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This article is part of our Essential Guide: LUN storage: Working with a SAN's logical unit numbers

What is a LUN, and why do we need one?

LUNs are a logical abstraction between a physical disk device and applications. Expert Randy Kerns outlines some LUN use cases and LUN security features.

A reader recently asked: What is a LUN and why do we have to create one rather than use the disk as it is? A logical...

unit number (LUN) can refer to an entire physical disk, or a subset of a larger physical disk or disk volume. The physical disk or disk volume could be an entire single disk drive, a partition (subset) of a single disk drive, or a disk volume from a redundant array of independent disks (RAID) controller comprising multiple disk drives aggregated together for larger capacity and redundancy. LUNs are an important factor in managing block storage arrays shared over a SAN. A LUN represents a logical abstraction or, if you prefer, virtualization layer between the physical disk device/volume and the applications.

The term virtualization has been used to mean many 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. For example, if you partition a disk drive into smaller pieces for your application or system needs (perhaps your server's operating system has a disk drive size limit) the sub-segments would share a common Small Computer System Interface (SCSI) target ID address with each partition being a unique LUN.

Another example would be a RAID controller where multiple disk drives are mirrored (RAID 1) or stripped with parity (RAID 5) to create a larger physical disk volume. A LUN would be used to represent the virtual disk devices made up of the disks in the RAID group. So, you may not need to create a LUN to use a disk device. For example, if you are simply attaching several JBOD disks to a server via SCSI, USB drive, Fibre Channel or whatever interface you prefer, LUNs may not have to be created. If you are using a SCSI disk and you have not subdivided it into partitions, the device would show up with a unique SCSI ID target and default LUN 0. Note that you will want to ensure the devices are properly addressed to avoid conflicts.

Unless you are using JBOD or a single disk device, you will use LUNs as an abstraction layer to access your storage.

When it comes down to deciding whether or not you need a LUN, you have to ask yourself what type of storage you have, how it is configured and how you will use it. Most likely, unless you are using JBOD or a single disk device, you will use LUNs as an abstraction layer to access your storage.

LUN zoning and LUN masking

Now that we've answered the question "What is a LUN?" the next step is to discuss security features that will ensure users and servers see only those storage resources they are allowed to access.

Logical unit number zoning isolates a single server to a specific logical unit or associates a group of servers with a logical unit. LUN zoning also enables a storage administrator to allocate bandwidth by assigning particular ports to a zone.

Once LUN zoning is completed, the LUN masking process subdivides port access so that only the logical unit numbers authorized to access a specific server can access the corresponding port.

Next Steps

LUN creation for storage-area networks

Five steps to managing LUNs

The role of LUNs in virtual environments

This was last published in July 2004

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Will there ever come a time when computing environments will be LUN-free?
An administrator configures 6 snapshots of a LUN and creates 8 clones of the same LUN. The administrator then creates 4 snapshots for each clone that was created. How many usable replicas are now available?