Server 2008 VMs freeze during Hyper-V snapshot processing for backup

We have a Server 2008 R2 Hyper-V host running a mix of Server 2008 and Server 2008 R2 guests.  When backing up from the host using either Windows Backup or Symantec Backup Exec 2012, the non-R2 guests seem to 'hang' during snapshot processing - they're completely unavailable for several minutes, then resume as if nothing happened.  This does not seem to happen on the R2 guests.  All Hyper-V Integration Services are installed on the guests.
mtxitAsked:
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jpgobertConnect With a Mentor Enterprise IT Systems ConsultantCommented:
Ok... have a look at the following links... one or more of the requirements for online backups aren't met... I'd review and check each one to make sure.

Why Hyper-v guests enter into a save state mode during the backup

Backing up and restoring virtual machines - MSDN

Altaro - Backing up and restoring virtual machines in Hyper-V
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jpgobertEnterprise IT Systems ConsultantCommented:
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mtxitAuthor Commented:
Yes we'd already done that (and I've now double-checked).  Vssadmin list writers shows the Microsoft Hyper-V VSS Writer as stable with no error.
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jpgobertEnterprise IT Systems ConsultantCommented:
Are there any common events logged to the event logs in the VM's when this occurs?  Can you start a ping loop from your workstation to one of the VM's and see any packet loss when this "pause" occurs?

Just trying to determine if the OS is truly freezing or if you have something else at play here...
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mtxitAuthor Commented:
Nothing obvious in the event log, no.  I did some testing earlier with pings to and from the device - they drop for the entire period.  I also monitored the Hyper-V Management Console and observed that the affected VMs go to a state of 'Saving' and then 'Restoring' before coming back online, which suggests that for some reason VSS isn't being used to back these ones up?
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mtxitAuthor Commented:
The requirement for the VM to use basic disks instead of dynamic wasn't something we knew about, and both problem servers are using basic disks.  Since it'll take a while to get those converted I'm happy to assume that this is the solution and close the question off, thanks very much.
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