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Date: Fri, 29 May 2015 11:15:35 -0400
From: Mike Snitzer <snitzer@redhat.com>
To: Ming Lin <mlin@kernel.org>
Cc: lkml <linux-kernel@vger.kernel.org>, Christoph Hellwig <hch@lst.de>,
        Kent Overstreet <kent.overstreet@gmail.com>,
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Subject: Re: [PATCH v4 01/11] block: make generic_make_request handle
 arbitrarily sized bios
Message-ID: <20150529151534.GA27800@redhat.com>
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On Fri, May 29 2015 at  3:05P -0400,
Ming Lin <mlin@kernel.org> wrote:

> On Wed, May 27, 2015 at 5:36 PM, Alasdair G Kergon <agk@redhat.com> wrote:
> > On Wed, May 27, 2015 at 04:42:44PM -0700, Ming Lin wrote:
> >> Here are fio results of XFS on a DM stripped target with 2 SSDs + 1 HDD.
> >> Does it make sense?
> >
> > To stripe across devices with different characteristics?
> >
> > Some suggestions.
> >
> > Prepare 3 kernels.
> >   O - Old kernel.
> >   M - Old kernel with merge_bvec_fn disabled.
> 
> How to disable it?
> Maybe just hack it as below?
> 
> void blk_queue_merge_bvec(struct request_queue *q, merge_bvec_fn *mbfn)
> {
>         //q->merge_bvec_fn = mbfn;
> }

Right, there isn't an existing way to disable it, you'd need a hack like
that.