" Research time !!"
I can not imagine how many times a disk of a linux server gets filled up ( I would say it is common with virtual machine since we normally size the disk to fit only the OS). Moreover, system administrators need to find out what happened to get that disk filled up. A good and powerful tool is du command so I will show you how to use it.
#du -hs *
It will show you the disk usage on that directory.
#du -s *|sort -n
It will sort the output so you can see the largest directory or file on the bottom ...
With only this two commands you can easily find the largest directory or file.