LUN, SAN

Logical Unit Number Context: Storage Subsystems Acronym: LUN A LUN or Logical Unit Number is a SCSI protocol term for an end device, usually a tape or disk drive. The LUN identifies a unique device on a SCSI bus. The SCSI bus consists of a SCSI controller, cable and the devices that are attached to that cable. For 8-bit SCSI implementations, up to 8 devices can be present on the bus (one of which must be the controller itself) and for 16-bit implementations, up to 16 devices may be present. I/O requests from a host computer, or initiator specify a SCSI target and a logical unit attached to that SCSI target. The logical unit number (LUN) is used to identify each of the devices accessible through that target. Within a storage area network (SAN), the fibre channel protocol passes SCSI requests and so fibre channel provides for initiators (usually HBAs) and targets (usually storage ports) within the SAN. A fibre channel storage port target will have a number of LUNs assigned. Note that although the term LUN refers to the logical unit number, the term LUN is usually used to refer to the device itself rather than just the assigned number.