Root filesystem 100% full

[Robert La Ferla]

I have a server running 2.4.17 and the root filesystem / is 100% full. 
 I did a 'du -x / | sort -rn' to see what could be taking up the space 
but I don't see anything obvious.  In fact, I've cleaned up and moved 
some directories to other drives and it keeps filling up.  I thought it 
was some sort of log file but I can't find one that could be causing the 
problem.  I am running a huge multi-threaded process that I redirected 
to a file in /tmp.  I did delete the file by mistake.  Could that be the 
cause of this?  Any ideas?  Unfortunately, I can't reboot because I have 
to wait till that process completes which may take days!

Thanks,
Robert