VMWare Fusion, Snapshots and Disk Space

My 40GB Windows XP VM had mysteriously grown to 50GB. I couldn't quite figure it out: 40GB disk, 1.5GB RAM, what more could it want to store?

Answer: I'd taken a VM snapshot prior to applying XP SP3.

Conceptually, a VMWare snapshot is a point-in-time image of your VM. However, you'll notice that taking a snapshot doesn't double the amount of disk space that your VM takes up. What actually appears to happen is the VMWare starts appending changes you make to your VM to a new 'differences' file within the VM package on disk, leaving your original VM file intact. If you ever revert to that snapshot, it can simply throw away this file containing the changes.

This also means, that as you change the contents of your VM, it will take more and more disk space as VMWare builds this 'differences' file. The solution to this is to discard the snapshot: select Discard Snapshot from the Virtual Machine menu. Be aware though that this operation can take a long time. VMWare has to go through the differences file and apply them to the original image. If a lot of data has changed, this will take a while. However, once the snapshot has been discarded, your VM will shrink back to its expected size.