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.

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    How many buses are involved? I'd guess that the limit you've reached is due to the bus. – Ashley Sep 1 '13 at 17:31
  • That's a good question. Anyone know how many internal SATA busses are in a 2010 Mac Pro? Each SATA II bus should be 375MBps, right? But I'm getting a total of ~530MBps. – Elliott Sep 1 '13 at 21:28

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