Diff on linux with lots of files

This is a post to record difference methods on linux or wherever for large numbers of files (3m to 4m)

https://stackoverflow.com/questions/4997693/given-two-directory-trees-how-can-i-find-out-which-files-differ

A utility that is useful for such compares is xargs.  A tutorial is below

http://www.linuxplanet.com/linuxplanet/tutorials/6522/1