I found a link to this on the MS Virtualisation Team Blog this morning. It’s an article on things you should consider when backing up VM’s. It’s well worth reading
There’s two ways you can approach backing up VM’s. The first is to treat each VM as a normal server by installing an agent in the guest OS and using that to backup data, configurations and the system state. This is the most comprehensive solution you’ll get.
The other solution is to take a snapshot of the machine itself. Hyper-V allows this by using Volume Shadow Copy Service (VSS). A VSS writer stops the VM for a couple of milliseconds to capture it in a consistent state. Consider a database being processed … do you want to snap the VM while things are happening? Maybe you’ll get one table showing part of a transaction and another table not showing the other half of the transaction. That’s not pretty at all! This process used by VSS is also how you can get consistent VSS snapshots of SQL and Exchange.
Another thing to consider is virtualised DC’s. Just like in VMware ESX, it’s recommended that you have at least one (preferably two) physical DC’s for your network to avoid potential chicken and egg scenarios. But you might have other DC’s that are virtualised. You should treat them just like physical DC’s. Trying to do clever things likes snapshots and rollbacks of virtualised DC’s will lead to horrible scenarios like the USN rollback.
This article also mentions pass through disks not being supported by VSS. If you think about it, of course they cannot. There is no host file system when using a pass through disk to perform VSS. The VM is using a raw LUN. Be very, very careful about using SAN snapshots here; it’s not supported for a consistent state snapshot if the VM is running.