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

[Hannes Reinecke]

On 07/15/2015 02:01 PM, James Bottomley wrote:
> On Wed, 2015-07-15 at 13:52 +0200, Hannes Reinecke wrote:
>> 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.
> 
> Have you already got this patch in SLES, if so, for how long?
> 
We haven't as of yet; I've come across this behaviour due to another
issue. And before I were to put this into SLES I thought I should be
asking those in the know ... persistent reservations _is_ an arcane
topic, after all.
I was just referring to the fact that I rarely got customer issues
with persistent reservations; and those I get tend to be tape-centric.

>> Anyway. I'll see if I can come up with something to restore the
>> original behaviour.
> 
> Or a way of verifying that nothing in the current enterprise uses path
> specific reservations ...  we can change the current behaviour as long
> as nothing notices.
> 
The only instance I know of is GPFS; someone in our company once
wrote an HA agent using persistent reservations, but I'm not sure if
it's deployed anywhere. But that agent is certainly aware of
multipathing, and doesn't issue per-path reservations.
(Well, actually it does, but it does it for every path :-)
I would think the same goes for GPFS.

Incidentally, the SVC docs have a section about persistent
reservations, but do not mention anything about internal use.
So if it does it'll be opaque to the user, otherwise I would assume
it to be mentioned there.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
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