VMware issues recall on new vSphere 5.0 UNMAP feature

Another failure for VMware!!!
  • When performing a Storage vMotion or a Virtual Machine Snapshot you experience poor system performance.
  • A Storage vMotion or Virtual Machine Snapshot fails or times out

VMware introduced a new feature in vSphere 5.0 called Space Reclamation, as part of VAAI Block Thin Provisioning. Space reclamation is a garbage collection process that helps storage partners to efficiently reclaim deleted space in coordination with vSphere 5.0.

ESXi 5.0 issues UNMAP commands for Space Reclamation in critical regions during several operations with the expectation that the operation would complete quickly. Due to varied response times from the storage devices, UNMAP command can result in poor performance of the system and should be disabled on the ESXi 5.0 host.

This article shows how to disable the UNMAP command used for the Space Reclamation.


VAAI Thin Provisioning is enabled by default on devices that adheres to T10 standards. ESXi identifies Thin Provisioned LUNs and issue UNMAP commands to reclaim deleted space on the storage. The implementation and response times for the UNMAP command may vary significantly among storage arrays.

This variation of response times in critical regions could potentially interfere with operations such as Storage vMotion and Virtual Machine Snapshot consolidation.


You can work around this issue on vSphere 5.0 hosts which have Thin Provisioned LUNs and T10 standard storage arrays.

Note: To verify that you have a T10 storage array, consult the VMware Compatibility Guide.

To avoid the use of UNMAP commands on Thin Provisioned LUNs:

  1. Log into your host using Tech Support mode. For more information on using Tech Support mode see Tech Support Mode in ESXi 4.1 and 5.0 (1017910).
  2. From your ESXi 5.0 host, issue this esxcli command:
    esxcli system settings advanced set --int-value 0 --option /VMFS3/EnableBlockDelete
    Note: This is a per-host setting and must be issued on each ESXi 5.0 host in your cluster.

Without disabling the UNMAP feature, you might experience timeouts with operations such as Storage vMotions and Virtual Machine Snapshot Consolidation.

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