A virtual LUN (virtual logical unit number) is a representation of a storage area that is not tied to any physical storage allocation. Virtual LUNs are used in thin provisioning (also known as virtual provisioning) for storage area network (SAN) management. A virtual LUN is also sometimes called a thin LUN.
A logical unit number (LUN) is a unique identifier for a physical storage allocation. A LUN could reference an entire RAID set, a single hard disk or partition, or multiple disks or partitions. Unlike a physical LUN, the virtual LUN does not map to a specific device or allocation of storage space but a virtualized space that can be created in excess of the actual physical space available.
In a SAN, hard disks are stored in physical racks. The number of physical LUNs that can be created is limited by the amount of space available and the RAID level. Virtual LUNs remove that limitation. Because virtual LUNs are not tied to physical devices or allocations, an administrator can create large numbers of them that will be allocated only when the data is written. That capacity helps optimize storage utilization because space is not allocated and then sitting idle. Virtual LUNs also increase fault tolerance because data is written across multiple disks.