Use
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.
Read more about this topic: Logical Unit Number
Famous quotes containing the word use:
“... it is use, and use alone, which leads one of us, tolerably trained to recognize any criterion of grace or any sense of the fitness of things, to tolerate ... the styles of dress to which we are more or less conforming every day of our lives. Fifty years hence they will seem to us as uncultivated as the nose-rings of the Hottentot seem today.”
—Elizabeth Stuart Phelps (1844–1911)