What is the best way to keep two file systems in synch?  I need to run 
two machines in parallel while I upgrade and migrate to the new 
machine.  I have nfs mounted the old drive but I want to be able to 
mirror that drive on a local device and be able to make changes on 
either side with a program to automatically copy changed files on one 
side to the other.  (Two way mirroring).

Suggestions?