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
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