linux-kernel - Re: [PATCH v3 14/16] Gut bio_add

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Date:	Mon, 28 May 2012 23:36:28 -0400
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Dave Chinner <david@...morbit.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Alasdair G Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>,
	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	dm-devel@...hat.com, linux-fsdevel@...r.kernel.org,
	axboe@...nel.dk, yehuda@...newdream.net, vgoyal@...hat.com,
	bharrosh@...asas.com, sage@...dream.net, drbd-dev@...ts.linbit.com,
	Dave Chinner <dchinner@...hat.com>, tytso@...gle.com,
	"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH v3 14/16] Gut bio_add_page()

On Tue, May 29, 2012 at 11:15:58AM +0900, Tejun Heo wrote:
> Hello,
> 
> On Tue, May 29, 2012 at 12:08:15PM +1000, Dave Chinner wrote:
> > > i thought a bit more about it and the only thing which makes sense to
> > > me is exposing the stripe granularity for striped devices -
> > > ie. something which says "if you go across this boundary, the
> > > performance characteristics including latency might get affected",
> > > which should fit nicely with the rest of topology information.
> > > Martin, adding that shouldn't be difficult, right?
> > 
> > We already have the optimal IO size/alignment field in the topology.
> > Doesn't this fit what you want exactly?
> 
> I don't know how xfs/ext4 is currently benefiting from
> merge_bvec_fn(), so I'm unsure.  If the existing ones are enough,
> great.

Excepting readahead I don't think they are at all.

For readahead all we need is a hint - call it "atomic IO size" or
something. Assuming one of the gazillion things in queue_limits doesn't
already mean that anyways.
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