text: 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. 