Back up the existing filesystem (just in case) then run 'mklv' and
  'growfs'. 'mklv' and 'growfs' are nondestructive, so you don't need
  to restore the backup unless you screw up. Don't use 'mkfs', which
  does destroy existing data.

  Under modern IRIX versions (w/ the XFS filesystem) lv (the original
  logical volume manager) has been replaced with xlv. The requisite
  subsystems are _not_ installed by default. If you wish to use logical
  volumes, you must install eoe.sw.xlv, and if you wish to use mirroring
  (RAID 1), you must also install eoe.sw.xlvplex. This option requires a
  license be purchased from sgi.

  To simply grow the filesystem to another disk, you must first create a
  concatenated logical volume. This will change the volume headers of the
  target disks and let the system know that they should be treated as one
  volume. You can then use an 'xfs_growfs' to expand the filesystem on to
  the new disk. see the manpages for xlv_make(1), xlv_mgr(1), and
  xfs_growfs(1) for more details.

  Please not that it is NOT POSSIBLE to have a successfully striped or
  concatenated root filesystem. The only available logical volume
  configuration for the root filesystem is a mirror (RAID1). Any
  attempt to use another logical volume configuration will most
  likely work until the next system reboot. Since the Logal volume
  management software funtions on the OS level, it is unavailable
  during the inital phases of the boot process. Syncing of the
  mirrored filesystem will occur after the system comes up.