Home > Storage > PowerMax and VMAX > Storage Admin > Dell PowerMax and VMware vSphere Configuration Guide > Dead Space Reclamation—UNMAP
Known as either Dead Space Reclamation by VMware or UNMAP, the SCSI UNMAP command (0x42) issues requests to deallocate extents on a thin device. The purpose behind UNMAP is the reclamation of space on the array. UNMAP can be used in two different scenarios. The first is to free space in a VMFS datastore - manually or automatically. The second is to free space in a thin VMDK that is part of a Guest OS in a VM. This method is known as Guest OS, or in-guest UNMAP. It supports both VMFS and vVol datastores.
As virtual machines are created and grow, then are moved or deleted, space is stranded in the thin pool. The array does not know that the blocks where the data once resided are no longer needed. Before the integration of this new Storage API in vSphere, reclamation of space required two steps:
With UNMAP support, vSphere tells the PowerMax what blocks can be unmapped or reclaimed. If those blocks span a complete extent or extents, they are deallocated and returned to the thin pool for reuse. If the range covers only some tracks in an extent, those tracks are marked as available on the PowerMax but the extent cannot be deallocated. This method is still beneficial, as those tracks do not have to be retrieved from the disks if a read request is performed. Instead, the PowerMax array immediately returns all zeros.
VMware reuses existing space in the datastore even if the data has not been reclaimed on the array. However, it may not be able to use all the space for various reasons. In addition, by reclaiming the space in the thin pool it becomes available to any device in the pool, not just the device backing the datastore. Using the SCSI command UNMAP, issued manually or automatically, is the only way to guarantee that the full amount of space is reclaimed for the datastore.
Guest OS or in-guest UNMAP supports both Windows and Linux VMs. There are several prerequisites that must be met to take advantage of Guest OS UNMAP. There are some differences between Windows and Linux that should be noted. They are covered in the next two sections.