[dm-devel] [PATCH] dm-mpath: always return reservation conflict

[Hannes Reinecke]

On 07/15/2015 01:35 PM, James Bottomley wrote:
> On Wed, 2015-07-15 at 13:23 +0200, Hannes Reinecke wrote:
>> If dm-mpath encounters an reservation conflict it should not
>> fail the path (as communication with the target is not affected)
>> but should rather retry on another path.
>> However, in doing so we might be inducing a ping-pong between
>> paths, with no guarantee of any forward progress.
>> And arguably a reservation conflict is an unexpected error,
>> so we should be passing it upwards to allow the application
>> to take appropriate steps.
> 
> If I interpret the code correctly, you've changed the behaviour from the
> current try all paths and fail them, ultimately passing the reservation
> conflict up if all paths fail to return reservation conflict
> immediately, keeping all paths running.  This assumes that the
> reservation isn't path specific because if we encounter a path specific
> reservation, you've altered the behaviour from route around to fail.
> 
That is correct.
As mentioned in the path, the 'correct' solution would be to retry
the offending I/O on another path.
However, the current multipath design doesn't allow us to do that
without failing the path first.
If we were just retrying I/O on another path without failing the
path first (and all paths would return a reservation conflict) we
wouldn't know when we've exhausted all paths.

> The case I think the original code was for is SAN Volume controllers
> which use path specific SCSI-3 reservations effectively to do traffic
> control and allow favoured paths.  Have you verified that nothing we
> encounter in the enterprise uses path specific reservations for
> multipath shaping any more?
> 
Ah. That was some input I was looking for.
With that patch I've assumed that persistent reservations are done
primarily from userland / filesystem, where the reservation would
effectively be done on a per-LUN basis.
If it's being used from the storage array internally this is a
different matter.
(Although I'd be very interested how this behaviour would play
together with applications which use persistent reservations
internally; GPFS springs to mind here ...)

But apparently this specific behaviour wasn't seen that often in the
field; I certainly never got any customer reports about mysteriously
failing paths.

Anyway. I'll see if I can come up with something to restore the
original behaviour.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
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