Using the unmapper to move space between storage pools

Storage Subsystem Administration Guide for Hitachi NAS Platform

Part Number

If, after considering the consequences associated with use of the unmapper, you decide it is worth the significant performance impact, you can use the following steps to move space between storage pools.

Unmapping chunks (using the unmapper) does not increase the amount of free space in a storage pool; instead, the unmapper:

  1. Takes chunks out of the VC list and returns the underlying HDP pages to the DP pool.
  2. Removes the capacity from the chunks listed in the vacated chunks list by unmapping the space.
  3. Zero-initializes the space.
  4. Returns the freed capacity to the underlying DP pool.

After being added to the DP pool, the freed space can be used by other storage pools that reside on the same DP pool.

Note: See the CLI man pages, or the Command Line Reference for detailed information about the commands mentioned below.

The following procedure describes how to move space from storage pool S to storage pool T. This procedure is based on a configuration where both storage pools are based on DP-Vols from the same DP pool, and that DP pool is thinly provisioned.

  1. Delete and recycle a file system from storage pool S (Span S).
  2. Run span-unmap-vacated-chunks on storage pool S.
  3. Run the span-list --sds T command, and look at the amount of free space in the DP pool.
    When the output from the span-list --sds T command shows that the DP pool has enough free space, create a new file system in storage pool T and/or expand one or more file systems in storage pool T.
    Note: It may take a significant amount of time for the zero-initialization process inside the storage to complete and the amount of space free in the DP pool to increase. The amount of time it takes for this process is dependent on the type of storage, the amount of space being initialized, and utilization of the storage system.
    If it takes too long to add the freed space to the DP pool, expand storage pool T onto a different DP pool (one that has available space).