Re: [PATCH] SATA NCQ support

[Jens Axboe <axboe@suse.de>]

On Mon, May 30 2005, Greg Stark wrote:
> > ATA has a limitation of 32 tags, so queued write cache off won't beat
> > unqueued write cache on in any modern drive.
> 
> People earlier were quoting 30-40% gains with NCQ enabled. I assumed
> those were with the same drive in otherwise the same configuration,
> presumably with write-caching enabled.

If you are talking about the numbers I quoted, those were for random
read performance.

> Without any form of command queueing write-caching imposes a severe
> performance loss, the question is how much of that loss is erased when
> NCQ is present.

I'll try some random write tests with write caching disabled.

> 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.

-- 
Jens Axboe