Comment on May 2015 CU for SharePoint 2013 has been released by Stefan Goßner

Hi Conway,
the only snapshots we support are cold snapshots. Means all machines in the farm have to be in a shutdown state. Then take a snapshot of the machines. I would also recommend to take the DB backup while the sharepoint machines are shutdown to ensure 100% consistency between DB and SharePoint servers.
Cheers,
Stefan