Storage terminology

This section provides a brief guide to some of the terminology used in storage.

Provisioning

Provisioning refers to the availability and allocation of the physical storage (or blocks on disk).

Traditional provisioning—Physical storage is allocated and available to consuming hosts at the time the storage is set up. Also called fat or thick provisioning.
Thin provisioning—Physical storage is allocated on demand. Unused storage is available but it has not been allocated. Thin provisioning is designed to avoid the hardware and energy costs incurred where preallocated storage remains unused. Enables storage to be over-subscribed.

Size and capacity refer to the amount of data that a storage entity can store.

Size—The underlying size of the device, the number of bytes of data that can fit on the device.
Capacity—The amount that can actually be used.
For example, if a storage entity has two 1 TB disks in a RAID 1 pair, the size of the storage is 2 TB, but the capacity is 1 TB.

Visible capacity—The amount of storage that is available to the consuming host.
Consumed capacity—The amount of capacity that the consuming host has used.
- With traditional provisioning, the consumed capacity equals the visible capacity. The physical storage was allocated before it is used, at the time the storage was set up.
- For thin provisioning, this is less than the visible capacity. The consuming host can see the full amount of storage available to them even though some of it is not allocated.

Capacity—The total amount of storage that is in the pool.
Consumed capacity—The amount of capacity that has been used by the volumes and subsidiary pools allocated from this pool.
Available capacity—The unused storage in the pool. The capacity minus the consumed capacity.

Subscribed capacity—The amount of capacity that can be used by volumes and subsidiary pools allocated from this pool. With thin provisioning, the subscribed capacity can be greater than the pool capacity. This is described as over-subscribed.
Over-subscribed capacity—The subscribed capacity minus the capacity.
Over-subscription warning—A warning flag set if the consumed capacity of an over-subscribed storage entity reaches 75%.

The following diagram shows a graphical representation of subscribed capacity, over-subscribed capacity, and the over-subscription warning.