Search the VMware Knowledge Base (KB)
View by Article ID

Disabling VAAI Thin Provisioning Block Space Reclamation (UNMAP) in ESXi 5.0 (2007427)

  • 22 Ratings


  • Performing a Storage vMotion or a Virtual Machine Snapshot results in poor system performance.
  • A Storage vMotion or Virtual Machine Snapshot fails or times out.
  • In the /var/log/vmkernel.log file, you see entries similar to:

    2011-12-07T08:55:04.792Z cpu40:8232)ScsiDeviceIO: 2305: Cmd(0x41248092e240) 0x42, CmdSN 0x13bb23 to dev "naa.60000970000292602427533030304536" failed H:0x0 D:0x2 P:0x0 Valid sense data: 0xb 0x44 0x0.
    2011-12-07T08:55:04.932Z cpu40:8232)NMP: nmp_ThrottleLogForDevice:2318: Cmd 0x42 (0x41248092e240) to dev "naa.60000970000292602427533030304536" on path "vmhba2:C0:T0:L3" Failed: H:0x0 D:0x2 P:0x0 Valid sense data: 0xb 0x44 0x0.Act:NONE


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.


This issue is resolved in ESXi 5.0 Patch 02. For more information, see VMware ESXi 5.0 Patch Image Profile ESXi-5.0.0-20111204001-standard (2009330).

To download the ESXi 5.0 Patch 02, see the VMware Download Center.

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 in to your host using the 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, run this command:

    esxcli system settings advanced set --int-value 0 --option /VMFS3/EnableBlockDelete

  3. To verify this setting, run this command :

    esxcli system settings advanced list --option /VMFS3/EnableBlockDelete
       Path: /VMFS3/EnableBlockDelete
       Type: integer
       Int Value: 0  <<<<<<<<<< 0 means Disabled
       Default Int Value: 1
       Min Value: 0
       Max Value: 1
       String Value:
       Default String Value:
       Valid Characters:
       Description: Enable VMFS block delete



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

See Also

Update History

01/10/2012 - Added error messages in the Symptoms section 02/10/2012 - Added information that this issue is resolved in ESXi 5.0 Patch 02

Request a Product Feature

To request a new product feature or to provide feedback on a VMware product, please visit the Request a Product Feature page.


  • 22 Ratings

Did this article help you?
This article resolved my issue.
This article did not resolve my issue.
This article helped but additional information was required to resolve my issue.

What can we do to improve this information? (4000 or fewer characters)

Please enter the Captcha code before clicking Submit.
  • 22 Ratings