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Re: [PATCH] MD: make bio mergeable

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  Re: [PATCH] MD: make bio mergeable Holger Kiehl <Holger.Kiehl@dwd.de> - 2016-04-28 22:10 +0200
    Re: [PATCH] MD: make bio mergeable Shaohua Li <shli@fb.com> - 2016-04-28 23:20 +0200
      Re: [PATCH] MD: make bio mergeable Holger Kiehl <Holger.Kiehl@dwd.de> - 2016-04-29 11:30 +0200

#1390488 — Re: [PATCH] MD: make bio mergeable

Hello,

On Mon, 25 Apr 2016, Shaohua Li wrote:

> blk_queue_split marks bio unmergeable, which makes sense for normal bio.
> But if dispatching the bio to underlayer disk, the blk_queue_split
> checks are invalid, hence it's possible the bio becomes mergeable.
> 
> In the reported bug, this bug causes trim against raid0 performance slash
> https://bugzilla.kernel.org/show_bug.cgi?id=117051
> 
This patch makes a huge difference. On a system with two Samsung 850 Pro
in a MD Raid0 setup the time for fstrim went down from ~30min to 18sec!

However, on another system with two Intel P3700 1.6TB NVMe PCIe SSD's
also setup as one big MD Raid0, the patch does not make any difference
at all. fstrim takes more then 4 hours!

Any idea what could be wrong?

Regards,
Holger


> Reported-by: Park Ju Hyung <qkrwngud825@gmail.com>
> Fixes: 6ac45aeb6bca(block: avoid to merge splitted bio)
> Cc: stable@vger.kernel.org (v4.3+)
> Cc: Ming Lei <ming.lei@canonical.com>
> Cc: Jens Axboe <axboe@fb.com>
> Cc: Neil Brown <neilb@suse.de>
> Signed-off-by: Shaohua Li <shli@fb.com>
> ---
>  drivers/md/md.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 194580f..14d3b37 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -284,6 +284,8 @@ static blk_qc_t md_make_request(struct request_queue *q, struct bio *bio)
>  	 * go away inside make_request
>  	 */
>  	sectors = bio_sectors(bio);
> +	/* bio could be mergeable after passing to underlayer */
> +	bio->bi_rw &= ~REQ_NOMERGE;
>  	mddev->pers->make_request(mddev, bio);
>  
>  	cpu = part_stat_lock();
> -- 
> 2.8.0.rc2
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

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#1390540

On Thu, Apr 28, 2016 at 08:00:22PM +0000, Holger Kiehl wrote:
> Hello,
> 
> On Mon, 25 Apr 2016, Shaohua Li wrote:
> 
> > blk_queue_split marks bio unmergeable, which makes sense for normal bio.
> > But if dispatching the bio to underlayer disk, the blk_queue_split
> > checks are invalid, hence it's possible the bio becomes mergeable.
> > 
> > In the reported bug, this bug causes trim against raid0 performance slash
> > https://bugzilla.kernel.org/show_bug.cgi?id=117051
> > 
> This patch makes a huge difference. On a system with two Samsung 850 Pro
> in a MD Raid0 setup the time for fstrim went down from ~30min to 18sec!
> 
> However, on another system with two Intel P3700 1.6TB NVMe PCIe SSD's
> also setup as one big MD Raid0, the patch does not make any difference
> at all. fstrim takes more then 4 hours!

Does the raid0 cross two partitions or two SSD?

can you post blktrace data in the bugzilloa, I'll track the bug there.

Thanks,
Shaohua

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#1390923

On Thu, 28 Apr 2016, Shaohua Li wrote:

> On Thu, Apr 28, 2016 at 08:00:22PM +0000, Holger Kiehl wrote:
> > Hello,
> > 
> > On Mon, 25 Apr 2016, Shaohua Li wrote:
> > 
> > > blk_queue_split marks bio unmergeable, which makes sense for normal bio.
> > > But if dispatching the bio to underlayer disk, the blk_queue_split
> > > checks are invalid, hence it's possible the bio becomes mergeable.
> > > 
> > > In the reported bug, this bug causes trim against raid0 performance slash
> > > https://bugzilla.kernel.org/show_bug.cgi?id=117051
> > > 
> > This patch makes a huge difference. On a system with two Samsung 850 Pro
> > in a MD Raid0 setup the time for fstrim went down from ~30min to 18sec!
> > 
> > However, on another system with two Intel P3700 1.6TB NVMe PCIe SSD's
> > also setup as one big MD Raid0, the patch does not make any difference
> > at all. fstrim takes more then 4 hours!
> 
> Does the raid0 cross two partitions or two SSD?
> 
Two SSD's. Where it works, for the two Samsung 850 Pro SATA SSD it was
via partitions.

> can you post blktrace data in the bugzilloa, I'll track the bug there.
> 
I did the blktrace on the two md raid0 devices /dev/nvme[01]n1 for 2 minutes
and attached them to the bug 117051 as a tar.bz2 file:

   https://bugzilla.kernel.org/show_bug.cgi?id=117051

Please just ask if I have forgotten anything. And many thanks for looking
at this and all the good work!

Regards,
Holger

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