SCSI (Small Computer System Interface) is implemented in a client/server model. Computer acts as client (initiator) and storage device acts as server (target). The SCSI command processing entity within the storage target represents a logical unit and assigned a number (LUN). SCSI targets are assigned a 3-part bus/target/LUN descriptor. Bus refer to one of the several SCSI that installed on the hot (e.g. HBA, iSCSI network card). Bus supported multiple daisy-chained disks (target). LUN represents a SCSI server within the target.
Operating systems expect to boot from LUN 0. For multiple server to boot from the same disk array, the array needs to support multiple LUN 0s. This is done by mapping the actual LUN to virtual LUN. LUN mapping also include LUN masking used to isolate set of LUN from other hosts or users.
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