Clear free disk with zeroes

Done in xubuntu 20.04

1. Open GRUB menu and select recovery mode
2. Open root shell prompt¹

3. Remount / in readonly mode:

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

4. Clean with zeroes

   zerofree -v /dev/sda1
   

This is to compress the data in the disk, as removing a file doesn’t get it’s contents erased. Now with free space being all zeroes you can compress to a nice small .ova.