Appendix C. Terminology 771
Capacity
IBM applies the following definitions to capacity:
/SM590000Raw capacity
The reported capacity of the drives in the system before formatting or RAID
/SM590000Usable capacity
The amount of capacity after formatting and RAID available for storing data on a system, 
pool, array, or MDisk. Usable capacity is the total of used and available capacity. For 
example, 50 TiB used, 50 TiB available is a usable capacity of 100 TiB.
/SM590000Used capacity
The amount of usable capacity taken up by data in a system, pool, array, or MDisk after 
data reduction techniques were applied.
/SM590000Available capacity
The amount of usable capacity that is not yet used in a system, pool, array, or MDisk. 
/SM590000Effective capacity
The amount of provisioned capacity that can be created in the system or pool without 
running out of usable capacity given the current data reduction savings being achieved. 
This capacity equals the physical capacity divided by the data reduction savings 
percentage.
/SM590000Provisioned capacity
Total capacity of all volumes in a pool or system.
/SM590000Written capacity
The amount of usable capacity that would be used to store written data in a pool or system 
before data reduction is applied.
/SM590000Overhead capacity
The amount of usable capacity occupied by metadata in a pool or system and other data 
used for system operation.
/SM590000Total capacity savings
The total amount of usable capacity saved in a pool, system, or volume through 
thin-provisioning and data reduction techniques. This capacity saved is the difference 
between the used usable capacity and the provisioned capacity.
/SM590000Data reduction
The techniques used to reduce the size of data including deduplication and compression.
/SM590000Data reduction savings
The total amount of usable capacity saved in a pool, system, or volume through the 
application of a compression or deduplication algorithm on the written data. This capacity 
saved is the difference between the written capacity and the used capacity
/SM590000Thin provisioning savings
The total amount of usable capacity saved in a pool, system, or volume by using usable 
capacity when needed as a result of write operations. The capacity saved is the difference 
between the provisioned capacity minus the written capacity.
/SM590000Over provisioned
A storage system or pool where there is more provisioned capacity than there is usable 
capacity.