A performance issue has recently been discovered in the VAAI Thin Provisioning Block Reclaim mechanism introduced in vSphere 5.0. This feature allows an ESXi host to tell the array that a block on a Thin Provisioned datastore is no longer needed and can be reclaimed. This could be the result of a file delete operation, a Storage vMotion, a Snapshot Consolidate operation, etc. I blogged about the reclaim feature some time ago.
What we have found is that the time taken to complete the Block Reclaim operation could perform poorly.
Unfortunately, in light of this performance issue, VMware are advising that the UNMAP feature be disabled in ESXi 5.0 until further notice.
A KnowledgeBase article, 2007427, has now been published and contains an additional description of the issue and details on how to disable the UNMAP feature. Click here to read the KB.