linux-kernel - Re: [PATCH v4 08/11] block: kill merge_bvec

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Date:	Mon, 25 May 2015 17:08:27 +0200
From:	Christoph Hellwig <hch@....de>
To:	Ilya Dryomov <idryomov@...il.com>
Cc:	Ming Lin <mlin@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Jens Axboe <axboe@...nel.dk>, Dongsu Park <dpark@...teo.net>,
	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 Development <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,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Alex Elder <elder@...aro.org>
Subject: Re: [PATCH v4 08/11] block: kill merge_bvec_fn() completely

On Mon, May 25, 2015 at 06:02:30PM +0300, Ilya Dryomov wrote:
> I'm not Alex, but yeah, we have all the clone/split machinery and so we
> can handle a spanning case just fine.  I think rbd_merge_bvec() exists
> to make sure we don't have to do that unless it's really necessary -
> like when a single page gets submitted at an inconvenient offset.
> 
> I have a patch that adds a blk_queue_chunk_sectors(object_size) call to
> rbd_init_disk() but I haven't had a chance to play with it yet.  In any
> case, we should be fine with getting rid of rbd_merge_bvec().  If this
> ends up a per-driver patchset, I can make rbd_merge_bvec() ->
> blk_queue_chunk_sectors() a single patch and push it through
> ceph-client.git.

Hmm, looks like the new blk_queue_split_bio ignore the chunk_sectors
value, another thing that needs updating.  I forgot how many weird
merging hacks we had to add for nvme..

While I'd like to see per-driver patches we'd still need to merge
them together through the block tree.  Note that with this series
there won't be any benefit of using blk_queue_chunk_sectors over just
doing the split in rbd.  Maybe we can even remove it again and do
that work in the drivers in the future.
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