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Chapter 6. Managing multipathed volumes

You can manage multipath volumes by using the multipath, dmsetup, and multipathd commands, which are provided by DM Multipath.

6.1. Resizing an online multipath device
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You can resize an online multipath device and a file system by using the multipad and resize2fs commands .

Procedure

  1. Resize your physical device.

  2. Find the paths to the logical unit number (LUN): 

    # multipath -l
    Copy to Clipboard Toggle word wrap

  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/path_device/device/rescan
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    Ensure that you run this command for each of the path devices. For example, if your path devices are sda, sdb, sde, and sdf, run the following commands: 

    # echo 1 > /sys/block/sda/device/rescan
# echo 1 > /sys/block/sdb/device/rescan
# echo 1 > /sys/block/sde/device/rescan
# echo 1 > /sys/block/sdf/device/rescan
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  4. Resize your multipath device: 

    # multipathd resize map multipath_device
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  5. Resize the file system, assuming no LVM or DOS partitions are used: 

    # resize2fs /dev/mapper/mpatha
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6.2. Moving a root file system from a single path device to a multipath device
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If you have installed your system on a single-path device and later added another path to the root file system, move your root file system to a multipathed device.

Prerequisites

  • You have installed the device-mapper-multipath package.

Procedure

  1. Create the /etc/multipath.conf configuration file: 

    # mpathconf --enable
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  2. Enable the multipathd service: 

    # systemctl enable multipathd.service
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  3. If the find_multipaths configuration parameter is not set to on, edit the blacklist and blacklist_exceptions sections of the /etc/multipath.conf file, as described in Preventing devices from multipathing.

  4. Add the WWID of the device to the /etc/multipath/wwids file: 

    # multipath -a /dev/sdb
wwid '3600d02300069c9ce09d41c4ac9c53200' added
    Copy to Clipboard Toggle word wrap

    Replace /dev/sdb with the root device name.

  5. Confirm that your configuration file is set up correctly: 

    # multipath -d 3600d02300069c9ce09d41c4ac9c53200
: mpatha (3600d02300069c9ce09d41c4ac9c53200) undef 3PARdata,VV
size=446M features='1 queue_if_no_path' hwhandler='1 alua' wp=undef
`-+- policy='service-time 0' prio=50 status=undef
  `- 5:0:0:0 sdb 8:16 undef ready running
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    Replace 3600d02300069c9ce09d41c4ac9c53200 with the WWID of your swap device.

  6. Rebuild the initramfs file system with multipath: 

    # dracut --force --add multipath
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  7. Shut the machine down.
  8. Boot the machine.
  9. Make the other paths visible to the machine.

Verification

  • Check if the multipath device is created by running the following command: 

    # multipath -l | grep 3600d02300069c9ce09d41c4ac9c53200
mpatha (3600d02300069c9ce09d41c4ac9c53200) dm-0 3PARdata,VV
    Copy to Clipboard Toggle word wrap

6.3. Moving a swap file system from a single path device to a multipath device
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By default, swap devices are set up as logical volumes. This does not require any special procedure for configuring them as multipath devices if you set up multipathing on the physical volumes that constitute the logical volume group. If your swap device is not an LVM volume, however, and it is mounted by device name, you might need to edit the /etc/fstab file to switch to the appropriate multipath device name.

Procedure

  1. Create the /etc/multipath.conf configuration file: 

    # mpathconf --enable
    Copy to Clipboard Toggle word wrap

  2. Enable the multipathd service: 

    # systemctl enable multipathd.service
    Copy to Clipboard Toggle word wrap
  3. If the find_multipaths configuration parameter is not set to on, edit the blacklist and blacklist_exceptions sections of the /etc/multipath.conf file, as described in Preventing devices from multipathing.

  4. Add the WWID of the device to the /etc/multipath/wwids file: 

    # multipath -a /dev/sdb
wwid '3600d02300069c9ce09d41c4ac9c53200' added
    Copy to Clipboard Toggle word wrap

    Replace /dev/sdb with the swap device name.

  5. Confirm that your configuration file is set up correctly: 

    # multipath -d 3600d02300069c9ce09d41c4ac9c53200
: mpatha (3600d02300069c9ce09d41c4ac9c53200) undef 3PARdata,VV
size=446M features='1 queue_if_no_path' hwhandler='1 alua' wp=undef
`-+- policy='service-time 0' prio=50 status=undef
  `- 5:0:0:0 sdb 8:16 undef ready running
    Copy to Clipboard Toggle word wrap

    Replace 3600d02300069c9ce09d41c4ac9c53200 with the WWID of your swap device.

  6. Set up an alias for the swap device in the /etc/multipath.conf file: 

    multipaths {
    multipath {
        wwid WWID_of_swap_device
        alias swapdev
    }
}
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  7. Edit the /etc/fstab file and replace the old device path to the root device with the multipath device.

    For example, if you had the following entry in the /etc/fstab file: 

    /dev/sdb2 swap                    swap    defaults        0 0
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    Change the entry to the following: 

    /dev/mapper/swapdev swap          swap    defaults        0 0
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  8. Rebuild the initramfs file system with multipath: 

    # dracut --force --add multipath
    Copy to Clipboard Toggle word wrap
  9. Shut the machine down.
  10. Boot the machine.
  11. Make the other paths visible to the machine.

Verification

  • Verify if the swap device is on the multipath device: 

    # swapon -s

Filename                Type          Size Used    Priority
/dev/dm-3               partition     4169724 0    -2
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    The file name should match the multipath swap device. 

    # readlink -f /dev/mapper/swapdev
/dev/dm-3
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6.4. Determining device mapper entries for multipath devices
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You can use the multipathd command to discover which device mapper entries match the multipathed devices.

Procedure

  • Display all device mapper devices: 

    # multipathd show maps format "%n %d"

name   sysfs
mpathd dm-4
mpathb dm-3
mpatha dm-2
mpathh dm-9
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6.5. Administering the multipathd daemon
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The multipathd commands can be used to administer the multipathd daemon.

Procedure

  • View the default format for the output of the multipathd show maps command: 

    # multipathd show maps
name sysfs uuid
mpathc dm-0 360a98000324669436c2b45666c567942
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  • Some multipathd commands include a format option followed by a wildcard. Display a list of available wildcards with the following command: 

    # multipathd show wildcards
multipath format wildcards:
%n  name
%w  uuid
%d  sysfs
...
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  • Display the multipath devices that multipathd is monitoring. Use wildcards to specify the shown fields: 

    # multipathd show maps format "%n %w %d %s"
name   uuid                              sysfs vend/prod/rev
mpathc 360a98000324669436c2b45666c567942 dm-0  NETAPP,LUN
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  • Display the paths that multipathd is monitoring. Use wildcards to specify the shown fields: 

    # multipathd show paths format "%n %w %d %s"
target WWNN        uuid                              dev vend/prod/rev
0x50001fe1500d2250 3600508b4001080520001e00011700000 sdb HP,HSV210
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  • Display data in a raw format: 

    # multipathd show maps raw format "%n %w %d %s"
mpathc 360a98000324669436c2b45666c567942 dm-0 NETAPP,LUN
    Copy to Clipboard Toggle word wrap

    In raw format, no headers are printed and the fields are not padded to align the columns with the headers. This output can be more easily used for scripting.

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