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Online resizing of multipath devices in Linux dm-multipath

Linux doesn’t automatically re-size multipath devices, so this procedure must be used to have online re-sizing of multipath.
(Offline re-size is automatic, just remove the mapping and reload)

  • this example will use multipath device /dev/mpath/testdisk, with scsi disks /dev/sdx and /dev/sdy
  • Resize the lun on the underlying storage layer (iscsi / san)
  • Check which sd? devices are relevant, and re-scan these:
    • multipath -ll testdisk
    • blockdev –rereadpt /dev/sdx
    • blockdev –rereadpt /dev/sdy
    • blockdev –getsz /dev/sdx
    • blockdev –getsz /dev/sdy
  • Take note of the new size returned by getsz.
  • Dump the dmsetup table to a file (and a backup)
    • dmsetup table testdisk | tee mapping.bak mapping.cur
  • Edit the table stored in ‘mapping.cur’
    • vi mapping.cur, replace field 2 (size) with the new size from getsz
  • Suspend I/O, reread the table, and resume I/O
    • dmsetup suspend testdisk; dmsetup reload testdisk mapping.cur; dmsetup resume testdisk
  • The multipath device should now be resized:
  • multipath -ll

You can now resize the filesystem on the multipath device, or the LVM-PV if you use LVM on the LUN.

IBM has a slightly more complex, but possibly ‘better’ procedure at http://www-03.ibm.com/support/techdocs/atsmastr.nsf/WebIndex/TD105262

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