When I copy a file foo to foo2 I get a transfer rate of X. If I then copy foo to foo3 and simultaneously copy foo2 to foo4 the transfer rates drop to about X/2 for both processes. I have not even come close to maxing out the IOPS from my SAN so I feel it must have something to do with how cp is written. I have tried dd with similar results. I can see in top that I have two cp processes running but is it really two threads or are the two processes sharing some kind of resource back and fourth that slows both down? Is there anything I can do to increase this?