disk usage
 

I am trying to find a tool or a way that will let me trace a process that is writting so much of data on an NFS mount (someone is abusing the disk usage by 1GB every 5 minutes). du is not an option cause this is an over 5TB system? any ideas or tool?

lsof /mountpoint

Will show you all processes that currently have files open on a given mountpoint.

You didn't mention whether you know the process writing to the NFS mount is a local process or a remote process. For local processes, you can try lsof to see what processes have open files under the mount point. For remote processes, you can try nfsstat -s -n to see if will give you per client statistics in order to see which client might be generating the data.

disk usage
 

yeah the process is local.

lsof will show you "local" processes using the (remote) NFS mount. That is to say you don't have to go to the server that exported the mount if you suspect another server that mounted the mount to be the culprit. You can run lsof on that other server mountpoint to see what processes have it open.

On the flip side you can NOT use lsof on the exporting server to tell you anything (other than the basic network connection information) from the other server. The "process" detail can only be obtained from the server where the "process" itself is running.