to set a configuration value for all of your devices. For example, you may want all devices to set no_path_retry to fail. This section supports all of the 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.

For information on the attributes that are set in the devices and defaults sections of the multipath.conf configuration file see the multipath.conf(5) man page.

5.8. Additional resources

  • Further information about the DM Multipath configuration file can be found on the multipath.conf(5) man page.