linux-kernel - Re: [PATCH 07/10] block: kill merge_bvec

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Message-ID: <20150428173137.GC12975@agk-dp.fab.redhat.com>
Date:	Tue, 28 Apr 2015 18:31:37 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Ming Lin <mlin@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Dongsu Park <dongsu.park@...fitbricks.com>,
	Lars Ellenberg <drbd-dev@...ts.linbit.com>,
	drbd-user@...ts.linbit.com, Jiri Kosina <jkosina@...e.cz>,
	Yehuda Sadeh <yehuda@...tank.com>,
	Sage Weil <sage@...tank.com>, Alex Elder <elder@...nel.org>,
	ceph-devel@...r.kernel.org, Alasdair Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
	Neil Brown <neilb@...e.de>, linux-raid@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH 07/10] block: kill merge_bvec_fn() completely

On Mon, Apr 27, 2015 at 11:48:34PM -0700, Ming Lin wrote:
> As generic_make_request() is now able to handle arbitrarily sized bios,
> it's no longer necessary for each individual block driver to define its
> own ->merge_bvec_fn() callback. Remove every invocation completely.

merge_bvec_fn is also about telling the code building up bios what optimum
maximum size to use.
 
Have you got some test data to demonstrate that we won't find some situations
where performance is now lost by code building up bios that are far too big
and are always going to have to be split up later?

Alasdair

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