LUN masking, like zoning, prevents servers from seeing all but specific storage resources, but it is more efficient and granular since it can be used to control LUNs within a storage device. LUNs are either individual disks, groups of disks, or individual parts of multiple disks defined by a RAID controller. LUNs, which most people commonly refer to as partitions or logical disks, are granular storage entities that are carved out of a single storage system (be it a RAID or JBOD or even a tape library). Because multiple LUNs can reside on a single storage system, multiple computers can access the LUNs through a single wire connection to a storage system with LUN masking, a situation that is far more scalable than zoning, which is hampered by its 1:1 setup (one port: one connection).
"With LUN Masking you can use a single Fibre Channel link to split up a RAID unit into multiple logical parts."
LUN Masking, which is usually carried out by intelligent Fibre Channel RAID controllers, ensures that the host operating systems can only see the LUNs that have been explicitly assigned to them.
LUN masking can be implemented quickly (in a matter of seconds), but it is important to note that for most RAID units like MTIs S200, the alteration of LUN masking settings usually requires a little bit of downtime as the controller reapplies its masks. As a common sense practice, you should never switch LUN masks while an application running on a server is still using the data on a LUN--even if the application seems to be idle. The easiest way to avoid data corruption is by scheduling downtime before doing any major activities.
LUN masking works well in small isolated SANs, but as the number of hosts and targets escalate, they can become somewhat unwieldy.