How to shrink vdi virtual disk in VirtualBox to reduce filesize

Problem:

I use VirtualBox and I want to share my Virtual Machine (MV), the export file (.ova) is too big.
When I try to share in Google Drive (or any other cloud service), it takes many time to upload.

Solution:

You shouldn’t share the .ova file, you can share the virtual hard disk (.vdi) and then create a new MV with the same content.

Step by step:

Start you guest virtual machine with root.

Reboot to GRUB menu (Press Esc as Ubuntu is booting up).
```
 Advanced options for Ubuntu > Recovery Mode (choose highest version number)
```
From the Recovery Menu select Drop to root shell prompt (root - Consola de superusuario)

df to get your partitions (to see the partition you want to shrink e.g. sda1, sda4 etc. usually its /dev/sda1)
```
 df
```
IMPORTANT! In next step, REVIEW and set the partition you want to shrink.

In Terminal, execute:

systemctl stop systemd-journald.socket && systemctl stop systemd-journald.service && systemctl stop systemd-journald-dev-log.socket systemd-journald-audit.socket && sudo swapoff -a && mount -n -o remount,ro -t ext2 /dev/sda1 / && zerofree /dev/sda1

Shutdown your guest in VirtualBox.
```
halt
```
after that clic in Machine -> Power off
Finally, compact the .vdi to get space. In your host (in Linux) open a Terminal and type:
```
VBoxManage modifymedium --compact '/path/to/thedisk.vdi'
```
IMPORTANT! remember specify the absolute path to .vdi file.

If your host is Windows OS:
```
 VBoxManage.exe modifymedium disk "C:\path\to\disk.vdi" --compact
```
If your host is MacOS (example, Ubuntu OS):
```
 VBoxManage modifyhd /path/to/thedisk.vdi --compact
```

Source:

https://askubuntu.com/questions/1092812/zerofree-on-ubuntu-18-04
http://manpages.ubuntu.com/manpages/focal/en/man1/systemctl.1.html
https://www.howtogeek.com/312883/how-to-shrink-a-virtualbox-virtual-machine-and-free-up-disk-space/
https://wiki.ubuntu.com/RecoveryMode
https://gist.github.com/bitstarr/01e4946d9ab4ca368953300415cb934a
https://blog.lazy-evaluation.net/posts/reclaiming-vm-space-virtualbox.html
https://bbs.archlinux.org/viewtopic.php?id=281224

========

(bonus!) brief explanation:

First, stop all processes writing to the disk so you don’t get mount /: mount busy error

systemctl stop systemd-journald.socket
systemctl stop systemd-journald.service
systemctl stop systemd-journald-dev-log.socket 
systemctl stop systemd-journald-audit.socket

Check if any swap partitions are enabled:

swapon -s

If enabled, then disable them:

sudo swapoff -a

Then finally you should be able to mount dev/sda1 as read-only. (Your filesystem type might be different from ext2. To find the filesystem, run df -T)

mount -n -o remount,ro -t ext2 /dev/sda1 /

Then finally run zerofree

zerofree -v /dev/sda1

Whens zerofree is done, shutdown Ubuntu

halt