Sometimes our Hyper-V guests were being backed up offline as opposed to online or backed up using saved state as opposed to using child partition snapshots. If the guest is not aware it’s being backed up then applications like Exchange don’t clear their logs, not good!
DISKSHADOW will tell you how a VM is going to be backed up and depending on what host level operating system you’re using, you’re looking for different things.
For Windows Server 2012 R2 run the following:
ECHO list writers >c:\script.txt diskshadow /s c:\script.txt | find /i "Caption: O"
This will list either offline or online
For Windows Server 2008/2012 run the following:
ECHO list writers >c:\script.txt diskshadow /s c:\script.txt | find /i "Caption: Backup Using"
This will list either “Backup Using Saved State” or “Backup Using Child Partition Snapshot”
A bit more research found that flipping this state was as simple as restarting the “Hyper-V Volume Shadow Copy Requestor” service within the guest
net stop vmicvss && net start vmicvss
or for the Powershell people