Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
, a logical unit number or LUN is a number used to identify a logical unit, which is a device addressed by the SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
In computing, iSCSI , is an abbreviation of Internet Small Computer System Interface, an Internet Protocol -based storage networking standard for linking data storage facilities. By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage...
. A LUN may be used with any device which supports read/write operations, such as a tape drive
Tape drive
A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...
, but is most often used to refer to a logical disk
Logical disk
A logical disk is a device that provides an area of usable storage capacity on one or more physical disk drive components in a computer system. Other terms that are used to mean the same thing are partition, logical volume, and in some cases a virtual disk .The disk is described as logical because...
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
. Though not technically correct, the term "LUN" is often also used to refer to the drive itself.
Examples
To provide a practical example, a typical disk array
Disk array
A disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization.Components of a typical disk array include:...
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
ports, each with one SCSI target address assigned. Then the disk array is formatted as a RAID and then this RAID is partitioned into several separated storage volumes. To represent each volume, a SCSI target is configured to provide a logical unit. Each SCSI target may provide multiple logical units and thus represent multiple volumes, but this does not mean that those volumes are concatenated
Concatenation
In computer programming, string concatenation is the operation of joining two character strings end-to-end. For example, the strings "snow" and "ball" may be concatenated to give "snowball"...
. The computer that accesses a volume on the disk array identifies which volume to read or write with the LUN of the associated logical unit.
Another example is a single disk drive with one physical SCSI port. It usually provides just a single target, which in turn usually provides just a single logical unit whose LUN is zero. This logical unit represents the entire storage of the disk drive.
Use
How to select a LUN: In the early versions of SCSI, an initiator delivers a Command Data Block (CDB) to a target (physical unit) and within the CDB is a 3-bit LUN field to identify the logical unit within the target. In current SCSI, the initiator delivers the CDB to a particular logical unit, so the LUN appears in the transport layer data structures and not in the CDB.
LUN vs. SCSI Device ID: The LUN is not the only way to identify a logical unit. There is also the SCSI Device ID, which identifies a logical unit uniquely in the world. Labels or serial numbers stored in a logical unit's storage volume often serve to identify the logical unit. However, the LUN is the only way for an initiator to address a command to a particular logical unit, so initiators often create, via a discovery process, a mapping table of LUN to other identifiers.
Context sensitive: The LUN identifies a logical unit only within the context of a particular initiator. So two computers that access the same disk volume may know it by different LUNs.
LUN 0: There is one LUN which is required to exist in every target: zero. The logical unit with LUN zero is special in that it must implement a few specific commands, most notably Report LUNs, which is how an initiator can find out all the other LUNs in the target. But LUN zero need not provide any other services, such as a storage volume.
Many SCSI targets contain only one logical unit (so its LUN is necessarily zero). Others have a small number of logical units that correspond to separate physical devices and have fixed LUNs. A large storage system may have up to thousands of logical units, defined logically, by administrative command, and the administrator may choose the LUN or the system may choose it.
cXtXdXsX nomenclature in Unix
From the computer perspective, SCSI LUN is only a part of the full SCSI address. The full device's address is made from the:
c-part: controller ID of the host bus adapter,
t-part: target ID identifying the SCSI target on that bus,
In Sun Microsystems' Solaris computer operating system, disk partitions are sometimes known as slices. This is a conceptual reference to the slicing of a cake into several pieces...
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s, these IDs are often combined into a single "name". For example, /dev/dsk/c1t2d3s4 would refer to controller 1, target 2, disk 3, slice 4. Presently Solaris, HP-UX
HP-UX
HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984...
NCR Corporation is an American technology company specializing in kiosk products for the retail, financial, travel, healthcare, food service, entertainment, gaming and public sector industries. Its main products are self-service kiosks, point-of-sale terminals, automated teller machines, check...
, and others continue to use "cXtXdXsX" nomenclature, while AIX
AIX operating system
AIX AIX AIX (Advanced Interactive eXecutive, pronounced "a i ex" is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms...
has abandoned it in favor of more familiar names.
Other uses
The term logical unit number also applies to an input/output access channel within certain programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
, some input/output statements such as the READ or WRITE statements contain an ordered pair of numbers which identifies the LUN of the file or other data source or destination, and (usually) also, the FORMAT of the data to be read or written, as in this example: WRITE (5,32) where 5 is the LUN of the target file or device, and 32 is the label of the FORMAT statement for the write.
The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.
