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Date: Wed, 26 Feb 2003 13:17:21 +0100
From: Helge Hafting <helgehaf@aitel.hist.no>
Organization: AITeL, HiST
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To: Andrew Morton <akpm@digeo.com>
CC: Patrick Mansfield <patmans@us.ibm.com>, linux-kernel@vger.kernel.org
Subject: Re: 2.5.62-mm2 slow file system writes across multiple disks
References: <20030224120304.A29472@beaverton.ibm.com>	<20030224135323.28bb2018.akpm@digeo.com>	<20030224174731.A31454@beaverton.ibm.com>	<20030224204321.5016ded6.akpm@digeo.com>	<20030225112458.A5618@beaverton.ibm.com> <20030226004454.2bfd8deb.akpm@digeo.com>
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Andrew Morton wrote:
> Patrick Mansfield <patmans@us.ibm.com> wrote:
[...]
> 
>>The larger queue depths can be nice for disk arrays with lots of cache and
>>(more) random IO patterns.
> 
> 
> So says the scsi lore ;)  Have you observed this yourself?  Have you
> any numbers handy?

I believe deep queues might work well if the drives did their own
anticipatory scheduling.  After all, they know both true geometry and
excact rotational position and latency.
But current drives aren't that good so all the deep queue achieves is
extra seeks which sometimes kills performance.  The fix is simple -
shorten the queues.  Long ones aren't really a goal by itself -
performance is.

Helge Hafting

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