What is LUN?

What is LUN?  Logical Unit Number is a unique identifier used on a SCSI bus that enables it to differentiate between up to eight separate devices (each of which is a logical unit).On a SCSI parallel bus (a type of inter-connect used to connect SAN Storage devices, it is different from Fibre Channel) it is possible to have different devices, and each one is given an address called a logical unit number (LUN). The LUN on a SCSI parallel bus is actually used to electrically address the various devices. The concept of a LUN has been adapted to Fibre Channel devices to allow multiple SCSI devices to appear on a single Fibre Channel connection.

The LUN is required to be unique among all devices that are visible to each other. When a LUN is assigned to a server, it acts as a physical disk drive for the server. The server can perform read and write operations on it. Storage Administrators can assign access and control privileges on a per-LUN basis. The different types of LUN supported by the storage subsystem are: Simple, Spanned, Striped, Mirrored, and Striped with Parity.

In the SAN Storage environment, the definition is much broader since it is possible, although usually not desirable, that all devices attached to a SAN can see each other. A RAID controller that simulates multiple SCSI devices with different LUN’s is said to have multi-LUN support. This allows a single RAID controller to present its storage as many smaller devices. LUNs represent a logical abstraction or, virtualization layer between the physical disk device/volume and the applications.

The term virtualization has been used to mean many different things; thus, the virtualization police may or may not agree to a LUN being a layer of virtualization. A LUN can be used to present a larger or smaller view of the disk storage to the server. In Windows server, the Storage administrator will have to do the DISKPART in order to “activate” the drive.