linux-kernel - Re: [Drbd-dev] FLUSH/FUA documentation & code discrepancy

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Message-ID: <20120906212952.GP29092@google.com>
Date:	Thu, 6 Sep 2012 14:29:52 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Philipp Reisner <philipp.reisner@...bit.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@....de>, drbd-dev@...ts.linbit.com
Subject: Re: [Drbd-dev] FLUSH/FUA documentation & code discrepancy

Hello,

On Wed, Sep 05, 2012 at 12:07:24PM +0200, Lars Ellenberg wrote:
> So reiterating the situation:
> 
> If I'd submit a non-empty bio with FLUSH/FUA set,
> on a queue that does support flush, we get to
> 	blk_queue_bio()
> 		if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
> 			spin_lock_irq(q->queue_lock);
> 			where = ELEVATOR_INSERT_FLUSH;
> 			goto get_rq;
> 
> This bio ends up *not* being merged or reordered by the elevator.
> (and, by means of flush/fua not by the hardware, either, obviously)
> 
> If the queue does not support it, flags are stripped away in
> generic_make_request_checks(), and we will not take that branch
> in blk_queue_bio(), but enter the normal elevator code path,
> attempting a merge, or doing ELEVATOR_INSERT_SORT.

which is an implementation detail.

> This same bio, happening to be submitted on a different IO stack,
> now *is* being reordered in the elevator already,
> even before being sent to the hardware.

and this is perfectly fine.

I really don't see what problem you're trying to solve here.  The
ordering requirement is weak.  Certain implementation path uses
stronger requirement for convenience / historical reasons.  If any
change makes sense, it's relaxing the unnecessarily strict ordering if
possible.

What actual problem are you seeing?

Thanks.

-- 
tejun
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