OK, thanks !
I did it for one machine just now NNMi-Test which was having 26GB snapshot file, it took 25 min to complete it, it was sitting at 95% giving me tension, but it worked finally.
Now for the one with 128 GB how long it will take I am thinking may be hours ?
by the way what is a good practice for snapshots, should be deleted after how much time , any threshold ? Because from my reading so far I understand in long run they are not good for performance of a virtual machine.
Many thanks for your support Mr. Andr'e