My understanding in a storage vmotion is that VMWare first copies the entire VM, verifies it's 100% good, and then deletes the source. So, it is possible you will need 2x the space to do the move (at least temporarily). If the VM has multiple vdisks then you might try doing an advanced storage vmotion and migrating the individual VMDK's one at a time. Before doing so, look at the available pool space, in the MSA, and confirm that the pool has enough free space for the individual VMDK you are moving each time. If both datastores are in the same pool, you might see a temporary reduction in free pool space during the copy but it should equal back out again after the move is complete.
Perhaps others, monitoring the thread, might have other suggestions but that's what I'd try.
Perhaps others, monitoring the thread, might have other suggestions but that's what I'd try.
