Over the last few months I have gotten a lot of questions about backing up Hyper-V virtual machines… Do you run the backup software in the virtual machine or on the parent or do you just “snapshot” the virtual machine?  Some of the confusion has come from that last question – SAN’s (large disk arrays) have had a concept of a “snapshot” for quite a while, it’s basically the same concept as a virtual machine snapshot you can tell the SAN to snapshot a running volume and then you can do things like revert back to that point or move that data to another SAN or make a new volume based on that etc…  Windows introduced a technology called VSS or Volume Shadow Service that can communicate with these SAN’s and tell them “hey some backup app wanted a snapshot please take one”, VSS also introduced a method where applications like SQL or Exchange can register to be informed when a back up is going to take place and participate in that process.  This has spawned the world of application aware and hardware aware backups where you can backup a running server while the applications are running directly to hardware.  Then latter you can restore specific parts of those applications to the same of a different server such as a single SQL table or a single Exchange mailbox.

So that’s all well in good what about Hyper-V?  Hyper-V has support to VSS built in – so if your backup software is VSS enabled when you request a backup of a the physical (parent/host) server Hyper-V will forward those requests to each virtual machine (requires integration components and OS VSS support) then the guest will forward the request to any VSS aware application.  In the end what you end up with is a back up of the parent/host will backup any virtual machines on that server and allow you to restore them individually or as a group.

Caption: After doing a backup using Windows Server Backup – I can now restore a specific virtual machine… I am showing the backup contains the ID’s of all the VM’s for the Hyper-V “application”…  (see the post by Rob Hefner linked to below to enabled Hyper-V in WSB)

 

 References

-How to enable Windows Server Backup support for the Hyper-V VSS Writer
    Rob Hefner (Support Escalation Engineer for Hyper-V) wrote a great post on enabling support for the Hyper-V VSS writer for Windows Server Backup.  This is a very helpful article!  
    Make sure you have the “Windows Server Backup” feature enabled or you won’t be able to create the registry key…

-Backup and Recovery Overview - TechNet

-Step-by-Step Guide for Windows Server Backup in Windows Server 2008 – TechNet

 

Taylor Brown
Hyper-V Integration Test Lead
http://blogs.msdn.com/taylorb