Posey's Tips & Tricks

How SP1 for Windows Server 2008 R2 Will Affect Hyper-V

Service Pack 1 (SP1) for Windows Server 2008 R2 was released late last month, and here are some steps to take for those planning to run Microsoft’s Hyper-V virtualization with the new update.

By far the biggest new feature in SP1 for virtualization administrators is the addition of dynamic memory support. Essentially, organizations using Hyper-V will be able to overcommit memory via this dynamic memory capability in a similar manner to what can be done already through VMware solutions. Dynamic memory should allow organizations to increase their virtual machine density without having to purchase additional physical memory for their virtualization hosts.

Although it may be tempting to apply the new service pack right away, it is critically important that you prepare your virtual machines before applying the service pack. Before I tell you how to prepare your virtual machines, I need to warn you that the process can result in a significant amount of down time. It also means giving up any virtual machine snapshots that you may have saved. I will discuss these issues more in a moment.

With that said, the first thing that you must do before you can apply the service pack is to completely shut down every virtual machine that is running on the server. You won't be able to place the virtual machine into a saved state because Windows Server 2008 R2 SP1 uses a different internal mechanism for placing virtual machines into a saved state. This means that if you were to place a virtual machine into a saved state and then apply the service pack, Hyper-V will not recognize the saved state after the upgrade.

Merge Your Snapshots
The next thing that you will have to do is to merge all of your snapshots. When you create a snapshot in Hyper-V, what really happens behind the scenes is that Hyper-V stops writing data to the corresponding virtual hard drive file and begins writing data to a differencing disk instead. That way, if you need to revert the machine to a previous state you can get rid of the snapshot and go back to using your original virtual hard drive file, which has been untouched since the time that the snapshot was created.

Although I have not seen a direct statement from Microsoft, it seems that snapshots created under Windows Server 2008 R2 are not compatible with SP1. That being the case, you are going to have to get rid of any snapshots that may exist before you apply the service pack. In most cases this will mean merging snapshots.

When you merge a snapshot, Windows takes the differencing disk that is associated with the snapshot and writes the differencing data to the previously untouched virtual hard drive file. This brings the virtual hard drive file up to a current state and eliminates the differencing disk. Depending on how much data has accumulated in the differencing file since the time that the snapshot was created, the merge process can take a very long time to complete.

While I'm sure many will be hesitant to apply the service pack because they like having the safety net that their existing snapshots provide, there is one benefit to merging your snapshots. Because of the way that Hyper-V uses differencing disks, the snapshot thing tends to kill virtual machine performance. However, after you merge the snapshots, you will likely notice a boost in performance, especially if you have multiple snapshots for each virtual machine.

If you don't want to merge your snapshots, then your other option is to delete them. As I said earlier, deleting a snapshot reverts the virtual machine to a previous state.

Apply the Service Pack
Once you have verified that all of the virtual machines are stopped and that all of the snapshots have either been merged or deleted, it is time to apply the service pack.

After the service pack has been applied, you still have a little bit of work to do. SP1 uses different drivers within the integration services than were used in the Windows Server 2008 R2 version of Hyper-V. As such, you will have to reapply the integration services to every virtual machine before your virtual machines will function properly. Most notably, the virtual machines will not be able to connect to the network until the integration services have been updated.

Of course if a virtual machine is running Windows Server 2008 R2, then you can achieve the same end result by applying SP1 to the virtual machine.

SP1 for Windows Server 2008 R2 has been long anticipated because of its memory sharing capabilities. However, organizations wanting to run Hyper-V with the new service pack will have a lot of planning to do before deploying it.

About the Author

Brien Posey is a 22-time Microsoft MVP with decades of IT experience. As a freelance writer, Posey has written thousands of articles and contributed to several dozen books on a wide variety of IT topics. Prior to going freelance, Posey was a CIO for a national chain of hospitals and health care facilities. He has also served as a network administrator for some of the country's largest insurance companies and for the Department of Defense at Fort Knox. In addition to his continued work in IT, Posey has spent the last several years actively training as a commercial scientist-astronaut candidate in preparation to fly on a mission to study polar mesospheric clouds from space. You can follow his spaceflight training on his Web site.


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