37.3. Updating the Size of Your Multipath Device

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If multipathing is enabled on your system, you will also need to reflect the change in logical unit size to the logical unit's corresponding multipath device (after resizing the logical unit). For Red Hat Enterprise Linux 5.3 (or later), you can do this through multipathd. To do so, first ensure that multipathd is running using service multipathd status. Once you've verified that multipathd is operational, run the following command:
# multipathd -k"resize map multipath_device"
The multipath_device variable is the corresponding multipath entry of your device in /dev/mapper. Depending on how multipathing is set up on your system, multipath_device can be either of two formats:
  • mpathX, where X is the corresponding entry of your device (for example, mpath0)
  • a WWID; for example, 3600508b400105e210000900000490000
To determine which multipath entry corresponds to your resized logical unit, run multipath -ll. This displays a list of all existing multipath entries in the system, along with the major and minor numbers of their corresponding devices.


Do not use multipathd -k"resize map multipath_device" if there are any commands queued to multipath_device. That is, do not use this command when the no_path_retry parameter (in /etc/multipath.conf) is set to "queue", and there are no active paths to the device.
If your system is using Red Hat Enterprise Linux 5.0-5.2, you will need to perform the following procedure to instruct the multipathd daemon to recognize (and adjust to) the changes you made to the resized logical unit:

Procedure 37.1. Resizing the Corresponding Multipath Device (Required for Red Hat Enterprise Linux 5.0 - 5.2)

  1. Dump the device mapper table for the multipathed device using:
    dmsetup table multipath_device
  2. Save the dumped device mapper table as table_name. This table will be re-loaded and edited later.
  3. Examine the device mapper table. Note that the first two numbers in each line correspond to the start and end sectors of the disk, respectively.
  4. Suspend the device mapper target:
    dmsetup suspend multipath_device
  5. Open the device mapper table you saved earlier (i.e. table_name). Change the second number (i.e. the disk end sector) to reflect the new number of 512 byte sectors in the disk. For example, if the new disk size is 2GB, change the second number to 4194304.
  6. Reload the modified device mapper table:
    dmsetup reload multipath_device table_name
  7. Resume the device mapper target:
    dmsetup resume multipath_device
For more information about multipathing, refer to the Red Hat Enterprise Linux 6 DM Multipath guide.
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