How to reduce disk space of .vdi VirtualBox hard-disk with a linux VM (debian)

So you are here because you want to compact your Debian VM .vdi file

Just using the vboxmanage modifyhd /path/to/my.vdi --compact does not do a lot of reduction alone.

You first have to group all the free space together, then --compact will work.

How to group all free space :

Ubuntu / Debian/ Mint :

We will use zerofree utility. To run this we must be running from a boot CD to have access to unmounted partitions. That is not too complicated. Whatever your distribution you can use a Debian live CD/DVD and boot with it.

You first need to modify your software source to use "sid" packages.

• Go into Administration > Software Sources
• Remove (uncheck) all software sources (including CD-ROMs) :
• Add these two software sources :
  • deb http://ftp.us.debian.org/debian/ sid main
  • deb-src http://ftp.us.debian.org/debian/ sid main
• proceed with installation

You can now install zerofree:

sudo apt-get install zerofree

Then verify which partition you want to free using the Applications > System Tools > Disk Utility

sudo zerofree -v /dev/sda1