WWR2 : About “capacity” and other key concepts : Capacity terminology
Capacity terminology
The following definitions for capacity concepts compliment the general definitions in this section: “General storage terminology used in Storage Modeller”.
Table 7 Capacity definitions used in Storage Modeller
Capacity-related term
Definition
Available (usable) capacity
The amount of usable capacity that is not yet used in a system, pool, array, or MDisk.
Data reduction
A set of techniques that can be used to reduce the amount of usable capacity that is required to store data. Examples of data reduction include data deduplication and compression.
Data reduction savings
The total amount of usable capacity that is saved in a pool, system, or volume through the application of a compression or deduplication algorithm on the written data. This saved capacity is the difference between the written capacity and the used capacity.
Effective capacity
Capacity of an array/pool/system after calculations for data reductions and additional overhead.
The amount of provisioned capacity that can be created in a system or pool without running out of usable capacity given the current data reduction savings being achieved. This capacity equals the usable capacity divided by the data reduction savings percentage. In some storage systems, restrictions in the system determine the maximum provisioned capacity allowed in a pool or system. In those cases, the effective capacity cannot exceed this limit. See Figure 76 and Figure 77.
Overhead capacity
The amount of usable capacity that is occupied by metadata in a pool or system and other data that is used for system operation.
Overprovisioned ratio
The ratio of provisioned capacity to usable capacity in a pool or system.
Overprovisioning
The result of creating more provisioned capacity in a storage system or pool than there is usable capacity. This is done when thin-provisioning or compression ensure that the used capacity of the provisioned volumes is less than their provisioned capacity
Provisioned capacity
The total capacity of all volumes in a pool or system.
Raw capacity
The reported capacity of the drives in the system before formatting or RAID is applied. See Figure 76 and Figure 77.
Thin-provisioning savings
The total amount of usable capacity that is 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.
Total capacity savings
The total amount of usable capacity that is saved in a pool, system, or volume through thin-provisioning and data reduction techniques. This saved capacity is the difference between the used usable capacity and the provisioned capacity.
Usable Capacity
Capacity of an array after RAID calculations (parity, redundancy, rebuild areas); capacity of an MDisk after extent calculations; capacity of a pool or system after pool calculations (reserved extents). See Figure 76 and Figure 77.
Used (usable) Capacity
The amount of usable capacity that is taken up by data in a system, pool, array, or MDisk after data reduction techniques have been applied.
Written capacity
The amount of usable capacity that would have been used to store written data in a pool or system if data reduction was not applied.
Figure 75 provides an example on how the different definitions relate to each other using a layered point of view.
Figure 75 A subset of IBM storage terminology
The exact definitions as used in Figure 75 are stated in Figure 76 on page 79.
Figure 76 Capacity calculation in Storage Modeller