Using HDP to expand a span

Storage Subsystem Administration Guide for Hitachi NAS Platform

Part Number

Using HDP to add space provides additional benefits.

These benefits include:
  • You can add disks in small increments, even just a single pool volume.
  • Data gets restriped.
  • Span gets faster and performance remains almost even.

Consider the following use case for using HDP to expand a span:

If you originally created a pool containing 10 TiB of real storage and eight DP-Vols of 2.5 TiB each, totalling 20 TiB, the pool is over-committed by a ratio of 2:1. As always, a storage pool (span on the CLI) resides on the DP-Vols. As time goes by, you make a series of expansions of 4 TiB each by adding new parity groups or pool volumes. The first expansion increases the amount of real storage in the pool to 14 TiB and the second expansion takes it to 18 TiB.

After each of these expansions, no further action is necessary. However, after a third 4 TiB expansion, the pool contains 22 TiB of real storage, but its DP-Vols total only 20 TiB. As a result, 2 TiB of the disk space that you have installed are inaccessible to the server.

More DP-Vols are needed, but any expansion should always add at least as many DP-Vols as were provided when the span was created. You must therefore create a further eight DP-Vols, preferably of the same 2.5 TiB as the original ones, and add them to the storage pool by using the span-expand command or NAS Manager equivalent. This addition brings the total DP-Vol capacity to 40 TiB. No further DP-Vols will be necessary unless the real disk space in the pool is expanded beyond 40 TiB.