Software VSS Provider - DPM 2010/2012 Brick Level Backup

Even entry level SAN's today from vendors such as HP, Dell etc are offering hardware VSS software as standard, which is great for brick level backups of your Hyper-V virtual environment. 

But... What happens if you find yourself using a SAN without H/W snapshots or your financial director has locked away the key to the stationary room. The dilemma is then whether or not to go ahead and submit the proposal for the H/W snapshot licenses or use the build in software VSS provider in Windows.

It is at this stage you need to undertake a comprehensive review of the perils of using software VSS, these being the increased demands on your Hyper-V hosts, long backup times and the dreaded redirected I/O in a clustered environment.

Redirected I/O

When backing up CSV's you will need to rethink your VM placement, this is due to the fact that low level operations on CSV's in Windows 2008 R2 Hyper-V result in redirected I/O. This process locks the CSV for exclusive access by the CSV owner in order to take a consistent backup, the bad news is every other host in the cluster will need to read and write data over cluster communications network rather than the storage network.

When using H/W VSS providers the amount of time for redirected I/O to occur during a backup is limited to the time required to create the hardware snapshot (typically a couple of seconds), but when using a S/W VSS provider redirection will occur for the entire length of the backup. Obviously backing up large numbers of virtual machines during business hours becomes a non runner, however S/W VSS can be used to backup VM's as long as you plan your environment to suit the solution.

VM Placement

When planning to use the S/W VSS provider you need to consider a couple of key area's. As mentioned earlier you want to limit the amount of time you incur redirected I/O so you should setup protection groups by CSV rather than by logical groupings such as SQL servers for instance. VM's with VHD's on multiple CSV's are also going to be an issue as when a backup is called, every CSV which is associated with the VM will also go into redirected I/O mode.

Using manual migration or Systems Center Virtual Machine Manager you want to get to a point where by manageable quantities of virtual machines are located for your soon to be built brick level protection groups.

Serialisation - Backup Jobs

Once you have sorted out your CSV / VM placement design the next stage is to serialise backups by using registry entries and the DSConfig powershell script. This process ensures that DPM is made aware of virtual machine placement at a storage level and limits DPM as to how many VM's it can backup at a particular time.

Conclusion...

Once you plan your environment and follow the guidelines outlined (http://technet.microsoft.com/en-us/library/hh757922.aspx), using DPM for brick level backups with the software VSS provider can help you get to a good place with your entire virtual environment ready to restore in a click of a button.