I ran into a server that showed 83% used on the / filesystem.
du -ak | sort -nr | more
showed no files or folders using the space.
The server had a back history of a missing mount, which had caused the filesystem to fill up to 100%. I suspected that the now-mounted filesystem was hiding folders and files under the mount point, that had not been deleted. How to look at these files, and better yet delete them if they existed? This is on Ubuntu Server 12.04.
mkdir /tmp/rootfilesys mount --bind / /tmp/rootfilesys then cd /tmp/rootfilesys du -ak | sort -nr | more
and the command showed a tree of files and folders under a directory that should only have been a mount point.
I deleted the directories under my mount point using
rm -rf /tmp/rootfilesys/mountpoint/
and this worked, the filesystem space was recovered without affecting the mounted filesystem at all…