Virtual PC: Using Differencing disks to role-back to a previous state

If you use Microsoft’s Virtual PC for anything, you should check out the Differencing Disks feature. Over the past few months, I’ve been making a effort to start leveraging Virtual PC as much as possible (side-note, I need more RAM in my main system), and the one of the features that I like most is the differencing disks feature!

Think of it like this… you have take a read-only base virtual hard disk (.VHD) image that you’ve built, running 2003 server for instance, and then you add any changes you make to a secondary, or child .VHD file called the differencing disk. You can then go back and add subsequent differencing disks off of that main base image for different types of services. In other words, you can create multiple points in time giving you the ability to easily role-back to a previous state.

One of the most effective ways that we’re using Virtual PC right now is in reproducing customer environments for testing, and the differencing disks feature goes a long way to making testing and documentation both easier and more comprehensive than was previously possible.

To add a differencing disk using Virtual PC 2004, SP1 just do the following:

Open Virtual PC, and select an instance, click “settings”. Next… hard disk 1 -> Virtual Disk Wizard ->Create a new Virtual disk -> A virtual hard drive -> Specify location -> Differencing -> Select the parent virtual disk

Managing your Virtual PC images is still a little more manual than it could be when compared to the Snapshot Manager in VMWare Workstation. For instance, I end up creating.TXT files in my Virtual PC directory structure to remind me what each .VHD is for, but Virtual PC certainly does provide for the basic necessities in terms of being able to easily role-back to a previous state.