Re: [PATCH v3 0/8] dm: add request-based blk-mq support

[Bart Van Assche <bvanassche@acm.org>]

On 12/24/14 20:26, Mike Snitzer wrote:
> This fixes it:
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index cdd84e9..138ffb2 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2030,6 +2030,8 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
>  		return -EIO;
>  
>  	if (q->mq_ops) {
> +		if (blk_queue_io_stat(rq->q))
> +			blk_account_io_start(rq, true);
>  		blk_mq_insert_request(rq, false, true, true);
>  		return 0;
>  	}
> 
> I've folded this fix into this commit (and rebased the 'for-next' and
> 'dm-for-3.20-blk-mq' branches):
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=1fd5e9c83c4ae6a5144783855e9b29a8f42bdc4a

Hello Mike,

Thanks, my tests confirm that this patch indeed fixes the issue I had
reported. Unfortunately this doesn't mean that the blk-mq multipath code
is already working perfectly. Most of the time I/O requests are
processed within the expected time but sometimes I/O processing takes
much more time than what I expected:

# /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null
0.02
# /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null
0.02
# /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null
8.68

However, if I run the same command on the underlying device it always
completes within the expected time.

Bart.