linux-kernel - Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

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Message-ID: <5322CC4D.8040509@suse.de>
Date:	Fri, 14 Mar 2014 10:30:53 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Christoph Hellwig <hch@...radead.org>,
	Mike Snitzer <snitzer@...hat.com>
CC:	Jeff Moyer <jmoyer@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Shaohua Li <shli@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

On 03/14/2014 10:25 AM, Christoph Hellwig wrote:
> On Thu, Mar 13, 2014 at 12:13:47PM -0400, Mike Snitzer wrote:
>> Pretty ironic that in the same email that you ask someone to "Let's make
>> this a little less personal." you start by asserting upstream
>> dm-multipath sees very little testing -- and use your commit that
>> recently broke dm-multipath as the basis.  Anyway, please exapnd on what
>> you feel is broken with upstream dm-multipath.
> 
> Getting a little upset, eh?  I didn't say it's broken, I said it gets
> very little testing.  The regression from me was found like so many
> before only after it was backported o some enterprise kernel.
> 
> I think the problem here is two-fold:
>  a) the hardware you use with dm-multipath isn't widely available.
>  b) it uses a very special code path in the block layer no one else uses
> 
> a) might be fixable by having some RDAC or similar emulation in qemu if
> someone wants to spend the effort.
> b) is a bit harder, but we should think hard about it when rewriting the
> multipath code to support blk-mq.  Talking about which I think trying to
> use dm-multipath on any blk-mq device will go horribly crash and boom at
> the moment.
> 
That was actually one of my plans, move dm-multipath over to use
blk-mq. But then I'd need to discuss with Jens et al how to best
achieve this; the current static hctx allocation doesn't play well
with multipaths dynamic path management.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@...e.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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