Hello,
Welcome to the community.
Prior to vSphere 5 and UNMAP when an operation occurred that deleted VM data from a VMFS volume, vSphere just looked up the inode on the VMFS file system and deleted the inode pointer which mapped to the blocks on disk. This shows the space as free on the file system but the disk blocks that were in use on the storage array are not touched and the array is not aware that they contain deleted data.
In vSphere 5 with UNMAP when those operations occur the process is more elaborate from a storage integration point of view, the inodes are still looked up like before, but instead of just removing the inode pointers a list of the logical block addresses (LBA) must be obtained.
The LBA list contains the locations of all the disk blocks on the storage array that the inode pointers map to. Once vSphere has the LBA list it can start sending SCSI UNMAP commands for each range of disk blocks to free up the space on the storage array. Once the array acknowledges the UNMAP, the process repeats as it loops through the whole range of LBA’s to UNMAP.
More details on this are available http://vsphere-land.com/news/all-about-the-unmap-command-in-vsphere.html
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Rgds
Kanishk