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Date:	Sat, 15 Jul 2006 19:41:46 -0600
From:	Robert Hancock <>
To:	Jonathan Baccash <>
Subject: Re: raid io requests not parallel?

Jonathan Baccash 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
> 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.

Because it's RAID 1, I'm assuming. The data has to be written to both 
drives, unlike reads which can be satisfied from either one.

Robert Hancock      Saskatoon, SK, Canada
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