Editing SCSI (section)

===General===
Note that a SCSI target device (which can be called a "physical unit") is sometimes divided into smaller "logical units". For example, a high-end disk subsystem may be a single SCSI device but contain dozens of individual disk drives, each of which is a logical unit. Further, a RAID array may be a single SCSI device, but may contain many logical units, each of which is a "virtual" disk—a stripe set or mirror set constructed from portions of real disk drives. The SCSI ID, WWN, etc. in this case identifies the whole subsystem, and a second number, the [[logical unit number]] (LUN) identifies a disk device (real or virtual) within the subsystem.

It is quite common, though incorrect, to refer to the logical unit itself as a "LUN".<ref>
  {{cite web
    | title      = na_lun(1) – Manual page for "lun" on NetApp DataONTAP
    | publisher  = [[NetApp]]
    | date       = July 7, 2009
    | quote      = The lun command is used to create and manage luns[...]
    | url        = https://library.netapp.com/ecmdocs/ECMP1511537/html/man1/na_lun.1.html
  }}
</ref> Sometimes, redundantly, the actual LUN may be called a "LUN number" or "LUN id".<ref>
  {{cite web
    | title      = na_lun(1) – Manual page for "lun" on NetApp DataONTAP
    | publisher  = [[NetApp]]
    | date       = July 7, 2009
    | quote      = If a LUN ID is not specified, the smallest number [...] is automatically picked.
    | url        = https://library.netapp.com/ecmdocs/ECMP1511537/html/man1/na_lun.1.html
  }}
</ref>

In modern SCSI transport protocols, there is an automated process for the "discovery" of the IDs. The SSA initiator (normally the host computer through the 'host adaptor') "walk the loop" to determine what devices are connected and then assigns each one a 7-bit "hop-count" value. [[Fibre Channel]] – Arbitrated Loop (FC-AL) initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN ([[World Wide Name]]). For iSCSI, because of the unlimited scope of the (IP) network, the process is quite complicated. These discovery processes occur at power-on/initialization time and also if the bus topology changes later, for example if an extra device is added.

SCSI has the CTL (Channel, Target or Physical Unit Number, Logical Unit Number) identification mechanism per [[host bus adapter]], or the HCTL (HBA, Channel, PUN, LUN) identification mechanism, one host adapter may have more than one channels.<ref>{{Cite web|title=SCSI Addressing|url=https://tldp.org/HOWTO/SCSI-2.4-HOWTO/scsiaddr.html|access-date=2021-03-04|website=tldp.org}}</ref>