Capacity terminology changes

IBM® Storage is making the capacity terminology that is used in IBM Storage products consistent. The new capacity terminology is clearer and simpler across IBM Storage products, including IBM Spectrum Control.

The following table lists the original capacity terms and the new terms:

Original terms New terms
(No existing term) Overhead Capacity
Allocated SAN Space Used SAN Capacity
Allocated Space Used Capacity, only on Block Storage Systems, Volumes, and Pools pages
Assigned SAN Space Mapped SAN Capacity
Assigned Volume Space Mapped Capacity
Available Disk Space Available Drive Capacity
Available File System Space Available File System Capacity
Available Pool Space Available Capacity
Available Repository Space Available Repository Capacity
Available Soft Space Available Soft Capacity
Available Space Available Capacity
Disk Capacity Capacity
Disk Space from name_storage_system Drive Capacity from name_storage_system
Effective Available Capacity Available Written Capacity
Effective Capacity Written Capacity Limit
Effective Unallocated Volume Space Available Volume Capacity
External Pool Used Space External Pool Used Capacity
External Used Space External Used Capacity
Effective Used Capacity Used Written Capacity
Enterprise HDD Available Space Enterprise HDD Available Capacity
File System Space from name_storage_system File System Capacity from name_storage_system
Inactive Used Space Inactive Used Capacity
Nearline HDD Available Space Nearline HDD Available Capacity
Overhead Space (Term removed)
Physical Allocation Used Capacity, only on Block Storage Systems, Volumes, and Pools pages
Pool Capacity Capacity
Raw Disk Capacity Raw Capacity
Reserved Pool Space Reserved Capacity
Soft Space Soft Capacity
Space Quota Capacity Quota
Tier 0 Flash Available Space Tier 0 Flash Available Capacity
Tier 1 Flash Available Space Tier 1 Flash Available Capacity
Total Capacity Capacity
Total Data Reduction Savings Total Capacity Savings
Total Disk Capacity Capacity
Total Disk Space Drive Capacity
Total File System Capacity File System Capacity
Total Space Capacity
Total Volume Capacity Provisioned Capacity
Unallocatable Volume Space Overprovisioned Capacity
Unallocated Space Available Capacity
Unallocated Volume Space Available Volume Capacity
Unassigned Volume Space Unmapped Capacity
Unreserved Pool Space Unreserved Capacity
Unused Space Reserved Volume Capacity
Used Allocated Space (Term removed, use Used Capacity instead)
Used File System Space Used File System Capacity
Used Pool Space (Term removed, use Used Capacity instead)
Used Space (Term removed, use Used Capacity instead)
Virtual Allocation Provisioned Capacity
Volume Capacity Capacity
Written Space Written Capacity
Restriction: The capacity term changes do not apply to the following functions:
  • REST API, with the exception of the following attributes that were removed:
    • Used Space on the Block Storage Systems, Pools, and Volumes pages
    • Used Pool Space on the Storage Systems page
    • Used Allocated Space (%) on the Volumes page
  • Reports in the Cognos® Analytics reporting tool
  • Command-line interface

Descriptions of new capacity terms

available capacity
The amount of usable capacity that is not yet used in a system, pool, array, or managed disk.
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 capacity
The extra capacity when there is 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.
reserved volume capacity
The amount of pool capacity that is reserved but has not been used yet to store data on the thin-provisioned volume.
total capacity 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.
unused capacity
The difference between the provisioned capacity of the volume and the used capacity of the volume.
usable capacity
The amount of capacity that is available for storing data on a system, pool, array, or managed disk after formatting and RAID techniques are applied.
used capacity
The amount of provisioned capacity that is taken up by data after data reduction techniques have been applied.
The amount of usable capacity that is taken up by data in a system, pool, array, or managed disk after data reduction techniques have been applied.
written capacity
The amount of provisioned capacity that would have been used to store written data on the thin-provisioned volume if data reduction techniques were not applied.
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.
written capacity limit
The amount of provisioned capacity that can be created in a system or pool without running out of usable capacity given the current total capacity savings being achieved. This capacity equals the usable capacity divided by the total capacity 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.