 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Logical unit number" – news ·newspapers ·books ·scholar ·JSTOR(March 2016) (Learn how and when to remove this message) |
Incomputer storage, alogical unit number (LUN) is a number used to identify alogical unit, which is a device addressed by theSCSI protocol or bystorage area network (SAN) protocols that encapsulate SCSI, such asFibre Channel (FC) oriSCSI.[1]
A LUN may be used with any device which supports read/write operations, such as atape drive, but is most often used to refer to alogical disk as created on a SAN. Though not technically correct, the term "LUN" is often also used to refer to thelogical disk itself.[2]
To provide a practical example, a typical multi-disk drive has multiple physical SCSI ports, each with one SCSI target address assigned. An administrator may format the disk array as aRAID and thenpartition this RAID into several separate storage-volumes. To represent each volume, a SCSI target is configured to provide a logical unit. Each SCSI target may provide multiple logical units and thus represent multiple volumes, but this doesnot mean that those volumes areconcatenated. The computer that accesses a volume on the disk array identifies which volume to read or write with the LUN of the associated logical unit.
In another example: a single disk-drive has one physical SCSI port. It usually provides just a single target, which in turn usually provides just a single logical unit whose LUN is zero. This logical unit represents the entire storage of the disk drive.
How to select a LUN: In the early versions of SCSI, an initiator delivers a Command Descriptor Block (CDB) to a target (physical unit) and within the CDB is a 3-bit LUN field to identify the logical unit within the target. In current SCSI, the initiator delivers the CDB to a particular logical unit, so the LUN appears in the transport layer data structures and not in the CDB.
LUN vs. SCSI Device ID: The LUN is not the only way to identify a logical unit. There is also the SCSI Device ID, which identifies a logical unit uniquely in the world. Labels or serial numbers stored in a logical unit's storage volume often serve to identify the logical unit. However, the LUN is the only way for an initiator to address a command to a particular logical unit, so initiators often create, via a discovery process, a mapping table of LUN to other identifiers.
Context sensitive: The LUN identifies a logical unit only within the context of a particular initiator. So two computers that access the same disk volume may know it by different LUNs.
LUN 0: There is one LUN which is required to exist in every target: zero. The logical unit with LUN zero is special in that it must implement a few specific commands, most notably Report LUNs, which is how an initiator can find out all the other LUNs in the target. But LUN zero need not provide any other services, such as a storage volume.
Many SCSI targets contain only one logical unit (so its LUN is necessarily zero). Others have a small number of logical units that correspond to separate physical devices and have fixed LUNs. A large storage system may have up to thousands of logical units, defined logically, by administrative command, and the administrator may choose the LUN or the system may choose it.
From the computer perspective, SCSI LUN is only a part of the full SCSI address. The full device's address is made from the:
In theUnix family ofoperating systems, these IDs are often combined into a single "name". For example,/dev/dsk/c1t2d3s4 would refer to controller 1, target 2, disk 3, slice 4. PresentlySolaris,HP-UX,NCR, and others continue to use "c_t_d_s_" nomenclature, whileAIX has abandoned it in favor of more familiar names. HP-UX refers to this as the Legacy Naming Model since version 11i v3.
The termlogical unit number also applies to an input/output access channel within certainprogramming languages.