Disaster Recovery for Hyper-V (Part 5)

by [Published on 22 Sept. 2009 / Last Updated on 22 Sept. 2009]

Continuing the Hyper-V disaster recovery series by talking about some of the benefits of running VSS backups at the host operating system level of your Hyper-V servers.

If you would like to read the next parts of this article series please go to:


In my previous article in this series, I spoke about numerous limitations associated with performing a host level VSS backup of Hyper-V. Some of these limitations are specific to Windows Server Backup, but even more so, the backup process is restrictive enough that you may have been left wondering if perhaps there might be a better way to backup your virtual machines.

I ended the previous article by saying that I was convinced by the advantages of using VSS backups on Hyper-V servers, but that VSS backups should only be a part of your overall backup strategy. Obviously, that’s a pretty bold statement, so I want to use this article to elaborate on why I made it.

Compatibility with Any Guest Operating System

The reason I recommend to use host level VSS backups of Hyper-V servers is that doing so allows you to get around operating system compatibility issues. Most of the backup applications that I have worked with over the years require agents to be deployed on the servers that are being backed up. Like any other type of software, the agents would not run unless certain minimum system requirements are met. This can be a real problem for organizations that are operating heterogeneous networks.

Imagine, for example, that the agents for your backup application are only compatible with Windows Server 2003 and Windows Server 2008. That is fine if those are the only server operating systems that you are using, but you have a real problem if you have any Linux servers or legacy Windows Servers.

The beauty of performing VSS backups at the host operating system level is that the backups do not care what kind of operating system is running on your virtual machines. Actually, that is not completely true. Even though your backup application neither knows nor cares what operating systems are running within your virtual machines, there is a small distinction between the way that some virtual machines are backed up.

If a virtual machine is running an operating system that supports VSS, then the Hyper-V integration services will facilitate the creating of a VSS snapshot of the virtual machine. If the virtual machine is not running a VSS aware operating system (or if the integration services are not installed onto the guest operating system) then a VSS snapshot is still made of the virtual machine. The difference is that because the guest operating system is not VSS compatible, it has no way to prepare itself for the snapshot (which is necessary for protecting the integrity of the backup). That being the case, Hyper-V momentarily hibernates the virtual machine while the snapshot is taken.

Unfortunately, there is a momentary interruption of service while the guest machine is hibernated, but the hibernation and snapshotting process usually happens fairly quickly. The reason why hibernation is necessary is because part of the hibernation process involves writing the contents of the virtual machine’s memory and in some cases even its virtual processor state to a file. This ensures that all of the data associated with the virtual machine is captured during the backup process and that transactions aren’t occurring during the backup, which would cause the backup to become corrupt.

Bare Metal Recovery

Another major benefit to performing VSS backups at the host operating system level of your Hyper-V server is that doing so allows you to perform the virtual machine equivalent to a bare metal recovery.

When you perform a full system backup of a physical server, you are backing up the server’s operating system, applications, data, and the system state. The same thing happens if you perform a full system backup of a virtual machine from within the virtual machine. The problem is that there is more to a virtual machine than there is to a physical machine, and running a typical full system backup from within a virtual machine does not capture this extra data.

So what is the extra data that I am talking about? It is the virtual machine’s configuration. When you create a virtual machine, you must tell Windows how much memory you want to assign to the virtual machine. You must also configure things like the name of the virtual hard drive file, the number of virtual processors to be used, and how you want to connect the virtual server to your network. All of this information is stored outside of the virtual machine, because the virtual machine can’t even boot without it.

My point is that if you are performing a typical full system backup from within a virtual machine, you are missing out on backing up the virtual machine’s configuration. That does not mean that your backup is useless. Certainly you can still restore the backup. You just would not be able to use the backup to completely recreate the virtual machine from scratch.

On the other hand, performing a backup at the host operating system level of your Hyper-V server captures all of the configuration information for each virtual machine, which allows you to perform the virtual machine equivalent to a bare-metal restore.

Backup Application Licensing Fees

These days everybody is worried about the cost of running a business (at least they are in the United States anyway). This leads me to another benefit in carrying out host level VSS backups that many people overlook. Obviously, every backup application is different but most of the backup applications that I have worked with over the years are licensed according to the number of servers that you are backing up. In other words, if you are backing up five servers, then you are typically going to need five licenses.

Many backup applications require you to install an agent onto the servers that you are going to be backing up. The agents not only facilitate communications between the backup application and the server, but they also typically provide the backup application with a handy way of counting how many servers are being backed up, and therefore how many licenses are being used.

When you perform a host level backup of a Hyper-V server, you are backing up multiple virtual machines, but the only agent that is required is the one that is running on the host operating system. If you have a lot of virtual machines, then this can amount to substantial cost savings.


As you can see, there are definitely benefits to performing host level VSS backups of your Hyper-V servers. One thing to keep in mind though, is that in order to avoid writing an article that is backup application specific, I generalized most of the concepts that I wrote about. To the best of my knowledge, the information in this article is accurate for most of the major backup applications on the market, but it is possible that some backup applications may do things a bit differently. With that said, the next article in this series will talk about some of the limitations of VSS host level backups, and how you can get around those limitations.

If you would like to read the next parts of this article series please go to:

See Also

The Author — Brien M. Posey

Brien M. Posey avatar

Brien Posey is an MCSE and has won the Microsoft MVP award for the last few years. Brien has written well over 4,000 technical articles and written or contributed material to 27 books.


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