From: Tejun Heo <tj@kernel.org>
To: Philipp Reisner <philipp.reisner@linbit.com>,
Jens Axboe <axboe@kernel.dk>,
linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
drbd-dev@lists.linbit.com
Subject: Re: [Drbd-dev] FLUSH/FUA documentation & code discrepancy
Date: Thu, 6 Sep 2012 14:29:52 -0700 [thread overview]
Message-ID: <20120906212952.GP29092@google.com> (raw)
In-Reply-To: <20120905100724.GA27527@soda.linbit>
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
next prev parent reply other threads:[~2012-09-06 21:30 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-09-04 12:32 FLUSH/FUA documentation & code discrepancy Philipp Reisner
2012-09-04 22:46 ` Tejun Heo
2012-09-05 8:44 ` [Drbd-dev] " Philipp Reisner
2012-09-05 8:49 ` Tejun Heo
2012-09-05 10:07 ` Lars Ellenberg
2012-09-06 21:29 ` Tejun Heo [this message]
2012-09-07 8:42 ` Lars Ellenberg
2012-09-10 22:54 ` Tejun Heo
2012-09-10 23:06 ` Tejun Heo
2012-09-10 23:12 ` Kent Overstreet
2012-09-10 23:31 ` Kent Overstreet
2012-09-11 5:58 ` NeilBrown
2012-09-11 8:25 ` Lars Ellenberg
2012-09-11 14:41 ` Vivek Goyal
2012-09-12 18:58 ` Tejun Heo
2012-09-12 23:12 ` Joseph Glanville
2012-09-12 23:20 ` Tejun Heo
2012-09-12 23:53 ` Joseph Glanville
2012-09-13 0:17 ` Joseph Glanville
2012-09-13 3:10 ` Joseph Glanville
2012-09-13 19:25 ` Tejun Heo
2012-09-11 14:34 ` Vivek Goyal
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