Logical unit number
A Logical Unit Number (LUN) is a logical unit that turns purely physical hard disk space into logical storage space for use by the operating system of a host server. Every Windows user should know the logical drive letters that are assigned to their hard drives. The operating system is usually loaded from drive C: whereas user files can be stored on a possible second logical drive with the designation D :. LUNs do the same job: they differentiate between different pieces of disk space and are the part of the address of storage that is presented to a host server.
A LUN is a unique identifier for individual or a group of hard disks . These can be controlled via SCSI , iSCSI , fiber optic cables or similar interfaces. LUNs are central to managing block storage arrays that are distributed over a storage area network (SAN).
The term LUN stems from the early days of SCSI when each device (up to eight) was identified with a logical number. Today, servers with a dozen or more LUNs are common. In addition, it has become rare to connect them to a conventional internal SCSI disk array. However, the basic elements of the server storage still refer to the LUN.
Each LUN identifies a specific logical unit that can be part of a hard disk, a complete hard disk or several hard disks in a storage group. LUN can thus refer to a complete RAID, individual hard disks or partitions or several hard disks or partitions. In either case, the logical unit is treated as a single device and identified by the LUN.
LUNs are created as a fundamental part of the storage provisioning process with the help of software tools that are usually part of a storage platform. There is no 1: 1 ratio between available drives and LUNs: numerous LUNs can easily be created on a single drive.
