Thin provisioning is a storage array feature that optimizes storage use by allocating and reclaiming the storage on demand. With thin provisioning, the array allocates storage to applications only when the storage is needed, from a pool of free storage. Thin provisioning solves the problem of under-utilization of available array capacity. Administrators do not have to estimate how much storage an application requires. Instead, thin provisioning lets administrators provision large thin or thin reclaim capable LUNs to a host. When the application writes data, the physical storage is allocated from the free pool on the array to the thin-provisioned LUNs.

The two types of thin provisioned LUNs are thin-capable or thin-reclaim capable. Both types of LUNs provide the capability to allocate storage as needed from the free pool. For example, storage is allocated when a file system creates or changes a file. However, this storage is not released to the free pool when files get deleted. Therefore, thin-provisioned LUNs can become 'thick' over time, as the file system starts to include unused free space where the data was deleted. Thin-reclaim capable LUNs address this problem with the ability to release the once-used storage to the pool of free storage. This operation is called thin storage reclamation.

The thin-reclaim capable LUNs do not perform the reclamation automatically. The server using the LUNs must initiate the reclamation. The administrator can initiate a reclamation manually, or with a scheduled reclamation operation.

Storage Foundation Cluster File System High Availability provides several features to support thin provisioning and thin reclamation, and to optimize storage use on thin provisioned arrays.