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From: Ming Lei <ming.lei@redhat.com>
To: Keith Busch <kbusch@kernel.org>
Cc: axboe@kernel.dk, martin.petersen@oracle.com, jdorminy@redhat.com,
	Mike Snitzer <snitzer@redhat.com>,
	bjohnsto@redhat.com, linux-block@vger.kernel.org,
	dm-devel@redhat.com
Subject: Re: [dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking
Date: Fri, 4 Dec 2020 09:45:35 +0800	[thread overview]
Message-ID: <20201204014535.GC661914@T590> (raw)
In-Reply-To: <20201203162738.GA3404013@dhcp-10-100-145-180.wdc.com>

On Thu, Dec 03, 2020 at 08:27:38AM -0800, Keith Busch wrote:
> On Thu, Dec 03, 2020 at 09:33:59AM -0500, Mike Snitzer wrote:
> > On Wed, Dec 02 2020 at 10:26pm -0500,
> > Ming Lei <ming.lei@redhat.com> wrote:
> > 
> > > I understand it isn't related with correctness, because the underlying
> > > queue can split by its own chunk_sectors limit further. So is the issue
> > > too many further-splitting on queue with chunk_sectors 8? then CPU
> > > utilization is increased? Or other issue?
> > 
> > No, this is all about correctness.
> > 
> > Seems you're confining the definition of the possible stacking so that
> > the top-level device isn't allowed to have its own hard requirements on
> > IO sizes it sends to its internal implementation.  Just because the
> > underlying device can split further doesn't mean that the top-level
> > virtual driver can service larger IO sizes (not if the chunk_sectors
> > stacking throws away the hint the virtual driver provided because it
> > used lcm_not_zero).
> 
> I may be missing something obvious here, but if the lower layers split
> to their desired boundary already, why does this limit need to stack?
> Won't it also work if each layer sets their desired chunk_sectors
> without considering their lower layers? The commit that initially
> stacked chunk_sectors doesn't provide any explanation.

There could be several reasons:

1) some limits have to be stacking, such as logical block size, because
lower layering may not handle un-aligned IO

2) performance reason, if every limits are stacked on topmost layer, in
theory IO just needs to be splitted in top layer, and not need to be
splitted further from all lower layer at all. But there should be exceptions
in unusual case, such as, lowering queue's limit changed after the stacking
limits are setup.

3) history reason, bio splitting is much younger than stacking queue
limits.

Maybe others?


Thanks,
Ming

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  parent reply	other threads:[~2020-12-04  1:46 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-30 17:18 [dm-devel] [PATCH] block: revert to using min_not_zero() when stacking chunk_sectors Mike Snitzer
2020-11-30 20:51 ` John Dorminy
2020-11-30 23:24   ` [dm-devel] " Mike Snitzer
2020-12-01  0:21     ` John Dorminy
2020-12-01  2:12       ` Mike Snitzer
2020-12-01 16:07 ` [dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking Mike Snitzer
2020-12-01 17:43   ` John Dorminy
2020-12-01 17:53   ` Jens Axboe
2020-12-01 18:02   ` Martin K. Petersen
2020-12-02  3:38   ` [dm-devel] [PATCH] dm: " Jeffle Xu
2020-12-02  3:38     ` Jeffle Xu
2020-12-02  3:57       ` JeffleXu
2020-12-02  5:03         ` [dm-devel] " Mike Snitzer
2020-12-02  5:14           ` Mike Snitzer
2020-12-02  6:31             ` JeffleXu
2020-12-02  6:35               ` JeffleXu
2020-12-02  6:28           ` JeffleXu
2020-12-02  7:10           ` JeffleXu
2020-12-02 15:11             ` Mike Snitzer
2020-12-03  1:48               ` JeffleXu
2020-12-03  3:26   ` [dm-devel] [PATCH v2] block: " Ming Lei
2020-12-03 14:33     ` Mike Snitzer
2020-12-03 16:27       ` Keith Busch
2020-12-03 17:56         ` Mike Snitzer
2020-12-04  1:45         ` Ming Lei [this message]
2020-12-04  2:11           ` Mike Snitzer
2020-12-04  6:22             ` Damien Le Moal
2020-12-04  1:12       ` Ming Lei
2020-12-04  2:03         ` Mike Snitzer
2020-12-04  3:59           ` Ming Lei
2020-12-04 16:47             ` Mike Snitzer
2020-12-04 17:32               ` [dm-devel] [RFC PATCH] dm: fix IO splitting [was: Re: [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking] Mike Snitzer
2020-12-04 17:49                 ` Mike Snitzer

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