Hotfix: Hyper-V backup can cause slow system boot / large registry files

A while ago I made an open offer to people to contact me if they had experienced issues using Hyper-V backup.  A number of you contacted me with reports of large registry files in the parent partition – that would slow down system boot over time.

Well, after a lot of investigation we have figured out the cause and a hotfix is now available.  You can download it from here:

I recommend that if you are using backup regularly on Hyper-V that you install this hotfix.

Note – that despite the title of the KB article, this fix applies to Windows Server 2008 R2 (not Windows Server 2008).

So what was the cause of the problem?

A short description is as follows:

Whenever we backup a virtual machine using VSS, we momentarily connect the backup copy of the virtual machines virtual hard disks to the parent partition in order to clean them up after backup.  Unfortunately with Windows Server 2008 R2 a new plug-and-play entry would get created in the Windows registry each time that we did this.  Over time this would cause the registry to get larger and larger, which would in turn slow down the Windows boot process.