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

[juncheng bai]

On 2015/6/15 21:03, Ilya Dryomov wrote:
> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
> <baijuncheng@xxxxxxxxxxxxxxx> wrote:
> >  From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00 2001
> > From: juncheng bai <baijuncheng@xxxxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxxxxx>
> > ---
> >   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.
Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
request can be merged from multiple bios. We can see the below function:
ll_back_merge_fn, ll_front_merge_fn and etc.
And I test in kernel 3.18 use this patch, and do:
echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
We use systemtap to trace the request size, It is upto 4M.

Thanks.
---------
juncheng bai
> Thanks,
>
>                  Ilya
> --
> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/