From mboxrd@z Thu Jan  1 00:00:00 1970
From: Chris Mason <chris.mason@oracle.com>
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] a few storage topics
Date: Tue, 24 Jan 2012 10:15:04 -0500
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Cc: Andrea Arcangeli <aarcange@redhat.com>, Jan Kara <jack@suse.cz>,
	Boaz Harrosh <bharrosh@panasas.com>,
	Mike Snitzer <snitzer@redhat.com>, linux-scsi@vger.kernel.org,
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	"Darrick J. Wong" <djwong@us.ibm.com>
To: Jeff Moyer <jmoyer@redhat.com>
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On Mon, Jan 23, 2012 at 01:28:08PM -0500, Jeff Moyer wrote:
> Andrea Arcangeli <aarcange@redhat.com> writes:
> 
> > On Mon, Jan 23, 2012 at 05:18:57PM +0100, Jan Kara wrote:
> >> requst granularity. Sure, big requests will take longer to complete but
> >> maximum request size is relatively low (512k by default) so writing maximum
> >> sized request isn't that much slower than writing 4k. So it works OK in
> >> practice.
> >
> > Totally unrelated to the writeback, but the merged big 512k requests
> > actually adds up some measurable I/O scheduler latencies and they in
> > turn slightly diminish the fairness that cfq could provide with
> > smaller max request size. Probably even more measurable with SSDs (but
> > then SSDs are even faster).
> 
> Are you speaking from experience?  If so, what workloads were negatively
> affected by merging, and how did you measure that?

https://lkml.org/lkml/2011/12/13/326

This patch is another example, although for a slight different reason.
I really have no idea yet what the right answer is in a generic sense,
but you don't need a 512K request to see higher latencies from merging.

-chris