Disaster Recovery for Hyper-V (Part 6)

by [Published on 16 Dec. 2009 / Last Updated on 16 Dec. 2009]

Continuing the series on disaster recovery for Hyper-V by discussing the advantages and disadvantages of performing a guest level backup of your virtual servers.

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Introduction

In my previous article in this series, I talked about some of the advantages to performing VSS backups at the host level. Since I had already discussed the primary disadvantages of host level backups in Part 4 of this series, I want to turn my attention to guest level backups. In other words, I will take this opportunity to talk about the advantages and disadvantages of backing up individual virtual machines rather than backing up your Hyper-V server as a whole.

Granularity

If you have read some of my previous articles in this series, then you may already have a pretty good idea of what some of the advantages and disadvantages are of performing guest machine backups, but I want to discuss them anyway, just to make sure that we are all on the same page.

By far the biggest advantage to guest machine level backups is the granularity that it provides you with. Unlike a host level backup, you are free to pick and choose exactly which files, folders, applications, etc. you would like to backup. Likewise, a guest level backup also provides you with the ability to restore individual files and folders as opposed to forcing you to restore the server as a whole.

Simplicity

Another advantage to guest level backups is the simplicity involved in creating them. I will be the first to concede that some backup applications are anything but simple, but that is not really what I am talking about. What I mean is that from the standpoint of your backup application, performing a guest level backup of a virtual server is basically the same as backing up a physical server. There aren’t a lot of special considerations that you have to take into account just because you are backing up a virtual machine.

Compatibility

One possible disadvantage to guest machine backups is compatibility. I recently saw a situation in which an organization had set up a Hyper-V server with a few guest machines, and then connected a tape drive directly to the host server. The problem with this configuration was that the tape drive was only accessible to the server’s parent partition. The virtual machines that were hosted on the server were not able to communicate with the tape drive. Likewise, the backup application that was running on the host operating system was not Hyper-V aware, so it had no way of reliably backing up the guest operating systems.

License Consumption

One possible disadvantage to performing a guest level backup is that you may consume a lot more licenses for your backup application than you would have had you just performed a host level backup. Most of the backup applications that I have worked with require you to purchase an agent license for every server that you are backing up. Performing a host level backup only requires a single agent (which typically requires one license), while backing up individual virtual machines requires a separate agent for each VM which typically requires (multiple licenses).

No Bare Metal Recovery

Perhaps the biggest disadvantage of performing a backup of individual virtual machines rather than backing up Hyper-V as a whole is that you would not be able to perform a bare metal recovery should it become necessary. OK, I know that there is really no such thing as a bare metal recovery when you are dealing with virtual machines, but let me explain what I am talking about.

Imagine for a moment that your Hyper-V server dies a horrible death, and that you have to completely replace the server hardware. Once you have the new hardware in place your next task is to install Hyper-V and then return all of your virtual servers to a functional state. Easy enough, right? Well, not quite.

The problem with backing up individual virtual machines is that if you have to perform a restoration, your backup software would not recognize the fact that you are restoring a virtual server.  While it is true that your backup still contains a copy of the Hyper-V Integration Services, there is more to a virtual machine than the Integration Services and the Windows operating system.

If you think back to the time when you originally set up the virtual machine in question, you will recall that you had to start out by giving Hyper-V some basic information about it. For example, you had to tell Hyper-V how much memory to allocate to the virtual machine, which files would be used as virtual hard drives, where those files would be located at, and how the virtual machine would connect to your network. This (among other information) becomes a part of the virtual machine.

What some administrators do not realize though, is that when you perform a backup of a guest operating system what you are really backing up is the contents of the virtual server’s virtual hard drive files (the .VHD files). The low level configuration information that I mentioned above does not reside on a virtual hard drive. After all, the low level configuration information tells Hyper-V which virtual hard drives to use, so windows certainly can’t embed the information within a virtual hard drive.

So what does all of this mean to someone who has to recover a failed Hyper-V server? Well, it is not impossible to recover a virtual server using guest operating system backups. It is just that the recovery process is going to take longer to perform and it is going to involve a lot more work for the administrator. An administrator will have to manually recreate each virtual machine prior to restoring the individual backups. Of course this is only true in the event of a catastrophic failure. If you only need to recover some individual files then performing a virtual machine restoration using a guest backup is no different than performing a restoration on a physical server.

Conclusion

So now that I have discussed all of the pros and cons associated with host level and guest level backups, the big question becomes which backup method should you use. My recommendation is to use both methods, but don’t do it blindly.

What I mean is that most organizations have to make sure that their backups complete within a specific amount of time (known as a backup window). There is also a finite amount of space available on the backup media. The problem is that if you are performing both host level and guest level backups, then you are backing up most of the data on the server twice. That often means that backups are going to take twice as long to complete, and that they are going to consume twice as much space on your backup media.

Believe it or not, there are some techniques that you may be able to use if you want to perform both types of backups. I will discuss these techniques in the next part of this series, as well as what you should do if you absolutely can’t perform both types of backups.

If you would like to be notified of when Brien Posey releases the next part in this article series please sign up to our VirtualizationAdmin.com Real Time Article update Newsletter.

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

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