The most common use case, for example, is a host application, such as VMware freeing  storage within a file system. The storage controller can then optimize the space, such as  reorganizing the data on the volume so that space is better used.  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 SAN Volume Controller can  pass the SCSI unmap command through to back-end storage controllers that support the  function. The system can generate and send SCSI Unmap commands to specific back-end storage  controllers. This process 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  sent only to IBM A9000, IBM FlashSystem FS900 AE3, IBM FlashSystem FS9100, IBM  Storwize, and Pure storage systems. This feature helps prevent a thin-provisioning storage controller from running out of free  capacity for write I/O requests. Therefore, when you are using supported thin-provisioned  back-end storage, SCSI Unmap normally is left enabled. By default, this feature is turned on, and it is recommended to keep back-end unmap enabled,  especially if a system is virtualizing a thin-provisioned storage controller or is using 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. The Spectrum Virtualize system can advertise support for SCSI Unmap to hosts. With it,  some host types (for example, Windows, Linux, or VMware) then change their behavior when  creating a file system on a volume and issuing SCSI Unmap commands to the whole capacity  of the volume. This process causes the system to overwrite the whole capacity with  zero-data, and the format completes when all of these writes completed. 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 reported by the DRP, when received  by thin-provisioned or compressed volumes. This does not apply to standard storage pools.  Also, IBM SAN Volume Controller sends back-end SCSI Unmap commands to controllers that  support them if host unmaps for corresponding blocks are received. 442  Implementing the IBM System Storage SAN Volume Controller with IBM Spectrum Virtualize V8.2.1 10.3.3 Host SCSI Unmap Example 10-7 Verifying back-end unmap support status IBM_2145:ITSO-SV1:superuser>lssystem | grep backend_unmap backend_unmap on 10.3.2 Back-end SCSI Unmap