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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
228:(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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59:introducing citations to additional sources
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146:protocols that encapsulate SCSI, such as
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49:Relevant discussion may be found on the
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378:"What is a Logical Unit Number (LUN)?"
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256:c_t_d_s_ nomenclature in Unix
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343:programming languages
70:"Logical unit number"
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144:Storage Area Network
55:improve this article
398:James Long (2006).
339:logical unit number
128:logical unit number
18:Logical Unit Number
239:Context sensitive:
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307:operating systems
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382:. Retrieved
380:. Techopedia
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384:2016-03-30
361:References
333:Other uses
305:family of
159:tape drive
111:March 2016
81:newspapers
337:The term
295:partition
277:target ID
191:partition
189:and then
51:talk page
430:Category
349:See also
291:slice ID
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177:Examples
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301:In the
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183:SCSI
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140:SCSI
126:, a
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327:AIX
323:NCR
217:Use
203:not
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