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Logical Unit Number (LUN)

Definition - What does Logical Unit Number (LUN) mean?

A logical unit number (LUN) is a number used for identifying a logical unit relating to computer storage. A logical unit is a device addressed by protocols and related to fiber channel, small computer system interface (SCSI), Internet SCSI (iSCSI) and other comparable interfaces.

LUNs are essential for managing the block storage arrays of a storage area network (SAN). A typical LUN is used with any component supporting read/write processes. LUNs are commonly used for logical discs produced on a SAN.

The term LUN was initiated from the SCSI protocol and provided a methodology for identifying specific disc drives within a regular component such as a disc array. Frequently, the term LUN is used in reference to the actual disc drive, which is not technically accurate. Additionally, a LUN may refer to an input/output (I/O) access channel within selected programming languages. Today, LUNs are found not only on disc drives, but also on virtual partitions or on volumes of redundant arrays of independent disks (RAID) using multiple drives.

Techopedia explains Logical Unit Number (LUN)

A standard disc array has several SCSI ports. Each SCSI port has an assigned target address. The disc array is formatted as RAID and partitioned into various storage units. Each volume is configured with a logical unit. There can be numerous logical units characterizing several volumes. Likewise, a disc drive with a single SCSI port typically has one target with a singular logical unit with a zero LUN. The zero signifies the entire storage of the disc drive.

Each device is given an exclusive number between zero and seven for an 8-bit bus or a number between eight and 16 for a 16-bit bus. A device initiating an I/O request is an initiator. A device executing the request is the target. An individual target has the ability to interconnect with up to eight or more components using one controller. These components are the logical units.

An SCSI LUN can be addressed with a combination of the controller ID, the target ID, a disk ID, and occasionally the slice ID. The identifications (IDs) in a UNIX OS are generally joined as one word. A typical example is the address c1t2d3s4. This refers to controller 1, target 2, disk 3 and slice 4. Full device addresses are as follows:

  • c-part: controller ID of host bus adapter
  • t-part: target ID classifying SCSI target on the bus
  • d-part: disk ID classifying LUN on the target
  • s-part: slice ID classifying exact slice on the disk
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