we've had terrible experience in particular versions of linux (i'd rather not specify which here), where a 'cp' running on the background for a few GB causes another process running a write() on the same partition to wait 5-20 seconds!. move from CFQ to deadline changed the formula a bit (to about 1.5 seconds to 8 seconds)!. I'm talking 8GB, 2 cpu, 3 GHz, Xeons (HT) running SCSI, and just 2 user processes (1 cp + 1 write()). talk about real-time linux. This is repeatable on any similar config.