[dm-devel] [PATCH 2/3] dm rq: add DM_MAPIO_DELAY_REQUEUE to delay requeue of blk-mq requests

[Hannes Reinecke]

On 09/13/2016 06:01 PM, Mike Snitzer wrote:
> Otherwise blk-mq will immediately dispatch requests that are requeued
> via a BLK_MQ_RQ_QUEUE_BUSY return from blk_mq_ops .queue_rq.
> 
> Delayed requeue is implemented using blk_mq_delay_kick_requeue_list()
> with a delay of 5 secs.  In the context of DM multipath (all paths down)
> it doesn't make any sense to requeue more quickly.
> 
> Signed-off-by: Mike Snitzer <snitzer at redhat.com>
> ---
>  drivers/md/dm-rq.c            | 35 ++++++++++++++++++++---------------
>  include/linux/device-mapper.h |  1 +
>  2 files changed, 21 insertions(+), 15 deletions(-)
> 
Hmm. Not sure if I agree with the reasoning for a 5 seconds delay; this
seems to be a rather broad estimate.
Won't it delay I/O resumption when we have intermittent path failure (eg
on iSCSI)?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare at suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)