linux-kernel - Re: [PATCH 2/5] dm: implement REQ_FLUSH/FUA support for bio-based dm

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Message-ID: <20100901135433.GB25251@redhat.com>
Date:	Wed, 1 Sep 2010 09:54:34 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jaxboe@...ionio.com, k-ueda@...jp.nec.com, j-nomura@...jp.nec.com,
	jamie@...reable.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-raid@...r.kernel.org,
	hch@....de, dm-devel@...hat.com
Subject: Re: [PATCH 2/5] dm: implement REQ_FLUSH/FUA support for bio-based dm

On Wed, Sep 01 2010 at  9:50am -0400,
Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> On 09/01/2010 03:43 PM, Mike Snitzer wrote:
> > On Mon, Aug 30 2010 at  5:58am -0400,
> > Tejun Heo <tj@...nel.org> wrote:
> > 
> >> This patch converts bio-based dm to support REQ_FLUSH/FUA instead of
> >> now deprecated REQ_HARDBARRIER.
> >>
> >> * -EOPNOTSUPP handling logic dropped.
> > 
> > Can you expand on _why_ -EOPNOTSUPP handling is no longer needed?  And
> > please at it to the final patch header.
> 
> It just doesn't happen anymore.  If the underlying device doesn't
> support FLUSH/FUA, the block layer simply make those parts noop.  IOW,
> it no longer distinguishes between writeback cache which doesn't
> support cache flush at all and writethrough cache.  Devices which have
> WB cache w/o flush very difficult to come by these days and there's
> nothing much we can do anyway, so it doesn't make sense to require
> everyone to implement -EOPNOTSUPP.
> 
> One scheduled feature is to implement falling back to REQ_FLUSH when
> the device advertises REQ_FUA but fails to process it, but one way or
> the other, the goal is encapsulating REQ_FLUSH/FUA support in block
> layer proper.  If FLUSH/FUA can be retried using a different strategy,
> it should be done inside request_queue proper instead of pushing retry
> logic to all its users.

OK, so maybe add this info to the patch header one of the primary
FLUSH+FUA conversion patches?

Thanks for the detailed explanation!

Mike
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