I have installed 4 brand-new SATA hard drives in a mac pro. My OS is running on a fifth drive (SSD). I run this command
pv /dev/zero -tpreb | sudo dd of=/dev/rdisk1 bs=512k
for many hours to stress test the new drives. Right away it maxes out the drive write speed at 170MBps. I open a second terminal and run the command on the second drive simultaneously, and they both go around 170MBps. But as I add the third and fourth drives, the speed drops to 135MBps each. This makes me believe that the read speed of /dev/zero is the limiting factor, but why? I have 28GB free memory, and my CPU is hardly being used.