See Drive Capacity and Speed for why drives slow down across the capacity.
John L writes:
I just bought two new HGST 8TB NAS drives from OWC. I want to run fill-volume on both of them as new drives. This will be done using NewerTech Docks from OWC.
I assume that in order to run both of them at the same time, I will need to use the command line as I don’t see an option for multiple instances in the application.
Is that correct?
Will I still get the output in a format to put into the Excel spreadsheet for graphing?
I have one running in Terminal now: it’s writing 7.41 GB files! It is running about 200 MiB/s right now and a Voyager Q connected by eSATA.
MPG: drives can be tested across their capacity using the fill-volume command of diglloydTools DiskTester. The output pastes into the supplied spreadsheet to show drive performance (read and write) over the entire capacity. Testing is trivially invoked in Terminal, for a drive named Drive1:
disktester fill-volume Drive1
Many drives can be done simultaneously using one invocation per Terminal window. The only caveat is that if the drives are on the same bus, testing can only show full speed if the bus over which the testing is done has enough bandwidth to support all the drives at the same time (such as in an OWC Thunderbay 4). So for proper results, be sure not to overload the bus bandwidth while testing (also disable Spotlight and TimeMachine, at least on the drives to be tested). If the bus is overloaded, bus bandwidth will throttle/flatline performance on a hard drive, which is “wrong” (see the graph below for what it should look like).
With Thunderbolt 3 and suitable 6-bay enclosures which MPG hopes will arrive this year, as many as 13 fast hard drives could be simultaneously tested on just one TB3 bus.
The Voyager Q takes only one drive so that’s not an issue unless something else is using the bus—then the bus might limit performance—but in this case eSATA is one dedicated channel. If it were USB3, two hard drives might work, depending on the dock in use and whether other things were using bus bandwidth. The OWC Thunderbolt 2 Drive Dock is sufficient for full speed with two fast hard drives.
Particularly for RAID, it is wise to test all the drives and then overlay the graphs as seen below—one bad apple (so to speak) can bring the whole RAID performance down. Below, there is a tight grouping, though clearly there is one standout performer a little faster than the others. But MPG has seen up to 15% variance and that is not good if performance is a goal. Also, a drive with bad blocks may remap them, creating canyons and peaks of performance at inappropriate places.