Re: [PATCH] SATA NCQ support

[Jens Axboe]

On Mon, May 30 2005, Jens Axboe wrote:
> 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.

The Maxtor drive shipped with write back caching off, I actually knew
and forgot that... So that of course changes the picture, same test as
the Seagate above run on the Maxtor:

Write cache off:

                Depth 1         Depth 30        Diff
Seq read:       19.83           22.46           + 13%
Ran read:        0.73            1.33           + 82%
Seq write:      10.51            5.65           - 47%
Ran write:       0.55            0.56           +  1%

Write cache on:

                Depth 1         Depth 30        Diff
Seq read:       19.83           34.35           + 73%
Ran read:        0.86            1.54           + 79%
Seq write:      25.82           35.21           + 36%
Ran write:       3.12            3.27           +  5%

Still something fishy going on. Eric, this is with both B0 and BM
firmware on these drives, any known bugs there?

-- 
Jens Axboe

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