I say 60x because vmstat was reporting 700k to 1000k per second. Real throughput may or may not have been far worse. It looked like it was going to take hours so I stopped it after 45 minutes.
The track to track seek on this drive is ~0.8ms. And /sys/block/sdX/queue/nr_requests is at the default 128. (Changing it to 200,000 doesn't help.) At 128 one would expect one pass over the drive platter surface, read/writing random requests, to take less than 0.1 seconds and read/write 1 MB for a read/write total of ~ 10,000KB/s. This assumes all requests to be on different cylinders.
Clearly, however, Dave is incorrect about Mixed Workload being a random read/write test. There is no way I would be seeing 60,000KB/s throughput with a 16k record size on a dataset 2x the size of ram on such a workload. And in the 4k record case I would expect to see a lot better than I do.
Clearly there is something odd going on. I'm curious what it might be.