https://www.ibm.com/support/pages/scsi-unmap-support-spectrum-virtualize-systems

https://www-01.ibm.com/support/docview.wss?uid=ibm10717303

What is SCSI Unmap ?

• This enables hosts to notify the storage controller of capacity that is no longer required, which may improve capacity savings.

The Spectrum Virtualize system will advertise support for SCSI Unmap to hosts.

Some host types (for example, Windows and Linux) will then change their behaviour when creating a new filesystem on a volume, issuing SCSI Unmap commands to the whole capacity of the volume. This causes the system to overwrite the whole capacity with zero-data - and the format will complete when all of these writes have completed.

Some host types run a background process (for example fstrim on Linux), which periodically issues SCSI Unmap commands for regions of a filesystem that are no longer required.

Support introduced in SVC / V7000 V8.1.1.2 or later

If formatting time becomes a problem, or if host SCSI Unmap requests trigger performance issues, it may be desirable to disable SCSI Unmap support. There are three ways to do this.

Note: Before using either option 2 or option 3, it is recommended to upgrade to V8.1.1.2, V8.1.3.0 or a later release. Some host types do not tolerate concurrently disabling SCSI Unmap on earlier code levels, requiring a host reboot to complete the change.

1. Disable SCSI Unmap by reconfiguring the host.

Host operating systems have different mechanisms for disabling SCSI Unmap / TRIM commands on a volume. Refer to the host documentation for full details.

On Linux, mounting a filesystem with "-o nodiscard " will disable SCSI Unmap

2. Change the host type to generic_no_unmap

If the host type is generic, change it to generic_no_unmap by issuing the following CLI command:

svctask chhost -type generic_no_unmap <host_id_or_name>

To re-enable SCSI Unmap for the host, change the host type back to generic .

If the host type is NOT generic (for example, HPUX or adminlun/VVOLS), SCSI Unmap cannot be disabled for that individual host.

3. Disable SCSI Unmap for the whole Spectrum Virtualize system

This is not recommended if:

• the system is virtualizing a thin-provisioned storage controller (for example, IBM A9000 or IBM Flashsystem FS900-AE3), or
• Data Reduction Pools are being used on 8.1.2 or later software.

In these circumstances, disabling SCSI Unmap will increase the amount of physical capacity that is used, and make it more likely that the storage will run out of free capacity.

SCSI Unmap can be disabled using the CLI command:

svctask chsystem -backendunmap off

svctask chsystem -hostunmap off

To re-enable SCSI Unmap, use the

svctask chsystem -backendunmap on

svctask chsystem -hostunmap on