Virtualbox

Option 1: Shrink VDI disk using dd

1. Start the VM and delete anything you don't need in the VM.
2. Then create a file with just zeros.
3. Run sync and delete the file.

dd if=/dev/zero of=junk
sync
rm junk

1. Poweroff the VM and navigate to the folder containing the disk on the host and run the following command.

VBoxManage modifymedium disk ./diskname.vdi -compact

Option 2: Shrink VDI disk using zerofree

About zerofree

First step is to install zerofree. On archlinux it can be installed from the AUR: yay -S zerofree.

1. Start the VM and delete anything you don't need in the VM.
2. Make sure you are allowed to edit command line parameters on boot.
3. When starting the VM, hit Esc repeatedly (grub) or hold space for the systemd boot menu to show up.
4. Press e to edit the parameters and append an init=/bin/bash to the command line arguments. Continue boot. You'll land in the bash shell.
5. Remount the root filesystem as read-only: mount -n -o remount,ro -t ext4 /dev/sda1 /
6. Finally run zerofree -v /dev/sda1
7. When finished, shutdown with halt

In the last step we run the VBoxManage command to shrink the VDI disk.

VBoxManage modifymedium disk ./diskname.vdi -compact