Important unmapper considerations

Storage Subsystem Administration Guide for Hitachi NAS Platform

Part Number

Although not recommended, if multiple storage pools (spans) exist on a single pool, use the unmapper feature to move space between the storage pools on that pool.

Important: Using the unmapper commands can have serious consequences. It is strongly recommended that you read the CLI man page for each command.
  • Unmapping vacated chunks does free space, but the performance of the storage is reduced until the server has zero-initialized all the space that you unmap. Never unmap chunks just to affect the appearance of available storage.
  • You can unmap space on any number of spans at one time.
  • The server has no commands for monitoring or managing the HDP zero-init process. Once the process starts, you must wait until it completes. The time can exceed many hours, even weeks in some cases.
Further reasons to avoid using the unmapper:
  • In most storage configurations, an HDP page cannot be reused immediately after being unmapped. For security reasons, the page must first be zero-initialized to overwrite the previous page with zeros. This process occurs inside the storage, and cannot be monitored or managed by commands on the server.
The unmapper feature uses the following commands:
  • span-vacated-chunks displays the number of vacated chunks in a storage pool and the progress of any unmapper.
  • span-stop-unmapping cancels an unmapper without losing completed work.
  • span-throttle-unmapping helps you avoid long queues of pages waiting to be zero-initialized.