How to Shrink your Dynamic Disk on VirtualBox Ubuntu guest:
Original instructions I found here:
https://www.howtogeek.com/312883/how-to-shrink-a-virtualbox-virtual-machine-and-free-up-disk-space/
I updated these commands to work for my system.
In Ubuntu Guest, grab zerofree:
sudo apt install zerofree
Now you will have to boot up Ubuntu in recovery mode:
Shut down Ubuntu.
Start it back up and hit “esc” while it is booting.
Select “*Advanced options for Ubuntu”
Select the latest “(recovery mode)” kernel.
Select “root”
Run df to find your root partition
df
It will look something like “/dev/sda1”
Now run zerofree:
zerofree -v /dev/sda1
When zerofree is done, halt the system:
halt
Close the Virtual Guest window and select “Power off the virtual machine”
Now you will need to run the “VBoxManage.exe” to shrink the physical snapshots and vdi file. Do the Snapshots first. Commands should look like this:
cd "C:\Users\Reynold\VirtualBox VMs\YourVirtualDiskName\Snapshots"
"C:\Program Files\Oracle\VirtualBox\VBoxManage.exe" modifymedium disk "{0e420dca-8044-4d3a-a256-cf90ca19de42}" --compact
cd "C:\Users\Reynold\VirtualBox VMs\YourVirtualDiskName"
"C:\Program Files\Oracle\VirtualBox\VBoxManage.exe" modifymedium disk "YourVirtualDiskName.vdi" --compact
If you have trouble with zerofree because /dev/sda1 is rw mode, then do the following. Found fix for this here:
https://ubuntuforums.org/showthread.php?t=2399268
In single user root mode again:
mount | grep "sda" echo "u" > /proc/sysrq-trigger mount /dev/mapper / -o remount,ro zerofree -v /dev/sda1