Deciding how far to over-provision storage

Storage Subsystem Administration Guide for Hitachi NAS Platform

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When using HDP, you should make a rough forecast of how much data storage capacity will be needed in the next 12 to 24 months, and then configure your DP-Vols to be larger than your estimate. If you overestimated your data storage requirements, not too much space will have been wasted.

The total capacity of the DP-Vols should exceed the total capacity of the parity groups or pool volumes by a factor of 2:1 or 3:1, depending on how far you expect the storage pool to expand. The total capacity of the DP-Vols created when the storage pool was initially set up does not constrain the eventual size of the storage pool.

For example, if you have 20 TiB of storage and the storage pool may need to expand to 50 TiB later on, you should set up 50 TiB of DP-Vols. If you ever need to grow the storage pool beyond 50 TiB, you can always add a second stripeset using the span-expand command, then continue to expand the DP pool in increments as small as required.

Limits on thin provisioning:
  • You can make the storage pool capacity larger than the total capacity of the DP-Vols that you created at the outset by adding more DP-Vols later.
  • Some storage systems and systems do not over-commit by more than a factor of ten to one.
  • For HDP, the storage requires an amount of memory that is proportional to the capacity of the large, virtual DP-Vols, rather than to the smaller, real parity groups or pool volumes. Therefore, consider the following:
    • Massive over-commitment causes storage to run out of memory prematurely.
    • Enterprise storage uses separate boards called shared memory.