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Backing up virtual servers: Top methods for Windows machines

If you are planning to add virtual servers to your organization, you must first figure out how to back them up. Let's take a look at two different methods for backing up virtual machines,

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with insight into the pros and cons of each one.

Method 1: Run backup software directly on the virtual machine

The first method for backing up a virtual server is to install a backup application or a backup agent on the virtual machine, and then back up the virtual server in the same way that you would back up a physical server.

The primary advantage of this method is consistency, as you can continue to use the backup process that your organization already has in place, rather than complicating things by adopting a special procedure for virtual machines. Another important advantage is that you can easily back up a virtual server while it is running.

Although virtual server performance has improved, a virtual server will never perform as well as a physical  [one] because of the extra overhead required by the emulation process.

The cons associated with running backup software directly on a virtual machine vary depending upon which virtual server product you're using. For the purposes of this article, I'm going to limit my discussion to Microsoft Virtual Server.

One disadvantage to running backup software directly on a virtual server is performance. Although virtual server performance has improved over the last year or two, a virtual server will never perform as well as a physical server because of the extra overhead required by the emulation process. That being the case, you may find that it takes longer to back up a virtual server then it does to back up a comparable physical server.

Performance can suffer even more if multiple virtual machines are running simultaneously. If multiple virtual machines are sharing a common hard disk or network adapter, then the backup's performance may be seriously impacted.

Fortunately, you can get around some of these performance issues by doing some planning ahead of time. For example, you should consider placing all of your virtual hard drives on high-performance disk arrays. That will help reduce the bottlenecks associated with having multiple virtual machines sharing a common pool of disk resources. Likewise, you might consider installing multiple network adapters and having each one service a specific virtual machine as a means of reducing the network bottleneck.

Another disadvantage to running backup software directly on a virtual machine is that virtual servers offer limited hardware support. Remember, the host operating system actually contains the drivers to communicate with the physical hardware. The guest operating system is running on an emulator that copies certain types of hardware. Because of this, backup hardware that is connected directly to the server that the virtual machines are running on may not be supported.

Here's an example of why this might be the case: Microsoft's virtual server products do not currently support USB devices. This is true even for the Hyper-V virtual server product designed for Windows Server 2008. If something as important as USB support has been left out, you can only imagine what would happen if you tried to link a virtual machine to more specialized hardware.

Method 2: Copy the .VHD and .VSV

The other method you can use to back up a virtual machine is to simply copy the virtual hard drive file (.VHD) and saved state file (.VSV). The primary benefit to using this method is simplicity. After all, how hard is it to copy a couple of files? Another advantage is that you can back up the virtual machine's current state, which is usually not the case with traditional backups.

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Still, there is one really big disadvantage to using this method -- you can only perform this type of backup when the virtual machine is shut down. If you try to use a Volume Shadow Copy Service snap shot to back up the virtual machine while it is running, data loss or even corruption may occur.

So which backup method should you use? It really depends on the nature of the virtual machines that you're backing up. For example, I have several virtual machines installed on my laptop and I back them up using the file copy method. This is feasible because I only use those virtual machines for lab purposes, so they are not running all the time. I also use a few virtual machines on my production network, which I back up using the traditional backup method because it's simply not practical for me to shut them down every time I want to run a backup.

The point is, when deciding which backup method you want to use, you will have to consider how the virtual servers are being used and which method would be most practical for you.

ABOUT THE AUTHOR
Brien M. Posey, MCSE, has received Microsoft's Most Valuable Professional Award four times for his work with Windows Server, IIS and Exchange Server. He has served as CIO for a nationwide chain of hospitals and healthcare facilities, and was once a network administrator for Fort Knox. You can visit his personal Web site at www.brienposey.com.

This was first published in July 2008

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