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7 Dell PS Series Arrays: Advanced Storage Features in VMware vSphere | TR1066
the volume, migrate virtual machines off the volume, or delete virtual machines from the volume. Once
space on the datastore has been made available, the stunned virtual machine can be un-stunned, and it
will continue operating again under normal conditions.
Note: VAAI thin provisioned stun requires Dell PS Series firmware 5.1 or above.
2.5 Thin provisioning space reclamation
The thin provisioning space reclamation primitive enables thin provisioned datastores to be manually re-
thinned to only consume the actual space they are using on a PS Series array—allowing thin provisioned
volumes to remain thin. Traditionally, the size of a thin provisioned volume, as shown at the storage layer,
reflects the maximum space consumption that occurred at some point since it was created. This is
because the operating system could not inform the array that particular blocks of data were no longer
required by the filesystem and were no longer needed to be stored by the array.
When the T10 SCSI committee added the unmap specification standard to the SCSI Block Commands 3
(SBC3), this allowed operating systems to communicate to the array, through the SCSI storage stack, that
blocks of data no longer needed to be stored. This unmap ability is referred to as thin provisioning space
reclamation by VMware.
With vSphere 5.0 update 1 and above, thin provisioning space reclamation has become available to
vSphere administrators as a command-line tool, enabling them to re-thin their thin provisioned datastores
so that they are only consuming the space that they require. This can improve efficiency of storage
capacity and lower the total cost of ownership of PS Series arrays.
Note: VMware recommends limiting thin provisioning space reclamation operations to maintenance
windows, as this can be a storage-intensive task on some arrays. Dell PS Series firmware manages unmap
operations as additional I/O, so unmap operations should be avoided during periods of peak I/O.
2.5.1 Space reclamation process on vSphere 5.0u1 and 5.1
To re-thin a thin provisioned datastore and reduce the amount of disk space consumed on an array, a
vSphere administrator must run the following command from the ESXi CLI from within the particular
datastore:
vmkfstools –y <percentage of deleted blocks to be reclaimed>
The percentage of deleted blocks to be reclaimed is a percentage of the free space of the datastore, and
not a percentage of the total size of the datastore. VMware suggests, but does not enforce, that a
maximum value of 60 percent be used. During the space reclamation operation, a temporary balloon file is
created using available storage on the datastore. If a large value such as 100 percent is specified, the
balloon file would consume all available space. This would prevent virtual machines located on that
datastore from powering on because the datastore would be unable to create its vswap file and a virtual
machine would not be able to grow a thin provisioned VMDK.