Inode

Inode Table:

All files in linux are having an inode number which is residing inside the inode table of a partition. For every partition there is an inode table, so an inode number will be unique for a partition, and thats the reason why we cannot hardlink between two partitions. Inode number contains a file's uid, gid, access time, modification time, change time and size of the file. inode number DOESNT hold a file's NAME. we can retrieve the contents of a file even if the file is deleted unless and until the inode number corresponding to that file is overwritten.
The inode number can be viewed with the -i switch with the 'ls' command..!
$ls -i filename

Script to determine where the inode usage was most :

for i in `ls -1A`; do echo “`find $i | sort -u | wc -l` $i”; done | sort -rn | head -5

With this you can Track it down to a directory then execute this command :

find ./ -type f -mtime +10 | wc -l

or 

find ./ -type f -mtime +10 | xargs rm

High inode usage is usually caused by a massive number of small files. In this case the session files are normally stored somewhere temporary and removed when not in use. Either there could have been a bug in the code not removing them or it was a higher traffic website.