[ClusterLabs] [ClusterLabs Developers] checking all procs on system enough during stop action?

[Ken Gaillot]

On 04/24/2017 10:32 AM, Jehan-Guillaume de Rorthais wrote:
> On Mon, 24 Apr 2017 17:08:15 +0200
> Lars Ellenberg <lars.ellenberg at linbit.com> wrote:
> 
>> On Mon, Apr 24, 2017 at 04:34:07PM +0200, Jehan-Guillaume de Rorthais wrote:
>>> Hi all,
>>>
>>> In the PostgreSQL Automatic Failover (PAF) project, one of most frequent
>>> negative feedback we got is how difficult it is to experience with it
>>> because of fencing occurring way too frequently. I am currently hunting
>>> this kind of useless fencing to make life easier.
>>>
>>> It occurs to me, a frequent reason of fencing is because during the stop
>>> action, we check the status of the PostgreSQL instance using our monitor
>>> function before trying to stop the resource. If the function does not return
>>> OCF_NOT_RUNNING, OCF_SUCCESS or OCF_RUNNING_MASTER, we just raise an error,
>>> leading to a fencing. See:
>>> https://github.com/dalibo/PAF/blob/d50d0d783cfdf5566c3b7c8bd7ef70b11e4d1043/script/pgsqlms#L1291-L1301
>>>
>>> I am considering adding a check to define if the instance is stopped even
>>> if the monitor action returns an error. The idea would be to parse **all**
>>> the local processes looking for at least one pair of
>>> "/proc/<PID>/{comm,cwd}" related to the PostgreSQL instance we want to
>>> stop. If none are found, we consider the instance is not running.
>>> Gracefully or not, we just know it is down and we can return OCF_SUCCESS.
>>>
>>> Just for completeness, the piece of code would be:
>>>
>>>    my @pids;
>>>    foreach my $f (glob "/proc/[0-9]*") {
>>>        push @pids => basename($f)
>>>            if -r $f
>>>                and basename( readlink( "$f/exe" ) ) eq "postgres"
>>>                and readlink( "$f/cwd" ) eq $pgdata;
>>>    }
>>>
>>> I feels safe enough to me. The only risk I could think of is in a shared
>>> disk cluster with multiple nodes accessing the same data in RW (such setup
>>> can fail in so many ways :)). However, PAF is not supposed to work in such
>>> context, so I can live with this.
>>>
>>> Do you guys have some advices? Do you see some drawbacks? Hazards?  
>>
>> Isn't that the wrong place to "fix" it?
>> Why did your _monitor  return something "weird"?
> 
> Because this _monitor is the one called by the monitor action. It is able to
> define if an instance is running and if it feels good.
> 
> Take the scenario where the slave instance is crashed:
>   1/ the monitor action raise an OCF_ERR_GENERIC
>   2/ Pacemaker tries a recover of the resource (stop->start)
>   3/ the stop action fails because _monitor says the resource is crashed
>   4/ Pacemaker fence the node.
> 
>> What did it return?
> 
> Either OCF_ERR_GENERIC or OCF_FAILED_MASTER as instance.
> 
>> Should you not fix it there?
> 
> fixing this in the monitor action? This would bloat the code of this function.
> We would have to add a special code path in there to define if it is called
> as a real monitor action or just as a status one for other actions.
> 
> But anyway, here or there, I would have to add this piece of code looking at
> each processes. According to you, is it safe enough? Do you see some hazard
> with it?
> 
>> Just thinking out loud.
> 
> Thank you, it helps :)

It feels odd that there is a situation where monitor should return an
error (instead of "not running"), but stop should return OK.

I think the question is whether the service can be considered cleanly
stopped at that point -- i.e. whether it's safe for another node to
become master, and safe to try starting the crashed service again on the
same node.

If it's cleanly stopped, the monitor should probably return "not
running". Pacemaker will already compare that result against the
expected state, and recover appropriately if needed.

The PID check assumes there can only be one instance of postgresql on
the machine. If there are instances bound to different IPs, or some user
starts a private instance, it could be inaccurate. But that would err on
the side of fencing, so it might still be useful, if you don't have a
way of more narrowly identifying the expected instance.