Virtual PC Differencing Disks Hierarchy

A long time ago, I talked about Differencing Disks in Virtual PC.

Most people just have a base image and difference from that. A fairly basic technique by now.

The next step, however, is to think about differencing from a differencing disk.

Using this technique you can set up the following collection:

• Clean Windows Server 2003 (normal VPC hard disk)
  • Differenced disk with SQL 2000 installed
    • Your differencing disks if you need to test 2003 + SQL 2000
  • Differenced disk with SQL 2005 installed
    • Your differencing disks if you need to test 2003 + SQL 2005
  • Your differencing disks if you have no need for SQL (the basic differencing technique)
• Clean Windows XP (normal VPC hard disk)
  • Differenced disk with SQL 2000 installed
    • Your differencing disks if you need to test XP + SQL 2000
  • Differenced disk with SQL 2005 installed
    • Your differencing disks if you need to test XP + SQL 2005
  • Your differencing disks if you have no need for SQL (the basic differencing technique)

So far we only talked about a base image. The term I’m using to refer to the above setup (the non-italic disks) is a base collection.

Creating these six Virtual PC hard disks and putting them on a share somewhere gives you a lot of flexibility if you need to test something on different platforms without having to use the undo disks as a differencing level to throw away after you’re done testing.

Now you simply create a new VPC, make a differencing disk from the platform you want (for example: Testing Community Server on Windows 2003 with SQL Server 2005), start it and install your software. And when you’re done testing you can simply delete that VPC if you want. You can enable undo disks on this disk if you want.

Knowing that you can create differencing disks from differencing disks lets you come up with some interesting schemes.