[ubuntu] zero free space - zero still available after files deleted

If your root reserve is larger than 5GB and it was used up, then it would still show 0 free after deleting 5GB of files, because it's still reserved for root. You can check the size of the root reserve using `tune2fs -l /dev/thing`. By default it is 5% which is a bit much for very large filesystems. It can be changed with `tune2fs -m`

Apart from that, if any process is still using the deleted files, the space is not freed.

You can check where the most space is wasted using xdiskusage. Example for the root partition /

sudo mount --bind / /mnt/root
sudo xdiskusage /mnt/root/