One of the features available in the latest Beta of Virtual Server 2005 R2 SP1 is the support for the Volume Shadow Copy Service (VSS). VSS is a feature of Windows 2003 server that takes “snaphots” of files, and allows you to quickly create a backup copy of a volume.
For an application to support VSS, it needs to get to a consistent state, freeze, and the perform the shadow copy. Once the shadow copy is created, the application thaws and resumes operations. If an application does not have a VSS provider, VSS cannot guarantee that the resulting shadow copy will be consistent. That is one of the best features of this technology – the actual applications (called “Writers” in VSS-speak) are involved in the creation of the shadow copy, so they can verify that whatever goes into the copy can be later restored without any problems.
By supporting VSS, backup programs (“Requestors”) can now tell Virtual Server that a backup is going to take place. Virtual Server can then make sure that the Virtual Machines are in a consistent state (I”m not sure if it suspends the VMs – I’m currently in the process of finding out that information), and tell the requestor that it is ready for the copy. According to the documentation, creating a shadow copy is very fast – for large volumes I’ve heard numbers of around 30 seconds to 4 minutes. This can also work for quickly cloning a Virtual Server host to another server, so that VMs can resume operations very quickly even if the system goes down.