An administrator attempts to manually reclaim space on thin-provisioned VMFS6 volumes but finds that no additional space is being recovered. Which could be causing this?

vSphere 6.5 fully supports UNMAP on VMFS6 volumes - VMFS6 was introduced in vSphere 6.5 specifically to improve thin-provisioned space reclamation through automated UNMAP.

Manual UNMAP via esxcli was deprecated for VMFS6 but was not relocated to the web client; the capability was replaced by the automated background UNMAP mechanism.

VASA (vSphere APIs for Storage Awareness) is used for vVols integration and storage policy communication and plays no role in the VMFS6 background UNMAP reclamation process.

Unlike VMFS5, which required administrators to manually run esxcli UNMAP commands to reclaim thin-provisioned space from the storage array, VMFS6 (introduced in vSphere 6.5) performs UNMAP automatically in the background on an ongoing basis. When a manual UNMAP is attempted, no space is recovered because all eligible deleted blocks have already been reclaimed by the automated process.