418 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
When a host allocates storage, the data is placed in a volume. To free the allocated space 
back to the storage pools, human intervention is usually needed on the storage controller. 
The SCSI Unmap feature is used to allow host operating systems to unprovision storage on 
the storage controller, which means that the resources can automatically be freed up in the 
storage pools and used for other purposes.
A SCSI unmappable volume is a volume that can have storage unprovision and space 
reclamation being triggered by the host operating system. IBM Storwize V7000 can pass the 
SCSI unmap command through to back-end storage controllers that support the function.
10.3.2  Back-end SCSI Unmap
The system can generate and send SCSI Unmap commands to specific back-end storage 
controllers.
This occurs when volumes are deleted, extents are migrated, or an Unmap command is 
received from the host. SCSI Unmap commands at the time of this writing are only sent to 
IBM A9000, IBM FlashSystem FS900 AE3, IBM FlashSystem FS9100, IBM Storwize, and 
Pure storage systems.
This helps prevent a thin-provisioning storage controller from running out of free capacity for 
write I/O requests. This means when you are using supported thin provisioned back-end 
storage, SCSI Unmap should normally be left enabled.
This feature is turned on by default. It is recommended to keep back-end unmap enabled, 
especially if a system is virtualizing a thin-provisioned storage controller or uses Flash Core 
Modules.
To verify that sending unmap commands to back-end is enabled, use the CLI command 
lssystem, as shown in Example 10-7.
Example 10-7   Verifying back-end unmap support status
IBM_Storwize:ITSOV7K:superuser>lssystem | grep backend_unmap
backend_unmap on
10.3.3  Host SCSI Unmap
The Spectrum Virtualize system might advertise support for SCSI Unmap to hosts. With it, 
some host types (for example, Windows, Linux, or VMware) then changes their behavior 
when creating a file system 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 completes when all of these writes complete. Some host types run 
a background process (for example fstrim on Linux), which periodically issues SCSI Unmap 
commands for regions of a file system that are no longer required.
Host Unmap commands can increase the free capacity that is reported by the Data Reduction 
Pool when received by thin-provisioned or compressed volumes. This does not apply to 
standard storage pools. 
Also, IBM Storwize V7000 sends back-end SCSI Unmap commands to controllers that 
support them if host unmaps for corresponding blocks are received.
Host SCSI Unmap commands drive more I/O workload to back-end storage. In some 
circumstances (for example, volumes on a heavily loaded nearline SAS array), this can cause