Using compression statistics

It is important to monitor compression-based storage to avoid running out of physical space.

The following commands display and control statistics for compression-based storage:

• span-list
• span-dump
• sd-list
• span-set-cap-warn-thresh

The span-list -s, span-dump and sd-list --scsi commands all display the following statistics for compressed storage:

• The amount of physical space that isn't in use

• The total amount of physical space

• The percentage saving by which compression has reduced the amount of physical space required to store data on the span

For example:

Compressed: 200GiB (0TiB) (3%) of 6843GiB (7TiB) is free; saving is 95% 

Using the span-list command

For all storage types except UVM, the span-list -s command also displays an 'available' figure, which is a conservative estimate of the amount of space that could be allocated from the stripeset.

For example:

Set 0: 4 x 2430GiB = 9720GiB, of which 9119GiB is free, 299GiB is vacated, 900GiB is available

Using the sd-list command

The sd-list command also contains the following options for compressed storage:

• --compression
• --is-compressed
• --is-uncompressed

The --compression parameter reports the compression information in tabular form and can be combined with --hdp switch.

To view compressed or uncompressed SDs only, use the --is-compressed and --is-uncompressed switches (-e and -E), respectively.

To see which SDs are most urgently in need of new physical storage, a sort criterion is available:

sd-list --compression --sort e

This command sorts SDs by the percentage of physical space that is still free. Users can reverse the sort order by using an upper case E , and can combine this sort criterion with others.

Using the span-set-cap-warn-thresh command

Use the span-set-cap-warn-thresh command to set the capacity warning threshold for spans and compressed HDP pools.

This threshold controls:

• The point at which span-list, trouble and the event log warn about low physical space.
• The safety margin that span-list -s uses when calculating the available free space on a stripeset.
• The point at which the server takes precautionary measures such as disabling filesystem auto-expansion and unmapping vacated chunks when physical space is low.

Parent Topic

• Storage Subsystem Administration Guide

  This guide provides information about managing the supported storage subsystems attached to the server/cluster. The guide includes information about tiered storage, storage pools, system drives (SDs), using Hitachi Dynamic Provisioning (HDP) with the server, and other storage-related configuration and management features and functions.

Child Topics

• Low physical space

  If the physical space on an HDP pool is exhausted, the HDP pool will fail, along with every span that uses it. Therefore, when physical space is low, the server automatically disables filesystem auto-expansion in order to help keep the system serving data.