> From: Richard Elling [mailto:richard.ell...@gmail.com] > Sent: Saturday, June 18, 2011 7:47 PM > > 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 :-(
Could you clarify what you mean by that? I was planning, in the near future, to go run iozone on some system with, and without the disk cache enabled according to format -e. If my hypothesis is right, it shouldn't significantly affect the IOPS, which seems to be corroborated by your message. I was also planning to perform sequential throughput testing on two disks simultaneously, with and without the disk cache enabled. If one disk is actually able to hog the bus in an idle state, it should mean the total combined throughput with cache disabled would be equal to a single disk. (Which I highly doubt.) > However the response time does [increase] :-( This comment seems to indicate that the drive queues up a whole bunch of requests, and since the queue is large, each individual response time has become large. It's not that physical actual performance has degraded with the cache enabled, it's that the queue has become long. For async writes, you don't really care how long the queue is, but if you have a mixture of async writes and occasional sync writes... Then the queue gets long, and when you sync, the sync operation will take a long time to complete. You might actually benefit by disabling the disk cache. Richard, have I gotten the gist of what you're saying? Incidentally, I have done extensive testing of enabling/disabling the HBA writeback cache. I found that as long as you have a dedicated log device for sync writes, your performance is significantly better by disabling the HBA writeback. Something on order of 15% better. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
