zhao, forrest wrote: ..
But initial test result of running iozone with O_DIRECT option turned on didn't show the visible performance gain with NCQ. In certain cases, NCQ even had a worse performance than without NCQ. So my question is in what usage case can we observe the performance gain with NCQ?
That's something I've been wondering for a couple of years, ever since implementing full NCQ/TCQ Linux drivers for several devices (most notably the very fast qstor.c driver). The observation with all of thses was that Linux already does a reasonably good enough job of scheduling I/O that tagged-queuing rarely seems to help, at least on any benchmark/test tools we've found to try (note that opposite results are obtained when using non-Linux kernels, eg. winxp). With some drives, the use of tagged commands triggers different firmware algorithms, that adversely affect throughput in favour of better random seek capability -- but since the disk scheduling already minimizes the randomness of seeking (very few back-and-forth flurries), this combination often ends up slower than without NCQ (on Linux). Cheers - : send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
