Configuring device mapper multipath

Red Hat Enterprise Linux 8.0 Beta

Using the Device Mapper Multipath feature

Red Hat Customer Content Services

Abstract

This documentation collection provides instructions on how to configure and manage the Device Mapper Multipath (DM-Multipath) feature on Red Hat Enterprise Linux 8.

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Chapter 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.

1.1. Basic DM Multipath setup

You set up DM Multipath with the mpathconf utility, which creates the multipath configuration file /etc/multipath.conf.

  • If the /etc/multipath.conf file already exists, the mpathconf utility will edit it.
  • If the /etc/multipath.conf file does not exist, the mpathconf utility will create the /etc/multipath.conf file from scratch.

For more information on the mpathconf utility, see the mpathconf(8) man page.

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 mpathconf 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. Enter 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(8) man page or enter the mpathconf command with the --help option specified.

    # mpathconf --help
    usage: /sbin/mpathconf <command>
    
    Commands:
    Enable: --enable
    Disable: --disable
    Set user_friendly_names (Default y): --user_friendly_names <y|n>
    Set find_multipaths (Default y): --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>
  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.

    The initial defaults section of the configuration file configures your system so that the names of the multipath devices are of the form /dev/mapper/mpathn; without this setting, the names of the multipath devices would be aliased to the WWID of the device. / /an alias of your choosing, see configuration file chapter If you do not want to use user friendly names, you can enter the following command:

    # mpathconf --enable --user_friendly_names n
  3. Save the configuration file and exit the editor, if necessary.
  4. Execute the following command:

    # systemctl start multipathd.service
Note

If you find that you need to edit the multipath configuration file after you have started the multipath daemon, you must execute the systemctl reload multipathd.service command for the changes to take effect.

1.2. 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 on, you should not have to blacklist these devices. For information on the find_multipaths configuration parameter and the meaning of the values to which you can set this parameter to, see the multipath.conf(5) man page.

If you do not set the find_multipaths configuration parameter to on, you 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 command shows the local disk, /dev/sda, in the multipath map.

    This examples specifies the -d option of the multipath command to indicate that this is a dry run that will not create the multipath devices.

    # multipath -v2 -d
    : 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  [---------
    
    : 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
    
    : 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
    
    : 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
    
    : 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 a 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.

    # systemctl reload multipathd.service

1.3. Configuring additional storage devices

By default, DM Multipath includes support for the most common storage arrays that themselves support DM Multipath. For information on the default configuration value, including supported devices, run either of the following commands.

# multipathd show config
# multipath -t

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. This example sets the device to queue for a minute (or 12 retries and 5 seconds per retry) after all paths have failed.

devices {
        device {
                vendor "HP"
                product "OPEN-V"
                no_path_retry 12
        }
}

1.4. Setting up multipathing in the initramfs file system

You can set up multipathing in the initramfs file system. After configuring multipath, you can rebuild the initramfs file system with the multipath configuration files by executing the dracut command with the following options:

# dracut --force --add multipath --include /etc/multipath

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.

Chapter 2. 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. For information on the default configuration values, including supported devices, run either of the following commands.

# multipathd show config
# multipath -t

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.

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.

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

2.1. 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 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.

2.2. Blacklisting devices from DM Multipath

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.

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 blacklisted. If the find_multipaths configuration parameter is set to on, then multipath will create a device only if one of three conditions are met:

  • There are at least two paths that are not blacklisted 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 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-blacklisted 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. For information on the find_multipaths_timeout parameter, see the multipath.conf(5) man page.

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.

For more information on the values you can set for find_multipaths, see the multipath.conf(5) man page.

With the find_multipaths parameter set to on, 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 blacklist devices by WWID, device name, device type, property, and protocol. For every device, these five blacklist criteria are evaluated in the the order "property, devnode, device, protocol, wwid". If a device turns out to be blacklisted by any criterion, it is excluded from handling by multipathd, and the later criteria are not evaluated. For each criterion, the whitelist takes precedence over the blacklist if a device matches both.

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 2.2.2, “Blacklisting by device name”.

2.2.1. 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
}

2.2.2. 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 all 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 2.2.6, “Blacklist exceptions”.

blacklist {
       devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
       devnode "^(td|ha)d[a-z]"
}

2.2.3. 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 and HP devices.

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

2.2.4. Blacklisting by udev property

The blacklist and blacklist_exceptions sections of the multipath.conf configuration file support the property parameter. This parameter allows users to blacklist certain types of devices. The property parameter takes a regular expression string that is matched against the udev environment variable name for the device.

The following example blacklists all all devices with the udev property ID_ATA.

blacklist {
        property "ID_ATA"
}

2.2.5. Blacklisting by device protocol

You can specify the protocol for a device to be excluded from multipathing in the blacklist section of the configuration file with a protocol section. The protocol strings that multipath recognizes are scsi:fcp, scsi:spi, scsi:ssa, scsi:sbp, scsi:srp, scsi:iscsi, scsi:sas, scsi:adt, scsi:ata, scsi:unspec, ccw, cciss, nvme, and undef. The protocol that a path is using can be viewed by running the command multipathd show paths format "%d %P".

The following example blacklists all all devices with an undefined protocol or an unknown SCSI transport type.

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

2.2.6. 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.

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 blacklisted. This parameter allows users to blacklist SCSI devices that multipath should ignore, such as USB sticks and local hard drives. To allow only 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)"
}

2.3. 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 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.

2.4. 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
        }
}

2.5. 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.

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
#	}
#}

2.6. 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.

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