Clustered Data ONTAP 8
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:
NTFS in Server 2012 and later will support the
UNMAP command. To enable the inline
UNMAP command, use HUK 6.0.2 or ONTAPDSM 4.0 or later
ESX 5.0U1 disables the
UNMAP command. It is recommended to use vmkfstools to reclaim space.
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 Storage Admin Guide
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.
AIX and HPUX:
These operating systems do not support the
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.