A logical unit number (LUN) is the logical unit of storage that a SAN or other disk subsystem can assign to metadata servers and clients. A volume is a LUN that is labeled by SAN File System for its use. Volumes are grouped together virtually to form storage pools, in which file data and metadata is stored.
An LUN becomes a SAN File System volume when you add it to a storage pool. It is automatically assigned a system-generated label that identifies it as a SAN File System volume. You must also give the volume a name that is unique among all the volumes used by a SAN File System cluster.
During startup, the metadata server scans all LUNs that it can access in the SAN, searching for the label that tells it that the LUN is a valid SAN File System volume. Clients perform this same search whenever they are started.
System-data LUN operations are performed by the metadata servers. All other data LUN operations are initiated from and coordinated by the metadata servers in the cluster but are actually performed by one or more clients; therefore, the metadata servers no longer need to see the data LUNs, and the clients only need to see the data LUNs that they need to access. This allows SAN File System to support a wide variety of SAN configurations, storage devices, and drivers, and also supports scaling to large numbers of storage devices and clients. This also allows SAN File System to support grouping clients and LUNs into SAN zones to provide enhanced security.
A volume must be empty to be removed from a storage pool. When you remove a volume, SAN File System moves the contents of that volume across other available volumes in the same storage pool. If the storage pool does not have sufficient space available in other volumes to move all of the data contained in the specified volume, the removal fails and the metadata server suspends the volume (the metadata server cannot allocate new data on that volume).