In a previous posting, I have talked about compressing a virtual hard disk for a Windows guest system running in VirtualBox on a Linux system. Since then, I have needed to do the same for a Linux guest following some housekeeping. The Linux distribution used is Debian so the instructions are relevant to that and maybe its derivatives such as Ubuntu, Linux Mint and their kind.
While there are other alternatives like dd, I am going to stick with a utility named zerofree to overwrite the newly freed up disk space with zeroes to aid compression later on in the process for this and the first step is to install it using the following command:
apt-get install zerofree
Once that has completed, the next step is unmount the relevant disk partition. Luckily for me, what I needed to compress was an area that I reserved for synchronisation with Dropbox. If it was the root area where the operating system files are kept, a live distro would be needed instead. In any event, the required command takes the following form with the mount point being whatever it is on your system (/home, for instance):
sudo umount [mount point]
With the disk partition unmounted, zerofree can be run by issuing a command that looks like this:
zerofree -v /dev/sdxN
Above, the -v switch tells zerofree to display its progress and a continually updating percentage count tells you how it is going. The /dev/sdxN piece is generic with the x corresponding to the letter assigned to the disk on which the partition resides (a, b, c or whatever) and the N being the partition number (1, 2, 3 or whatever; before GPT, the maximum was 4). Putting all this together, we get an example like /dev/sdb2.
Once, that had completed, the next step is to shut down the VM and execute a command like the following on the host Linux system ([file location/file name] needs to be replaced with whatever applies on your system):
vboxmanage modifyhd [file location/file name].vdi --compact
With the zero filling in place, there was a lot of space released when I tried this. While it would be nice for dynamic virtual disks to reduce in size automatically, I accept that there may be data integrity risks with those so the manual process will suffice for now. It has not been needed that often anyway.