[ClusterLabs] Two nodes cluster issue

Mon Aug 7 07:38:26 EDT 2017

Tomer Azran napsal(a):
> Just updating that I added another level of fencing using watchdog-fencing.
> With the quorum device and this combination works in case of power failure of both server and ipmi interface.
> An important note is that the stonith-watchdog-timeout must be configured in order to work.
> After reading the following great post: http://blog.clusterlabs.org/blog/2015/sbd-fun-and-profit , I choose the softdog watchdog since the I don't think ipmi watchdog will do no good in case the ipmi interface is down (If it is OK it will be used as a fencing method).
>
> Just for documenting the solution (in case someone else needed that), the configuration I added is:
> systemctl enable sbd
> pcs property set no-quorum-policy=suicide
> pcs property set stonith-watchdog-timeout=15
> pcs quorum device add model net host=qdevice algorithm=lms
>
> I just can't decide if the qdevice algorithm should be lms or ffsplit. I couldn't determine the difference between them and I'm not sure which one is the best when using two node cluster with qdevice and watchdog fencing.
>
> Can anyone advise on that?

I'm pretty sure you've read corosync-qdevice (8) man page where is quite 
detailed description of algorithms so if you were not able to determine 
the difference them there is something wrong and man page needs 
improvement. What exactly you were unable to understand?

Also for your use case with 2 nodes both algorithms behaves same way.

Honza

>
> -----Original Message-----
> From: Jan Friesse [mailto:jfriesse at redhat.com]
> Sent: Tuesday, July 25, 2017 11:59 AM
> To: Cluster Labs - All topics related to open-source clustering welcomed <users at clusterlabs.org>; kwenning at redhat.com; Prasad, Shashank <ssprasad at vanu.com>
> Subject: Re: [ClusterLabs] Two nodes cluster issue
>
>> Tomer Azran napsal(a):
>>> I tend to agree with Klaus – I don't think that having a hook that
>>> bypass stonith is the right way. It is better to not use stonith at all.
>>> I think I will try to use an iScsi target on my qdevice and set SBD
>>> to use it.
>>> I still don't understand why qdevice can't take the place SBD with
>>> shared storage; correct me if I'm wrong, but it looks like both of
>>> them are there for the same reason.
>>
>> Qdevice is there to be third side arbiter who decides which partition
>> is quorate. It can also be seen as a quorum only node. So for two node
>> cluster it can be viewed as a third node (eventho it is quite special
>> because it cannot run resources). It is not doing fencing.
>>
>> SBD is fencing device. It is using disk as a third side arbiter.
>
> I've talked with Klaus and he told me that 7.3 is not using disk as a third side arbiter so sorry for confusion.
>
> You should however still be able to use sbd for checking if pacemaker is alive and if the partition has quorum - otherwise the watchdog kills the node. So qdevice will give you "3rd" node and sbd fences unquorate partition.
>
> Or (as mentioned previously) you can use fabric fencing.
>
> Regards,
>     Honza
>
>>
>>
>>>
>>> From: Klaus Wenninger [mailto:kwenning at redhat.com]
>>> Sent: Monday, July 24, 2017 9:01 PM
>>> To: Cluster Labs - All topics related to open-source clustering
>>> welcomed <users at clusterlabs.org>; Prasad, Shashank
>>> <ssprasad at vanu.com>
>>> Subject: Re: [ClusterLabs] Two nodes cluster issue
>>>
>>> On 07/24/2017 07:32 PM, Prasad, Shashank wrote:
>>> Sometimes IPMI fence devices use shared power of the node, and it
>>> cannot be avoided.
>>> In such scenarios the HA cluster is NOT able to handle the power
>>> failure of a node, since the power is shared with its own fence device.
>>> The failure of IPMI based fencing can also exist due to other reasons
>>> also.
>>>
>>> A failure to fence the failed node will cause cluster to be marked
>>> UNCLEAN.
>>> To get over it, the following command needs to be invoked on the
>>> surviving node.
>>>
>>> pcs stonith confirm <failed_node_name> --force
>>>
>>> This can be automated by hooking a recovery script, when the the
>>> Stonith resource ‘Timed Out’ event.
>>> To be more specific, the Pacemaker Alerts can be used for watch for
>>> Stonith timeouts and failures.
>>> In that script, all that’s essentially to be executed is the
>>> aforementioned command.
>>>
>>> If I get you right here you can disable fencing then in the first place.
>>> Actually quorum-based-watchdog-fencing is the way to do this in a
>>> safe manner. This of course assumes you have a proper source for
>>> quorum in your 2-node-setup with e.g. qdevice or using a shared disk
>>> with sbd (not directly pacemaker quorum here but similar thing
>>> handled inside sbd).
>>>
>>>
>>> Since the alerts are issued from ‘hacluster’ login, sudo permissions
>>> for ‘hacluster’ needs to be configured.
>>>
>>> Thanx.
>>>
>>>
>>> From: Klaus Wenninger [mailto:kwenning at redhat.com]
>>> Sent: Monday, July 24, 2017 9:24 PM
>>> To: Kristián Feldsam; Cluster Labs - All topics related to
>>> open-source clustering welcomed
>>> Subject: Re: [ClusterLabs] Two nodes cluster issue
>>>
>>> On 07/24/2017 05:37 PM, Kristián Feldsam wrote:
>>> I personally think that power off node by switched pdu is more safe,
>>> or not?
>>>
>>> True if that is working in you environment. If you can't do a
>>> physical setup where you aren't simultaneously loosing connection to
>>> both your node and the switch-device (or you just want to cover cases
>>> where that happens) you have to come up with something else.
>>>
>>>
>>>
>>>
>>> S pozdravem Kristián Feldsam
>>> Tel.: +420 773 303 353, +421 944 137 535
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>>>
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>>> BIC: FIOBCZPPXX
>>> IBAN: CZ82 2010 0000 0024 0033 0446
>>>
>>> On 24 Jul 2017, at 17:27, Klaus Wenninger
>>> <kwenning at redhat.com<mailto:kwenning at redhat.com>> wrote:
>>>
>>> On 07/24/2017 05:15 PM, Tomer Azran wrote:
>>> I still don't understand why the qdevice concept doesn't help on this
>>> situation. Since the master node is down, I would expect the quorum
>>> to declare it as dead.
>>> Why doesn't it happens?
>>>
>>> That is not how quorum works. It just limits the decision-making to
>>> the quorate subset of the cluster.
>>> Still the unknown nodes are not sure to be down.
>>> That is why I suggested to have quorum-based watchdog-fencing with sbd.
>>> That would assure that within a certain time all nodes of the
>>> non-quorate part of the cluster are down.
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Mon, Jul 24, 2017 at 4:15 PM +0300, "Dmitri Maziuk"
>>> <dmitri.maziuk at gmail.com<mailto:dmitri.maziuk at gmail.com>> wrote:
>>>
>>> On 2017-07-24 07:51, Tomer Azran wrote:
>>>
>>>> We don't have the ability to use it.
>>>
>>>> Is that the only solution?
>>>
>>>
>>>
>>> No, but I'd recommend thinking about it first. Are you sure you will
>>>
>>> care about your cluster working when your server room is on fire?
>>> 'Cause
>>>
>>> unless you have halon suppression, your server room is a complete
>>>
>>> write-off anyway. (Think water from sprinklers hitting rich chunky
>>> volts
>>>
>>> in the servers.)
>>>
>>>
>>>
>>> Dima
>>>
>>>
>>>
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>>> --
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>>> Klaus Wenninger
>>>
>>>
>>>
>>> Senior Software Engineer, EMEA ENG Openstack Infrastructure
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