Quote Originally Posted by schragge View Post
I was hoping some big unlinked open files would show up in the output of lsof. But so far the largest open file is /usr/lib/firefox/libxul.so
Maybe this will help:
sudo find /proc/*/fd -type f -links 0 \! -empty -ls
Another possibility, albeit quite improbable, is that some big files lurk at the /home mountpoint hidden by mount. Try to unmount /home and see if there's something in there.

Well, I guess you can free up /usr only by uninstalling some packages. Besides, 6G doesn't look like too much to me.
Now, thats is strange. I booted from USB disk, mounted my root partition and df -h shows 7.4GB used on /dev/sda5 (which is my root partition) out of 18GB, that is 11GB free. This is even 400MB less then diskspace analyser shows.

And: there is nothing in the /home mountpoint, i.e. no overlay mount.

I found a quite extensive thread on http://serverfault.com/questions/570...-df-difference but nothing there helps me.