[dm-devel] Fwd: Re: [PATCH] dm-mpath: push back requests instead of queueing

[Junichi Nomura]

On 11/11/13 20:44, Hannes Reinecke wrote:
> On 11/11/2013 12:25 PM, Steffen Maier wrote:
>> One thing I'm still wondering: Would there be any benefit of actually stopping the request
>> queue until at least one path becomes available again?
> [blk_{start|stop}_queue()] I.e. stop
>> in map_io() after we're sure there is no path in any prio group,
> and restart in reinstate_path().
>>
> Hehe. Thought about that, too.
> 
> Problem with that approach is the way multipath currently works.
> Currently multipath does _not_ have a flag for 'all paths down and
> queue_if_no_path is active, please requeue'.
> It rather evaluates all possible paths during map_io, and only
> if it determines that no paths are present and queue_if_no_path
> is set it'll requeue the I/O.
> 
> So if we were to use start/stop queue here the block layer would
> never trigger 'map_io', and multipath would never check the path
> states and no I/O will be sent, ever.

I might misunderstand but if you stop the queue here:

        if ((pgpath && m->queue_io) ||
            (!pgpath && m->queue_if_no_path)) {
                /* Queue for the daemon to resubmit */

you can start the queue on completion of pg_init
for "pgpath && m->queue_io" case,
and can start on reinstate_path() message
for "!pgpath && m->queue_if_no_path" case.

> blk_start/stop_queue works best if you have _alternative_ means
> of setting those, besides the normal I/O path.
> (It's original idea was to be used from LLDDs, after all).
> When one of these functions is being used in the normal I/O path
> things are becoming iffy really fast.
> 
> That said, it _would_ make sense to use blk_start/stop_queue for
> pg_init; we cannot send I/O during pg_init anyway, so there's
> no point in retrying I/O here.

blk_stop/start_queue is used to implement dm suspend/resume.
So if you use them from target, care has to be taken not to
interfere with dm core.

-- 
Jun'ichi Nomura, NEC Corporation