Logical Unit Number

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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 is used in the SCSI protocol as a way to differentiate individual disk drives within a common SCSI target device like a disk array..

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

[edit] Nomenclature

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, Hewlett Packard's HP-UX and NCR Teradata's MP-RAS continue to use LUN slices, while IBM's AIX has abandoned the "ctd" nomenclature in favor of more familiar names.

[edit] Other uses

The term LUN also applies to a file access channel within certain programming languagues. For example in FORTRAN, the WRITE statement has a form which identifies the LUN of the target file and the FORMAT of the data to be written as in WRITE(5,32) where 5=the LUN of the file and 32 is the FORMAT statement for the write.