Differencing Disks in Virtual PC 2004

Last week I was wondering (together with Peter) how Microsoft distributes their VPC images. For example for a course which uses two VPC images, you get one "base" image, and two "delta" images which rely on the same "base" image. Suppose you could pull of the same trick for your own VPC images using your magic-super-tuned base image! This would give you at least two advantages:

• Less MB's
  Believe me, even a 250 GB hard disk seems a lot smaller when you use heavy 7GB VPC images. So storing only the delta would save some precious disk space.
• Spawn new VPC's faster
  Currently I have a couple of base images on my hard disk, when I need a new VPC instance to test something out for example, I copy one of my base images and create a new VPC that uses my copy. My base images are around 3GB, copying one of them takes op to 10 minutes. When using a "delta" image, you're up and running in less than a minute!

There is some good news, you can do the same trick with your own images! Welcome to the world of Differencing Disks. Matt Hausmann has a nice post that goes into detail. There's even a complete webcast that shows you everything. You can even chain multiple differenciated disks! A final caveat: you can't compact a differenciated (is this an English word?) disk. A recommendation which I've read quite a few times in the newsgroups, related to this issue is to place the differenciated disk into a compressed folder on a NTFS partition.