linux-kernel - Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

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Date:	Mon, 15 Jun 2015 16:03:25 +0300
From:	Ilya Dryomov <idryomov@...il.com>
To:	juncheng bai <baijuncheng@...tedstack.com>
Cc:	idryomov@...hat.com, Alex Elder <elder@...aro.org>,
	Josh Durgin <josh.durgin@...tank.com>, lucienchao@...il.com,
	jeff@...zik.org, yehuda@...newdream.net,
	Sage Weil <sage@...dream.net>, elder@...tank.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ceph Development <ceph-devel@...r.kernel.org>
Subject: Re: [PATCH RFC] storage:rbd: make the size of request is equal to
 the, size of the object

On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
<baijuncheng@...tedstack.com> wrote:
> From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00 2001
> From: juncheng bai <baijuncheng@...tedstack.com>
> Date: Mon, 15 Jun 2015 18:34:00 +0800
> Subject: [PATCH] storage:rbd: make the size of request is equal to the
>  size of the object
>
> ensures that the merged size of request can achieve the size of
> the object.
> when merge a bio to request or merge a request to request, the
> sum of the segment number of the current request and the segment
> number of the bio is not greater than the max segments of the request,
> so the max size of request is 512k if the max segments of request is
> BLK_MAX_SEGMENTS.
>
> Signed-off-by: juncheng bai <baijuncheng@...tedstack.com>
> ---
>  drivers/block/rbd.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
> index 0a54c58..dec6045 100644
> --- a/drivers/block/rbd.c
> +++ b/drivers/block/rbd.c
> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
>         segment_size = rbd_obj_bytes(&rbd_dev->header);
>         blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>         blk_queue_max_segment_size(q, segment_size);
> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>         blk_queue_io_min(q, segment_size);
>         blk_queue_io_opt(q, segment_size);

I made a similar patch on Friday, investigating blk-mq plugging issue
reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
AFAIU there is no point in setting to anything bigger since the bios
will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
256, this gives is 1M direct I/Os.

Thanks,

                Ilya
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