Home > Storage > PowerMax and VMAX > Storage Admin > Dell PowerMax and VMware vSphere Configuration Guide > Dead Space Reclamation—UNMAP
It is always possible to run manual UNMAP against VMware datastores whether automation is in use or not. VMware uses the esxcli command framework to unmap storage. The command is known as unmap, and is located under the esxcli storage vmfs namespace:
esxcli storage vmfs unmap
<--volume-label=<str>|--volume-uuid=<str>> [--reclaimunit=<long>]
Command options:
-n|--reclaim-unit=<long>: Number of VMFS blocks that should be unmapped per iteration.
-l|--volume-label=<str>: The label of the VMFS volume to unmap the free blocks.
-u|--volume-uuid=<str>: The uuid of the VMFS volume to unmap the free blocks.
The command has no output when run, as displayed in Figure 109.
When the user runs an unmap command, dead space is reclaimed in multiple iterations. A user can provide the number of blocks (the default is 200) to be reclaimed during each iteration. VMware issues UNMAP to all free blocks, even if those blocks have never been written to.
This command uses the UNMAP primitive to inform the array that blocks can be reclaimed. This method enables a correlation between what the array reports as free space on a thin-provisioned datastore and what vSphere reports as free space.
Dell Technologies recommends using the VMware default block size number (200), though fully supports any value VMware does. Dell has not found a significant difference in the time that it takes to unmap by increasing the block size number. If using manual UNMAP, Dell suggests reclaiming space in off-peak time periods to avoid any impact on production environments.