linux-kernel - Re: raid io requests not parallel?

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Message-ID: <17594.53980.497790.409035@cse.unsw.edu.au>
Date:	Mon, 17 Jul 2006 09:59:24 +1000
From:	Neil Brown <neilb@...e.de>
To:	"Jonathan Baccash" <jbaccash@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: raid io requests not parallel?

On Saturday July 15, jbaccash@...il.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.

NeilBrown



> MT Random read: 5.276 sec
> MT Random write: 19.839 sec
> 
> As expected, the multi-threaded reads are 2x as fast as single-threaded
> reads. But I would have expected (assuming the write to both disks can
> occur in parallel) that the random writes are about the same speed (10
> seconds) as the single-threaded random reads, for both the
> single-threaded and multi-threaded write cases. The fact that the
> multi-threaded reads were
> twice as fast indicates to me that read requests can occur in parallel.
> 
> So.... why doesn't the raid issue the writes in parallel? Thanks in
> advance for any help.
> 
> Jon.
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