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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