Last week, Parallels released VM Compactor 1.0 beta. They claim that the software can compact the size of a virtual disk up to 80 %. Disk space is always an issue, if you are running many virtual machines on one host. So I was curious what VM Compactor can really do.

The tool supports virtualization solutions from VMware, Microsoft and Parallels. It can’t be used with VMware ESX Server, and the guest systems have to be Windows 2000, 2003 or XP. The host system can also be Linux or Mac OS X. I only tested VM Compactor with VMware Workstation 5.5. My host system was Windows Server 2003.

The documentation says that VM Compactor has certain limitations for compacting virtual machines running in non-persistent mode. In this mode, VMware doesn’t write changes to the disk file which allows you to exclude virtual disks from the snapshot. Thus, I only tested virtual machines with disks in persistent mode.

There are further limitations. VM Compactor doesn’t support snapshots and linked clones (the cloned disk only stores changes). I tried a virtual machine with snapshots, nevertheless, and VM Compactor started trying to compact it. Only during the compacting process, it realized that it can’t handle the virtual machine.

The time to compact a virtual machine varied a lot in my tests. Once, I already thought that VM Compactor crashed, since the progress bar didn’t move for quite a while, and the corresponding process did not show any CPU utilization. I gave the tool the chance to finish its task during my lunch break, and it seized this opportunity. You can’t use the virtual machine while VM Compactor is shrinking the virtual disks. So, for a server system this long compacting process might be a problem.

VM Compactor did indeed shrink the occupied space on the host system. However, it was always far away from reaching 80 %; the gained disk space was between 10-20 %. My tests might not be representative though. Unfortunately, the documentation doesn’t say anything under which circumstances this fabulous 80 % can be accomplished. I suppose, it is only possible if your guest system is heavily-soiled with unnecessary files.

But all in all, I find this a useful tool. You get several methods to obtain new disk space in one easy-to-use tool. Even a 15 % gain for every virtual machine, might already be quite helpful in certain situations.