When you revert to a snapshot a USN rollback occurs. USNs are used to determine what changes need to be replicated to other DCs. USNs are unique to each DC, and use a monotonically increasing value. A USN is an Active Directory database instance counter which gets incremented each time an update to AD is made. Why is rolling back AD bad? I mean why is rolling back AD *REALLY* bad? Microsoft has these little things called USNs, or Update Sequence Numbers. But, there is at least one class of VMs that you almost NEVER want to roll back from a snapshot with, those which are vector-clock synchronized software such as Active Directory. I know during all my (failed) vSphere 5.1 installs I practically wore out the revert to snapshot button in vCenter. Invaluable feature in both the lab, and in production. Windows Server 2012 generation ID is a great new feature that will allow use to safely virtualize a domain controller, on specific hypervisors. One of the really great features that hypervisors have had for ages is the ability to perform snapshots, then roll back to a prior state with a click of a mouse.
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