Cannot unmount volume 'Datastore Name: vm3:xxx VMFS uuid: 517c9950-10f30962-931f-00304830a1ea' because file system is busy. Correct the problem and retry the operation.There is KB about this symptom. VSAN component VSANTRACE was using datastore. That was the reason of busy file system. It was pretty annoying issue as VSAN was not used nor enabled.
The solution is to disable vsantraced service so it is necessary to issue following command on evey ESX ...
chkconfig vsantraced off
Not so nice, right? That's the downside of fully integrated VSAN software into general ESX hypervisor. I'm not happy with this approach. In my opinion, it would be much better distribute VSAN as additional software installing as regular VIB (VMware Installable Bundle).