Richard Elling wrote:
Actually, all of the data I've gathered recently shows that the number of IOPS does not significantly increase for HDDs running random workloads. However the response time does :-( My data is leading me to want to restrict the queue depth to 1 or 2 for HDDs.
Thinking out loud here, but if you can queue up enough random I/Os, the embedded disk controller can probably do a good job reordering them into less random elevator sweep pattern, and increase IOPs through reducing the total seek time, which may be why IOPs does not drop as much as one might imagine if you think of the heads doing random seeks (they aren't random anymore). However, this requires that there's a reasonable queue of I/Os for the controller to optimise, and processing that queue will necessarily increase the average response time. If you run with a queue depth of 1 or 2, the controller can't do this.
This is something I played with ~30 years ago, when the OS disk driver was responsible for the queuing and reordering disc transfers to reduce total seek time, and disk controllers were dumb. There are lots of options and compromises, generally weighing reduction in total seek time against longest response time. Best reduction in total seek time comes from planning out your elevator sweep, and inserting newly queued requests into the right position in the sweep ahead. That also gives the potentially worse response time, as you may have one transfer queued for the far end of the disk, whilst you keep getting new transfers queued for the track just in front of you, and you might end up reading or writing the whole disk before you get to do that transfer which is queued for the far end. If you can get a big enough queue, you can modify the insertion algorithm to never insert into the current sweep, so you are effectively planning two sweeps ahead. Then the worse response time becomes the time to process one queue full, rather than the time to read or write the whole disk. Lots of other tricks too (e.g. insertion into sweeps taking into account priority, such as if the I/O is a synchronous or asynchronous, and age of existing queue entries). I had much fun playing with this at the time.
-- Andrew Gabriel _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
