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What are the recommendations for the host stacks supporting SCSI UNMAP (TRIM) commands for space reclamation?

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Applies to

  • ONTAP 9
  • Clustered Data ONTAP 8


The UNMAP command is a SCSI command standardized within the T10 SCSI command set used to reclaim space from blocks that have been deleted by the host OS, an application, or a virtual machine. Thin provisioning technology broadly adopted in the current computer industry uses the UNMAP command to achieve improved storage utilization, flexible capacity planning, and non-stop storage provisioning service. The UNMAP command can be used as an inline or as an offline operation.

Support for the UNMAP command has been added for Data ONTAP 8.1.3 and later.

Host stacks support the UNMAP command to different degrees.

The following is a summary of general recommendations for various host stacks:


SCSI unmap is enabled by default.  This can be verified with the following command. 

> fsutil behavior query disabledeletenotify

DisableDeleteNotify = 0 means in-band UNMAP is enabled for the host.

DisableDeleteNotify = 1 means in-band UNMAP is disabled for the host.


ESX 5.0U1 disables the UNMAP command. It is recommended to use vmkfstools to reclaim space, refer to vmware kb for more 2007427
ESX 6.0 and above Reclaiming VMFS deleted blocks on Thin Provisioned LUNs, refer to vmware KB 2014849


RHEL uses discard operations to reclaim space through the UNMAP command. It supports both online discard (fine-grained discard using the –o discard mount option) and offline discard (batch discard using the fstrim utility).

Red Hat recommends using batch discard operations unless the system's workload is such that the batch discard is not feasible, or online discard operations are necessary to maintain performance. For more information, see the RHEL6, RHEL7 and RHEL8

Solaris does not support SCSI unmap for file system UFS. Since Solaris 11.1 (Oct. 2012) ZFS supports SCSI unmap. This has been backported to Solaris 10 with patch 150400-16.


These operating systems do not support the UNMAP command.


Symantec does not support the inline UNMAP command for Solaris/Linux and AIX. For more information on using the UNMAP command, see the section on Thin Reclamation in the Symantec Storage Foundation Administrator's Guide.

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