linux-kernel - Re: absurdly high "optimal_io

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Date:	Fri, 7 Nov 2014 10:18:46 -0500
From:	worley@...m.mit.edu (Dale R. Worley)
To:	Chris Friesen <chris.friesen@...driver.com>
CC:	martin.petersen@...cle.com, axboe@...nel.dk,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	snitzer@...hat.com
Subject: Re: absurdly high "optimal_io_size" on Seagate SAS disk

> From: Chris Friesen <chris.friesen@...driver.com>

> Also, I think it's wrong for filesystems and userspace to use it for 
> alignment.  In E.4 and E.5 in the "sbc3r25.pdf" doc, it looks like they 
> use the optimal granularity field for alignment, not the optimal 
> transfer length.

Everything you say suggests that "optimal transfer length" means
"there is a penalty for doing transfers *larger* than this", but
people have been treating it as "there is a penalty for doing
transfers *smaller* than this".  But the latter is the "optimal
transfer length granularity".

Dale
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