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Compacting a Virtualbox image

My Laptop is on the move: I finally decided to reinstall it as a consequence from a self-inflicted filesystem crash. But this is also a good chance to switch filesystems. This was also already on my list for some time.

While coping all data to a safe place, I tried to figure out how to compact the Virtual Box image I use once in a while -- as its containing XP guest bloated during its lifetime. So when doing the autumn cleanup, why I shouldnt also try to clean up that to?

Disclaimer: The instructions are conserving space very aggressivly. You should not follow them if you are unsure what you are doing.  Repeat: No safety pins attached! It may explode right into your face! You did backup, right?

With some help of google, here are the instructions to shrink a vdi, the image containing the guest Windows XP:

1. Merge all snapshots (optional, but for best results you should do it).Note: Click the topmost "parent" snapshot and use "discard snapshot" to actually merge all together.....)

2. Start the Guest XP and clean it up:

• Remove all unused programs.
• To see space-hogs, use a tool like WinDirStat. If you see that there are programms you need seldom but takes lots of space, consider "compressing" it (Can be enabled on the files property dialog, "advanced settings")
• Remove all Service-Pack and Windows Update folder (In the Windows directory, something like $NTServicePackUninstall$ or $NTUninstallKB<number>) Check Google for detailed instructions. 
• Empty the trash can
• Disable suspend-2-disk (hibernate) and virtual memory
• reboot the guest
• Defrag the guests' drive. Do it twice.
• Use nullfile (get it here) to fill unused sector with "easy to compress" zeros.
• Turn on again virtual memory and hibernation support. 
• Turn off the guest OS

Now, the vdi is ready to be compressed. 

Start a shell and execute this command to complete the operation:

VBoxManage modifyhd <path-to-your-VDI> compact
(for me its: VBoxManage modifyhd /home/tobi/.VirtualBox/VDI/WindowsXP.vdi compact)

Beware: This takes some time. So do not use it if you are in a hurry or on battery power ;-)