From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] a few storage topics
Date: Mon, 23 Jan 2012 18:53:53 +0100
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To: Jan Kara <jack@suse.cz>
Cc: Boaz Harrosh <bharrosh@panasas.com>, Mike Snitzer <snitzer@redhat.com>, linux-scsi@vger.kernel.org, neilb@suse.de, dm-devel@redhat.com, linux-fsdevel@vger.kernel.org, lsf-pc@lists.linux-foundation.org, "Darrick J. Wong" <djwong@us.ibm.com>
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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).