Logical unit number

In computer storage, a logical unit number or LUN is an address for an individual disk drive and by extension, the disk device itself. The term originated in the SCSI protocol as a way of differentiating individual disk drives within a common SCSI target device like a disk array.

The term has become common in storage area networks and other enterprise storage fields. Today, LUNs are normally not an entire disk drives but rather virtual partitions (or volumes) of a RAID set.

In SCSI, LUNs are addressed in conjunction with the controller ID of the host bus adapter, the target ID of the storage array, and an optional (and no longer common) slice ID. In the UNIX family of operating systems, these IDs are often combined into a single "word". For example, "c1t2d3s4" would refer to controller 1, target 2, disk 3, slice 4. Only Sun's Solaris operating system continues to use LUN slices, and IBM's AIX has abandoned the "ctd" nomenclature in favor of more familiar names.