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

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Message-ID: <20140314124457.GA4859@infradead.org>
Date:	Fri, 14 Mar 2014 05:44:57 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Hannes Reinecke <hare@...e.de>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Mike Snitzer <snitzer@...hat.com>,
	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 Fri, Mar 14, 2014 at 10:30:53AM +0100, Hannes Reinecke wrote:
> 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.

I'd say it the other way around: the clone + insert hacks in
dm-multipath don't work well with blk-mq.  Not allowing non-owned
requests is fundamentally part of the blk-mq model to allow things
like the integrated tag allocator and queue depth limiting or the
preallocated driver specific data.

Instead dm-multipath should alway resubmit the request like it already
does for the slow path for the first step.  Longer term we might be able
to operate using a cheaper temporary structure, but I'm not sure that's
going to be worth it.
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