[Term of the Day]: Logical Unit Number

Term of the Day

"Logical Unit Number (LUN)"

Definition — What is LUN?

In data storage technology, a logical unit number, or LUN, is a virtual address which is used to identify SCSI devices, such as external hard drives, connected to a computer. Each device is assigned a LUN, from 0 to 7, which serves as the device's unique address.

This address tells the system which volume to send and read data from when it's addressed. In a single SCSI environment, no two devices can contain the same LUN or they will conflict, usually resulting in one of the devices becoming inaccessible or invisible on the SAN fabric until a unique LUN is assigned.

LUNs can also be used for identifying virtual hard disk partitions, which are used in RAID configurations. For example, a single hard drive may be partitioned into multiple volumes. Each volume can then be assigned a unique LUN. However, few modern computers use LUNs, since SCSI devices have mostly been replaced by USB and Firewire devices.