[Pacemaker] Adding Cluster Nodes in Pacemaker "Configuration Explained"

[Colin]

On Wed, Oct 21, 2009 at 12:14 PM, Andrew Beekhof <andrew at beekhof.net> wrote:
> On Oct 21, 2009, at 12:11 PM, Colin wrote:
>> On Wed, Oct 21, 2009 at 12:00 PM, Andrew Beekhof <andrew at beekhof.net> wrote:
>>> On Oct 21, 2009, at 11:50 AM, Colin wrote:
>>>> On Wed, Oct 21, 2009 at 11:12 AM, Andrew Beekhof <andrew at beekhof.net> wrote:
>>>>> On Oct 21, 2009, at 11:05 AM, Colin wrote:
>>>>>
>>>>>> perhaps the section on adding cluster nodes should mention adjusting
>>>>>> the expected-quorum-votes, otherwise you get something like:
>>>>>
>>>>> expected-quorum-votes is ignored for heartbeat based clusters.
>>>>> i should remove it from the crm_mon output when not using openais
>>>>
>>>> Erm ... so how do I tell heartbeat how many nodes are needed?
>>>
>>> You don't.
>>> It knows how many nodes it has ever seen and you get quorum when you have
>>> at least half that.
>>
>> Ok -- is that a "greater equal" or a "greater than"?
>
> greater than.  thats the definition of quorum
>
>> In a 8-node cluster we _don't_ want 4 nodes to have quorum, only 5
>> nodes or more, to strictly prevent a split brain...
>>
>> ...however a 2-node cluster wouldn't work with "greater than 50%", so
>> presumably it's the other (which we don't want).
>
> heartbeat always pretends two-node clusters have quorum.

Ah, I see, that's why I was unsure about the "greater than" vs. "greater equal".

Let's see whether I can summarise correctly:

For Pacemaker + CoroSync/OpenAIS, the Pacemaker attribute
"expected-quorum-votes" tells the cluster how many votes it needs for
quorum. (What happens when the option is not set?)

For Pacemaker + Heartbeat, the Pacemaker attribute
"expected-quorum-votes" is ignored, and Heartbeat's idea of quorum is
used, i.e. (a) for a two-node cluster (list of cluster nodes has two
entries as per "cl_status listnodes") every node that is up and
running believes to have quorum, whether alone or not, and (b) for an
n-node cluster with n greater than two a partition with strictly
greater than 50% of the nodes has quorum. (IIRC (a) can be changed by
making the vote more complicated, but I'm not interested in two-node
clusters and did not look into this.)

Regards, Colin