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

[juncheng bai <baijuncheng@unitedstack.com>]

On 2015/6/15 21:03, Ilya Dryomov wrote:
> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
> <baijuncheng@unitedstack.com> wrote:
>>  From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00 2001
>> From: juncheng bai <baijuncheng@unitedstack.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@unitedstack.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.
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
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