Thanks for the reply Richardson. I've looked at the links and those refer as to how to reclaim space from a existing thin provisioned VMDK disk. In our case, we deleted a VM and it's associated vmdk, but the datastore did not increased it's free space (double checked it was a "delete from disk" operation).
I've googled and seems like "vmkfstools -y" could do the trick, but I'm not sure.
It's a vmfs3 with 4.1 hosts
Thanks
