[Linux-cluster] FAQ correction ?!

[Robert Peterson]

Fajar Priyanto wrote:
> On Tuesday 13 March 2007 16:49, Hugues Lafarge wrote:
>> I'm not sure i correctly understand
>> http://sources.redhat.com/cluster/faq.html#cman_2to3 so i'm kindly asking
>> you if there is not a typo in the following sentence:
>>
>> 8. How do I add a third node to my two-node cluster?
>>
>>  Unfortunately, two-node clusters are a special case. Because a
>>  two-node cluster only needs two nodes to establish quorum, the only
>>  way to add a third node is to shut down the cluster, add the third
>>  node into your /etc/cluster/cluster.conf file and get rid of
>>  two_node="1", then restart all three nodes.
>>
>>
>> I would have expected that to be:
>>
>>  Unfortunately, two-node clusters are a special case. Because a
>>  two-node cluster only needs ONE NODE to establish quorum, ...
>>
>> Am i right there, or misunderstanding something ?
> 
> In my opinion, you're both correct.
> But, this sentence can be easier to understand:
> Two-node clusters are a special case. Because a
> two-node cluster only needs two nodes to establish quorum, the only
> way to add a third node is to shut down the cluster, add the third
> node into your /etc/cluster/cluster.conf file (either by hand or 
> system-config-cluster), copy it into the newly joined cluster, and restart 
> all of the nodes. (the two_node="1" will be removed automatically by 
> system-config-cluster).

Hi,

Actually, in a two-node cluster (without quorum disk/partition)
two nodes are necessary to establish quorum, but only one node
is enough to maintain quorum once it's established.
By design, a two-node cluster can't be started with only
one node, even with a quorum disk/partition that contributes votes.
That was done to guard against split-brain.

That still doesn't explain why you have to restart the cluster
when adding a third node:  it's because the two_node value
can't be undone once it's been set in memory.

It was just a bad explanation on my part, and I apologize.
I updated the FAQ so it's a little bit more accurate.

Regards,

Bob Peterson
Red Hat Cluster Suite