[ClusterLabs] Gracefully stop nodes one by one with disk-less sbd

[Klaus Wenninger]

On 8/12/19 2:30 PM, Yan Gao wrote:
> Hi Klaus,
>
> On 8/12/19 1:39 PM, Klaus Wenninger wrote:
>> On 8/9/19 9:06 PM, Yan Gao wrote:
>>> On 8/9/19 6:40 PM, Andrei Borzenkov wrote:
>>>> 09.08.2019 16:34, Yan Gao пишет:
>>>>> Hi,
>>>>>
>>>>> With disk-less sbd,  it's fine to stop cluster service from the cluster
>>>>> nodes all at the same time.
>>>>>
>>>>> But if to stop the nodes one by one, for example with a 3-node cluster,
>>>>> after stopping the 2nd node, the only remaining node resets itself with:
>>>>>
>>>> That is sort of documented in SBD manual page:
>>>>
>>>> --><--
>>>> However, while the cluster is in such a degraded state, it can
>>>> neither successfully fence nor be shutdown cleanly (as taking the
>>>> cluster below the quorum threshold will immediately cause all remaining
>>>> nodes to self-fence).
>>>> --><--
>>>>
>>>> SBD in shared-nothing mode is basically always in such degraded state
>>>> and cannot tolerate loss of quorum.
>>> Well, the context here is it loses quorum *expectedly* since the other
>>> nodes gracefully shut down.
>>>
>>>>> Aug 09 14:30:20 opensuse150-1 sbd[1079]:       pcmk:    debug:
>>>>> notify_parent: Not notifying parent: state transient (2)
>>>>> Aug 09 14:30:20 opensuse150-1 sbd[1080]:    cluster:    debug:
>>>>> notify_parent: Notifying parent: healthy
>>>>> Aug 09 14:30:20 opensuse150-1 sbd[1078]:  warning: inquisitor_child:
>>>>> Latency: No liveness for 4 s exceeds threshold of 3 s (healthy servants: 0)
>>>>>
>>>>> I can think of the way to manipulate quorum with last_man_standing and
>>>>> potentially also auto_tie_breaker, not to mention
>>>>> last_man_standing_window would also be a factor... But is there a better
>>>>> solution?
>>>>>
>>>> Lack of cluster wide shutdown mode was mentioned more than once on this
>>>> list. I guess the only workaround is to use higher level tools which
>>>> basically simply try to stop cluster on all nodes at once. It is still
>>>> susceptible to race condition.
>>> Gracefully stopping nodes one by one on purpose is still a reasonable
>>> need though ...
>> If you do the teardown as e.g. pcs is doing it - first tear down
>> pacemaker-instances and then corosync/sbd - it is at
>> least possible to tear down the pacemaker-instances one-by one
>> without risking a reboot due to quorum-loss.
>> With kind of current sbd having in
>> - 
>> https://github.com/ClusterLabs/sbd/commit/824fe834c67fb7bae7feb87607381f9fa8fa2945
>> - 
>> https://github.com/ClusterLabs/sbd/commit/79b778debfee5b4ab2d099b2bfc7385f45597f70
>> - 
>> https://github.com/ClusterLabs/sbd/commit/a716a8ddd3df615009bcff3bd96dd9ae64cb5f68
>> this should be pretty robust although we are still thinking
>> (probably together with some heartbeat to pacemakerd
>> that assures pacemakerd is checking liveness of sub-daemons
>> properly) of having a cleaner way to detect graceful
>> pacemaker-shutdown.
> These are all good improvements, thanks!
>
> But in this case the remaining node is not shutting down yet, or it's 
> intentionally not being shut down :-) Loss of quorum is as expected, so 
> is following no-quorum-policy, but self-reset is probably too much?
Hmm ... not sure if I can follow ...
If you shutdown solely pacemaker one-by-one on all nodes
and these shutdowns are considered graceful then you are
not gonna experience any reboots (e.g. 3 node cluster).
Afterwards you can shutdown corosync one-by-one as well
without experiencing reboots as without the cib-connection
sbd isn't gonna check for quorum anymore (all resources
down so no need to reboot in case of quorum-loss - extra
care has to be taken care of with unmanaged resources but
that isn't particular with sbd).

Klaus
>
> Regards,
>    Yan