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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
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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
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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
217:(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.
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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.
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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.
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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.
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From the computer perspective, SCSI LUN is only a part of the full SCSI address. The full device's address is made from the:
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has abandoned it in favor of more familiar names. HP-UX refers to this as the Legacy Naming Model since version 11i v3.
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ports, each with one SCSI target address assigned. An administrator may format the disk array as a
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target may provide multiple logical units and thus represent multiple volumes, but this does
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A LUN may be used with any device which supports read/write operations, such as a
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To provide a practical example, a typical multi-disk drive has multiple physical
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command, and the administrator may choose the LUN or the system may choose it.
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this RAID into several separate storage-volumes. To represent each volume, a
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would refer to controller 1, target 2, disk 3, slice 4. Presently
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also applies to an input/output access channel within certain
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In the early versions of SCSI, an initiator delivers a
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target is configured to provide a logical unit. Each
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268:identifying the SCSI target on that controller,
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48:introducing citations to additional sources
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135:protocols that encapsulate SCSI, such as
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38:Relevant discussion may be found on the
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367:"What is a Logical Unit Number (LUN)?"
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275:identifying a LUN on that target,
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332:programming languages
59:"Logical unit number"
211:How to select a LUN:
133:Storage Area Network
44:improve this article
387:James Long (2006).
328:logical unit number
117:logical unit number
228:Context sensitive:
300:/dev/dsk/c1t2d3s4
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371:. Retrieved
369:. Techopedia
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344:LUN masking
373:2016-03-30
350:References
322:Other uses
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148:tape drive
100:March 2016
70:newspapers
326:The term
284:partition
266:target ID
180:partition
178:and then
40:talk page
419:Category
338:See also
280:slice ID
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166:Examples
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290:In the
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308:HP-UX
141:iSCSI
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188:SCSI
184:SCSI
176:RAID
172:SCSI
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129:SCSI
115:, a
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316:AIX
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206:Use
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111:In
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