Hyper-V Backups Cause Large Registry Files

I saw this one in my tweet feed this morning.  It’s an important one for anyone doing host level backups of virtual machines on W2008 R2 Hyper-V host servers.  Ben Armstrong has explained how the registry file of the host will grow over time as you back up virtual machines.  This eventually leads to slow boot up times for the host.

Here’s how I understand the description:

  • A VSS backup at the host (W2008 R2) level backs up a VM
  • The backed up copy of each VHD is temporarily mounted by the parent partition for some cleanup work.
  • This mount causes a registry entry to be created.

Imagine doing this with a VSS enabled and optimized backup product like DPM 2010.   Maybe 15-20 VM’s, with an average of 2 VHD’s per VM, per host being backed up every 15 minutes.  That’s a lot of stuff going on.

Ben also warns that the fix, KB982210, is titled incorrectly.  It says Windows Server 2008 when it should say Windows Server 2008 R2.

By the way, this fix is thanks to the way that a number of MS people have embraced social media.  Ben asked people to contact him if they encountered issues with Hyper-V backups.  A number of people raised this issue, and we go this fix.

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