I’ve been an advocate of compressing VHD files for a long time now. As examples, I wrote an blog entry on “Compression and Creating Clusters in Virtual Server” and another on “Compressing Virtuals”. These have also been referenced in other blog entries such as “Tips for using Virtual PC and Virtual Server”. The summary from my perspective was this: They take up less space and can actually perform better than the uncompressed ones. That makes compressed virtuals optimal in environments such as lab environments. So recently I moved my virtuals from my Server 2008 Hyper-V to Server 2008 R2 Hyper-V. There’s nothing like after a long hard day, coming home to work on your lab environment. You opening up Hyper-V R2 to migrate over one of your VHD files and get your lab back online. Trying to add the drive and getting this:

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If you really want something fun to do try uncompressing some 13-17 gb VHD files! I started off by grabbing a cup of coffee, and ended up going out for dinner. It may be possible to re-compress a virtual after the VHD has been attached but I’m not willing to spend the time to find out today (and I’m pretty sure it would be unsupported).

Summary: While I’m sure that there was a good technical reason for it, don’t assume that you can use a compressed VHD file with Windows Server 2008 R2/Hyper-V. It’s a good idea to decompress all of your virtuals prior to trying to move them over.