As stated in a previous blog post, Hyper-V Backup Solution, I stated that the time taken for subsequent backups was comparable to the initial backup.
Well, I think I have found a solution to get it to do the fast differential backups that transfer only changed blocks to the backup device!
On your server, run Windows Explorer, and for each volume that you’ll be backing up, right click on the volume and select “Configure shadow copies”, and click “Enable”. If you do this for each volume, this turns on automatic shadow copy creation. This also seems to have the effect of enabling persistent shadow copies, which enables Windows Server Backup to perform the fast differentials.
On our test server, where we have approx. 232GB of data on our Server 2008 / Hyper-V Role machine, backups were taking around 57 minutes before the change (see below). However, after the change, the backup time was slashed to around 3 minutes!
We’re going to continue testing to establish exactly what was causing the slowdown in the first place, as it seems to be specific to Hyper-V. But in the meantime, everyone can now enjoy fast differential backups of Hyper-V servers!