linux-kernel - Re: [PATCH] md: Call blk_queue_flush() to establish flush/fua support

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Message-ID: <20101123012836.GV14383@tux1.beaverton.ibm.com>
Date:	Mon, 22 Nov 2010 17:28:36 -0800
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Neil Brown <neilb@...e.de>
Cc:	linux-raid@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] md: Call blk_queue_flush() to establish flush/fua
	support

On Tue, Nov 23, 2010 at 10:50:00AM +1100, Neil Brown wrote:
> On Mon, 22 Nov 2010 15:22:08 -0800
> "Darrick J. Wong" <djwong@...ibm.com> wrote:
> 
> > Before 2.6.37, the md layer had a mechanism for catching I/Os with the barrier
> > flag set, and translating the barrier into barriers for all the underlying
> > devices.  With 2.6.37, I/O barriers have become plain old flushes, and the md
> > code was updated to reflect this.  However, one piece was left out -- the md
> > layer does not tell the block layer that it supports flushes or FUA access at
> > all, which results in md silently dropping flush requests.
> > 
> > Since the support already seems there, just add this one piece of bookkeeping
> > to restore the ability to flush writes through md.
> 
> I would rather just unconditionally call
>    blk_queue_flush(mddev->queue, REQ_FLUSH | REQ_FUA);
> 
> I don't think there is much to be gained by trying to track exactly what the
> underlying devices support, and as the devices can change, that is racy
> anyway.
> 
> Thoughts?

I don't think there's anything that would get confused by an md that advertises
flush/fua support when any of the underlying devices don't support it, but that
was the only reason why I didn't just code it up your way to start with. :)

None of the in-kernel code checks queue->flush_flags except the block layer
itself, and the block layer silently strips off REQ_FLUSH/REQ_FUA if the device
doesn't support it.  I'm not sure I like /that/ behavior, but at the moment I
have no objection.  dm hardcodes the flags on as well.

--D
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