Re: raid io requests not parallel?

[Jens Axboe <axboe@suse.de>]

On Mon, Jul 17 2006, Neil Brown wrote:
> On Saturday July 15, jbaccash@gmail.com wrote:
> > I'm using kernel linux-2.6.15-gentoo-r1, and I noticed performance of
> > the software RAID-1 is not as good as I would have expected on my two
> > SATA drives, and I was wondering if anyone has an idea what may be
> > happening. The test I run is 1024 16k direct-IO reads/writes from
> > random locations within a 1GB file (on a RAID-1 partition), with my
> > disk caches set to
> > write-through mode. In the MT (multi-threaded) case, I issue them from
> > 8 threads (so it's 128 requests per thread):
> > 
> > Random read: 10.295 sec
> > Random write: 19.142 sec
> 
> Odd.  I would expect these two numbers to be a lot closer together.
> 
> Try changing the IO scheduler on the drives and see if it makes a
> difference.
> e.g.
>   cat /sys/block/XXX/queue/scheduler
>   echo cfq > /sys/block/XXX/queue/scheduler
>   echo deadline > /sys/block/XXX/queue/scheduler
> 
> See what works best.

His cache is set to write through, 16kb direct writes in that case will
be a lot slower than the equivalent reads. 10 vs 20 seconds does not
sounds out of the question.

-- 
Jens Axboe