root full Linux. No free blocks

df -h reports the '/' partition to be 100% full. While running du -hs * on each directory of this partition shows there is still lot of space.

tune2efs which reports just one block to be free. Ran fsck as well, which also shows all blocks being used.

ionode use is 14% on the '/' partition.

Please note that /var, /usr, /dev, /tmp, /home are mounted on different partitions and have space available in them.

Can you please let me know the possible causes for all blocks to be occupied and disk reporting to be full even tough there is a lot of space.

As well as the open files problem that commonly causes otherwise free space to be held unavailable, a not uncommon problem is files shielded by mount points. For instance if you have /tmp as a separate logical volume but still have files in the directory /tmp in the root filesystem, those files will be consuming space but will be hidden by the mount.

Try dropping into single-user mode at boot so nothing is running that might hold mounts open, unmount everything, and make sure there are no files hiding in the directories that are normally used as mount points.

This is often caused by having a file open for writing that has been deleted but the process writing to the file hasn't been restarted thus relinquishing the file. You can use lsof to find files that are open but unlinked (deleted)

lsof +L1

should do the trick. As the man page states:

A specification of the form +L1 will select open files that have been unlinked. A specification of the form +L1 <file_system> will select unlinked open files on the specified file system.