Ostensibly standing for X Logical Volumes, XLV is SGI's more modern implementation of logical volumes. See Linux's LVM for a FOSS example of an equivalent.
XLV is unfortunately locked with a license key. Information on how to crack the binaries is found here.
XLV supports mirror and stripe functions (RAID 1 and 0 respectively). On 32-bit SGI systems (those using a 32-bit kernel/n32 ABI) the maximum size is 1 terabyte. On 64-bit, Techpubs claims it to be "unlimited". It also allows for online volume reconfiguration.
XLV also allows filesystem journals to be placed on separate devices, increasing disk performance.