When dealing with the setup and configuration of Storage Area Networks (SANs), you may find yourself running into the term "LUN" more than a few times. Luckily, unless you're actually setting up the nuts and bolts portion of a new or reconfigured SAN, you shouldn't have to deal with LUNs very often. So what are they and what do they do?
What is a LUN?
LUN stands for "Logical Unit Number". It is a virtual address to a device in a SCSI environment. The short version here is that if you have a SCSI RAID device, each associated volume will have its own LUN assignment. 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.
Why do we need LUNs?
As mentioned above, a LUN is an address in a SCSI environment that allows one system to find a data store, typically a disk partition, in a larger network. It's possible to have multiple LUNs on a single physical drive, all pointing at different disk partitions. By targeting and masking certain LUNs, we can dictate which systems are exposed to each other in our SAN. This level of control allows us to shape our SAN environment to fit our facilities needs.
Managing LUNs
LUNs allow a system architect to selectively reveal a device or partition to one or more servers or workstations on the SAN fabric. By configuring which LUNs that a workstation or server can see on the network, you can restrict access to data or shape bandwidth paths based on zoned areas of the fabric and which LUNs those paths can reach. This allows for the configuration of redundant connection paths and paths with more bandwidth than others. Some SCSI based environments like Fibre Channel SAN system may have hundreds of individual LUNs associated with them. Most SAN software has some level of LUN management built into it, allowing you to adjust and refine what devices can see each other on the fabric.
Conclusion
LUNs are an integral part of the structure needed to properly deploy a SAN system. Set up properly, they allow us to control access and performance across a large environment of devices and storage. Once setup though, most users will not even know they exist, and given the somewhat complex nature of how they operate, that's not a bad thing.