Re: [PATCH] SATA NCQ support

[Jens Axboe <axboe@suse.de>]

On Mon, May 30 2005, Jens Axboe wrote:
> On Mon, May 30 2005, Jens Axboe wrote:
> > > People actually tend to report that IDE drives are *faster*. Until
> > > they're told they have to disable write-caching on their IDE drives to
> > > get a fair comparison, then the performance is absolutely abysmal. The
> > > interesting thing is that SCSI drives don't seem to take much of a
> > > performance hit from having write-caching disabled while IDE drives
> > > do.
> > 
> > NCQ will surely lessen the impact of disabling write caching, how much
> > still remains to be seen. You could test, if you have the hardware :)
> > Real life testing is more interesting than benchmarks.
> 
> With a few simple tests, I'm unable to show any write performance
> improvement with write back caching off and NCQ (NCQ with queueing depth
> of 1 and 16 tested). I get a steady 0.55-0.57MiB/sec with 8 threads
> random writes, a little over 5MiB/sec with sequential writes.
> 
> Reads are _much_ nicer. Sequential read with 8 threads are 23% faster
> with a queueing depth of 16 than 1, random reads are 85% (!!) faster at
> depth 16 than 1.
> 
> Testing was done with the noop io scheduler this time, to only show NCQ
> benefits outside of what the io scheduler can do for reordering.
> 
> This is with a Maxtor 7B300S0 drive. I would have posted results for a
> Seagate ST3120827AS as well, but that drive happily ignores any attempt
> to turn off write back caching. To top things off, it even accepts FUA
> writes but ignores that as well (they still go to cache).

Actually, I partly take that back. The Seagate _does_ honor drive write
back caching disable and it does show a nice improvement with NCQ for
that case. Results are as follows:

8 thread io case, 4kb block size, noop io scheduler, ST3120827AS.

Write cache off:

                Depth 1         Depth 30        Diff
Seq read:       18.94           21.51           +  14%
Ran read:        0.86            1.24           +  44%
Seq write:       6.58           19.30           + 193%
Ran write:       1.00            1.27           +  27%

Write cache on:

                Depth 1         Depth 30        Diff
Seq read:       18.78           21.58           +  15%
Ran read:        0.84            1.20           +  43%
Seq write:      24.49           23.26           -   5%
Ran write:       1.55            1.63           +   5%

Huge benefit on writes with NCQ when write back caching is off, with it
on I think the deviation is within standard jitter of this benchmark.


-- 
Jens Axboe