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SubjectRe: [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@us.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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