[Pacemaker] split brain - after network recovery - resources can still be migrated

[Digimer]

On 25/10/14 06:35 PM, Vladimir wrote:
> On Sat, 25 Oct 2014 17:30:07 -0400
> Digimer <lists at alteeve.ca> wrote:
>
>> On 25/10/14 05:09 PM, Vladimir wrote:
>>> Hi,
>>>
>>> currently I'm testing a 2 node setup using ubuntu trusty.
>>>
>>> # The scenario:
>>>
>>> All communication links betwenn the 2 nodes are cut off. This
>>> results in a split brain situation and both nodes take their
>>> resources online.
>>>
>>> When the communication links get back, I see following behaviour:
>>>
>>> On drbd level the split brain is detected and the device is
>>> disconnected on both nodes because of "after-sb-2pri disconnect" and
>>> then it goes to StandAlone ConnectionState.
>>>
>>> I'm wondering why pacemaker does not let the resources fail.
>>> It is still possible to migrate resources between the nodes although
>>> they're in StandAlone ConnectionState. After a split brain that's
>>> not what I want.
>>>
>>> Is this the expected behaviour? Is it possible to let the resources
>>> fail after the network recovery to avoid fürther data corruption.
>>>
>>> (At the moment I can't use resource or node level fencing in my
>>> setup.)
>>>
>>> Here the main part of my config:
>>>
>>> #> dpkg -l | awk '$2 ~ /^(pacem|coro|drbd|libqb)/{print $2,$3}'
>>> corosync 2.3.3-1ubuntu1
>>> drbd8-utils 2:8.4.4-1ubuntu1
>>> libqb-dev 0.16.0.real-1ubuntu3
>>> libqb0 0.16.0.real-1ubuntu3
>>> pacemaker 1.1.10+git20130802-1ubuntu2.1
>>> pacemaker-cli-utils 1.1.10+git20130802-1ubuntu2.1
>>>
>>> # pacemaker
>>> primitive drbd-mysql ocf:linbit:drbd \
>>> params drbd_resource="mysql" \
>>> op monitor interval="29s" role="Master" \
>>> op monitor interval="30s" role="Slave"
>>>
>>> ms ms-drbd-mysql drbd-mysql \
>>> meta master-max="1" master-node-max="1" clone-max="2"
>>> clone-node-max="1" notify="true"
>>
>> Split-brains are prevented by using reliable fencing (aka stonith).
>> You configure stonith in pacemaker (using IPMI/iRMC/iLO/etc, switched
>> PDUs, etc). Then you configure DRBD to use the crm-fence-peer.sh
>> fence-handler and you set the fencing policy to
>> 'resource-and-stonith;'.
>>
>> This way, if all links fail, both nodes block and call a fence. The
>> faster one fences (powers off) the slower, and then it begins
>> recovery, assured that the peer is not doing the same.
>>
>> Without stonith/fencing, then there is no defined behaviour. You will
>> get split-brains and that is that. Consider; Both nodes lose contact
>> with it's peer. Without fencing, both must assume the peer is dead
>> and thus take over resources.
>
> That split brains can occur in such a setup that's clear. But I would
> expect pacemaker to stop the drbd resource when the link between the
> cluster nodes is reestablished instead of continue running it.

DRBD will refuse to reconnect until it is told which node's data to 
delete. This is data loss and can not be safely automated.

>> This is why stonith is required in clusters. Even with quorum, you
>> can't assume anything about the state of the peer until it is fenced,
>> so it would only give you a false sense of security.
>
> Maybe I'll can use resource level fencing.

You need node-level fencing.

-- 
Digimer
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