[ClusterLabs] Pacemaker responsible of DRBD and a systemd resource

Wed Nov 15 15:30:27 EST 2017

I agree. Thing is, we have this kind of setup deployed largely and since a while. Never ran into any issue.
Not sure if something changed in Corosync/Pacemaker code or way of dealing with systemd resources.

As said, without a systemd resource, everything just work as it should… 100% of the time
As soon as a systemd resource comes in, it breaks.

-derek

--
Derek Wuelfrath
dwuelfrath at inverse.ca <mailto:dwuelfrath at inverse.ca> :: +1.514.447.4918 (x110) :: +1.866.353.6153 (x110)
Inverse inc. :: Leaders behind SOGo (www.sogo.nu <https://www.sogo.nu/>), PacketFence (www.packetfence.org <https://www.packetfence.org/>) and Fingerbank (www.fingerbank.org <https://www.fingerbank.org/>)

> On Nov 14, 2017, at 23:03, Digimer <lists at alteeve.ca> wrote:
> 
> Quorum doesn't prevent split-brains, stonith (fencing) does. 
> 
> https://www.alteeve.com/w/The_2-Node_Myth <https://www.alteeve.com/w/The_2-Node_Myth>
> 
> There is no way to use quorum-only to avoid a potential split-brain. You might be able to make it less likely with enough effort, but never prevent it.
> 
> digimer
> 
> On 2017-11-14 10:45 PM, Garima wrote:
>> Hello All,
>>  
>> Split-brain situation occurs due to there is a drop in quorum which leads to Spilt-brain situation and status information is not exchanged between both two nodes of the cluster. 
>> This can be avoided if quorum communicates between both the nodes.
>> I have checked the code. In My opinion these files need to be updated (quorum.py/stonith.py) to avoid the spilt-brain situation to maintain Active-Passive configuration.
>>  
>> Regards,
>> Garima
>>  
>> From: Derek Wuelfrath [mailto:dwuelfrath at inverse.ca <mailto:dwuelfrath at inverse.ca>] 
>> Sent: 13 November 2017 20:55
>> To: Cluster Labs - All topics related to open-source clustering welcomed <users at clusterlabs.org> <mailto:users at clusterlabs.org>
>> Subject: Re: [ClusterLabs] Pacemaker responsible of DRBD and a systemd resource
>>  
>> Hello Ken !
>>  
>> Make sure that the systemd service is not enabled. If pacemaker is
>> managing a service, systemd can't also be trying to start and stop it.
>>  
>> It is not. I made sure of this in the first place :)
>>  
>> Beyond that, the question is what log messages are there from around
>> the time of the issue (on both nodes).
>>  
>> Well, that’s the thing. There is not much log messages telling what is actually happening. The ’systemd’ resource is not even trying to start (nothing in either log for that resource). Here are the logs from my last attempt:
>> Scenario:
>> - Services were running on ‘pancakeFence2’. DRBD was synced and connected
>> - I rebooted ‘pancakeFence2’. Services failed to ‘pancakeFence1’
>> - After ‘pancakeFence2’ comes back, services are running just fine on ‘pancakeFence1’ but DRBD is in Standalone due to split-brain
>>  
>> Logs for pancakeFence1: https://pastebin.com/dVSGPP78 <https://pastebin.com/dVSGPP78>
>> Logs for pancakeFence2: https://pastebin.com/at8qPkHE <https://pastebin.com/at8qPkHE>
>>  
>> It really looks like the status checkup mechanism of corosync/pacemaker for a systemd resource force the resource to “start” and therefore, start the ones above that resource in the group (DRBD in instance).
>> This does not happen for a regular OCF resource (IPaddr2 per example)
>> 
>> Cheers!
>> -dw
>>  
>> --
>> Derek Wuelfrath
>> dwuelfrath at inverse.ca <mailto:dwuelfrath at inverse.ca> :: +1.514.447.4918 (x110) :: +1.866.353.6153 (x110)
>> Inverse inc. :: Leaders behind SOGo (www.sogo.nu <https://www.sogo.nu/>), PacketFence (www.packetfence.org <https://www.packetfence.org/>) and Fingerbank (www.fingerbank.org <https://www.fingerbank.org/>)
>> 
>> 
>> On Nov 10, 2017, at 11:39, Ken Gaillot <kgaillot at redhat.com <mailto:kgaillot at redhat.com>> wrote:
>>  
>> On Thu, 2017-11-09 at 20:27 -0500, Derek Wuelfrath wrote:
>> 
>> Hello there,
>> 
>> First post here but following since a while!
>> 
>> Welcome!
>> 
>> 
>> 
>> Here’s my issue,
>> we are putting in place and running this type of cluster since a
>> while and never really encountered this kind of problem.
>> 
>> I recently set up a Corosync / Pacemaker / PCS cluster to manage DRBD
>> along with different other resources. Part of theses resources are
>> some systemd resources… this is the part where things are “breaking”.
>> 
>> Having a two servers cluster running only DRBD or DRBD with an OCF
>> ipaddr2 resource (Cluser IP in instance) works just fine. I can
>> easily move from one node to the other without any issue.
>> As soon as I add a systemd resource to the resource group, things are
>> breaking. Moving from one node to the other using standby mode works
>> just fine but as soon as Corosync / Pacemaker restart involves
>> polling of a systemd resource, it seems like it is trying to start
>> the whole resource group and therefore, create a split-brain of the
>> DRBD resource.
>> 
>> My first two suggestions would be:
>> 
>> Make sure that the systemd service is not enabled. If pacemaker is
>> managing a service, systemd can't also be trying to start and stop it.
>> 
>> Fencing is the only way pacemaker can resolve split-brains and certain
>> other situations, so that will help in the recovery.
>> 
>> Beyond that, the question is what log messages are there from around
>> the time of the issue (on both nodes).
>> 
>> 
>> 
>> 
>> It is the best explanation / description of the situation that I can
>> give. If it need any clarification, examples, … I am more than open
>> to share them.
>> 
>> Any guidance would be appreciated :)
>> 
>> Here’s the output of a ‘pcs config’
>> 
>> https://pastebin.com/1TUvZ4X9 <https://pastebin.com/1TUvZ4X9>
>> 
>> Cheers!
>> -dw
>> 
>> --
>> Derek Wuelfrath
>> dwuelfrath at inverse.ca <mailto:dwuelfrath at inverse.ca> :: +1.514.447.4918 (x110) :: +1.866.353.6153
>> (x110)
>> Inverse inc. :: Leaders behind SOGo (www.sogo.nu <http://www.sogo.nu/>), PacketFence
>> (www.packetfence.org <http://www.packetfence.org/>) and Fingerbank (www.fingerbank.org <http://www.fingerbank.org/>)
>> -- 
>> Ken Gaillot <kgaillot at redhat.com <mailto:kgaillot at redhat.com>>
>> 
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> 
> -- 
> Digimer
> Papers and Projects: https://alteeve.com/w/ <https://alteeve.com/w/>
> "I am, somehow, less interested in the weight and convolutions of Einstein’s brain than in the near certainty that people of equal talent have lived and died in cotton fields and sweatshops." - Stephen Jay Gould
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