I have found this article which relates to what I am seeing:
VMware KB: Thin Provisioning Block Space Reclamation (VAAI UNMAP) does not work
When I check the VMware compatibility matrix it is showing as VAAI thin provisioning space reclamation is not supported. I am using a FAS8020 array, I would have thought that this SAN considering the age would support this feature? or is it an issue elsewhere with the environment? Looking at whether I upgrade to a newer version of ONTAP or VMware ESXi it doesn't make any difference.
Is the Thin Provisioning space reclamation only a feature on very high end SANs?
Is there anything I can use to reclaim space?
Thanks