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Date: Wed, 27 May 2015 10:20:18 +0200
From: Christoph Hellwig <hch@lst.de>
To: device-mapper development <dm-devel@redhat.com>,
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Subject: Re: [Drbd-dev] [dm-devel] [PATCH v4 01/11] block: make
	generic_make_request	handle arbitrarily sized bios
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On Wed, May 27, 2015 at 01:40:22AM +0100, Alasdair G Kergon wrote:
> It does sometimes make a significant difference to device-mapper stacks.
> DM only uses it for performance reasons - it can already split bios when
> it needs to.  I tried to remove merge_bvec_fn from DM several years ago but
> couldn't because of the adverse performance impact of lots of splitting activity.

Does it still?  Since the move to immutable biovecs the bio splits are
pretty cheap now, but I'd really like to see this verified by benchmarks.