From: Mike Snitzer <snitzer@redhat.com>
To: Ming Lei <ming.lei@redhat.com>, hare@suse.de
Cc: axboe@kernel.dk, martin.petersen@oracle.com, jdorminy@redhat.com,
bjohnsto@redhat.com, linux-block@vger.kernel.org,
dm-devel@redhat.com, Keith Busch <kbusch@kernel.org>
Subject: Re: [dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking
Date: Thu, 3 Dec 2020 21:11:08 -0500 [thread overview]
Message-ID: <20201204021108.GB32150@redhat.com> (raw)
In-Reply-To: <20201204014535.GC661914@T590>
On Thu, Dec 03 2020 at 8:45pm -0500,
Ming Lei <ming.lei@redhat.com> wrote:
> 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?
Hannes didn't actually justify why he added chunk_sectors to
blk_stack_limits:
commit 987b3b26eb7b19960160505faf9b2f50ae77e14d
Author: Hannes Reinecke <hare@suse.de>
Date: Tue Oct 18 15:40:31 2016 +0900
block: update chunk_sectors in blk_stack_limits()
Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Damien Le Moal <damien.lemoal@hgst.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@seagate.com>
Tested-by: Shaun Tancheff <shaun.tancheff@seagate.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Likely felt it needed for zoned or NVMe devices.. dunno.
But given how we now have a model where block core, or DM core, will
split as needed I don't think normalizing chunk_sectors (to the degree
full use of blk_stack_limits does) and than using it as basis for
splitting makes a lot of sense.
Mike
--
dm-devel mailing list
dm-devel@redhat.com
https://www.redhat.com/mailman/listinfo/dm-devel
next prev parent reply other threads:[~2020-12-04 2:11 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
2020-12-04 2:11 ` Mike Snitzer [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20201204021108.GB32150@redhat.com \
--to=snitzer@redhat.com \
--cc=axboe@kernel.dk \
--cc=bjohnsto@redhat.com \
--cc=dm-devel@redhat.com \
--cc=hare@suse.de \
--cc=jdorminy@redhat.com \
--cc=kbusch@kernel.org \
--cc=linux-block@vger.kernel.org \
--cc=martin.petersen@oracle.com \
--cc=ming.lei@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).