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

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Message-ID: <20120529033434.GC10175@dhcp-172-18-216-138.mtv.corp.google.com>
Date:	Mon, 28 May 2012 23:34:34 -0400
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	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, mpatocka@...hat.com, vgoyal@...hat.com,
	bharrosh@...asas.com, tj@...nel.org, sage@...dream.net,
	agk@...hat.com, drbd-dev@...ts.linbit.com,
	Dave Chinner <dchinner@...hat.com>, tytso@...gle.com
Subject: Re: [PATCH v3 14/16] Gut bio_add_page()

On Tue, May 29, 2012 at 11:54:38AM +1000, Dave Chinner wrote:
> It also allowed us to build IOs that span
> entire RAID stripe widths, thereby avoiding potential RAID RMW
> cycles, and even allowing high end raid controllers to trigger BBWC
> bypass fast paths that could double or triple the write throughput
> of the arrays...

merge_bvec_fn has nothing to do with that though, since for one there
aren't any merge_bvec_fn's being called in the IO paths on these high
end disk arrays, and for our software raid implementations their
merge_bvec_fns will keep you from sending them bios that span entire
stripes.
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