Good Post by Cormac Hogan (thank you)
I was involved in some conversations recently on how the VAAI UNMAP command behaved, and what were the characteristics which affected its performance. For those of you who do not know, UNMAP is our mechanism for reclaiming dead or stranded space from thinly provisioned VMFS volumes. Prior to this capability, the ESXi host had no way of informing the storage array that the space that was being previously consumed by a particular VM or file is no longer in use. This meant that the array thought that more space was being consumed than was actually the case. UNMAP, part of the vSphere APIs for Array Integration, enables administrators to overcome tho challenge by telling the array that these blocks on a thin provisioned volume are no longer in use and that they can be reclaimed.
Read on here