We use HyperV a lot at work, and for small scale Windows Server platform deployments I actually quite like it as our chosen virtualisation tech. However, backing up any virtual machine, regardless of platform, can be “fun” sometimes. We actually use a script that I put together using diskshadow (VSS) and a set of batch scripts, which works really well. However I hadn't actually really put much thought into what was happening and although I've done test restores for Windows machine I'm yet to do one for a *ix box.
How exactly does the virtual machine get to a state where it knows that it's being backed up? After all the last thing you want is a restore and the VM to believe it's the time when the backup occured (which would leave it in an inconsistant/messy state).
I went looking and came across a video from TechNet which, about half way through, has a high level technical explanation of what happens when you use VSS to backup VMs, and what happens when the guest OS isn't VSS aware. Certainly interesting and potentially worrying stuff.