Re: A question about NCQ

[Mark Lord <liml@rtr.ca>]

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