Reclaim “white space” – HP Lefthand SAN

Post from TeleData:

This is always one of the challenges (and limitations) to thin provisioning.

The technology used to provide thin provisioning in SAN/iQ is really more of a “high water mark”.Once a block has been marked as “written” you cannot recover unused space by deleting data from the volume. There is no communication facility in which the OS would “tell” the SAN that,
“Hey that block of data we were using yesterday, is now empty and you can have it back.”Since there is no way to “tell” the SAN the space is empty, those blocks of data, once written to cannot be reclaimed.

Your only option is to create a NEW volume, and migrate the data to the new volume, and then delete the old volume.

This can be challenging with direct native iSCSI mounted volumes, but if you are using a virtual machine (with virtual disks) you can reclaim storage by creating a new VMFS datastore, using sdelete to zero out unused space (within the Windows OS), then performing a storage migration and choosing “thin” provisioning on the virtual disk.

While still requiring a new (VMFS) volume, the virtualized disk can be left intact avoiding any reconfiguration within the Windows server itself.

The result would NOT be different if you had chosen thick vs thin. The blocks are still marked as used and a “high water mark” is still maintained. The only difference is when you mark it “thick” SAN/iQ reserves the entire space, and it cannot be used to provision to other volumes/snapshots.

This is why you can dynamically switch between thin and thick provisioning within the CMC.

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