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Re: [Linux-cluster] Questions related to cluster quorum and fencing



On 01/19/2011 01:19 AM, Parvez Shaikh wrote:
> Hi all,
> 
> *Quorum - *
> The questions are bit theoretical, I have gone through documentation and
> man pages and have understood that, a cluster is "quorate" if a cluster
> or its partition has nodes, with votes equal to or more than
> "expected_votes" in "cman" section of cluster.conf file (with no
> requirement mandating use of quorum disk)
> 
> So how does cluster being quorate or non-quorate affects functioning of
> a cluster or services? If cluster is non-quorate, does it indicate an
> alarming situation and why?
> 
> If a cluster is composed of resource groups which including only IP
> resource and script resource monitoring my application server listening
> on IP resource(no shared disk or shared resource between cluster nodes),
> then is cluster being "quorate" (or non quorate) important for services
> and/or cluster?

With the exception of 'two_nodes="1"', "expected_votes" should be the
total votes of all nodes. Quorum is achieved once 50%+1 votes coming
from the available nodes in a cluster. Without quorum, the cluster will
not work.

I've covered some of the theory here:

http://wiki.alteeve.com/index.php/Red_Hat_Cluster_Service_3_Tutorial#An_Overview_Before_We_Begin

> *Fencing - *
> Is fencing and cluster being quorate or non-quorate related? I tried one
> experiment, wherein I removed "fencing" for cluster nodes and shutdown
> one of the nodes in cluster. And I got message in /var/log/messages
> indicating fencing failed for node, and service was not failed over from
> that node. So is fencing mandatory even if there is no "shared" disk
> between two cluster nodes?
> 
> Also is a cluster non-quorate in a time window when a node has failed
> and has not been fenced successfully?
> 
> Yours gratefully

Without fencing, your cluster is unstable. Once a fence is needed but
fails, the entire cluster will block. Please check out the link above.
In the same section is a detailed example of why fencing is critical.

Cheers

-- 
Digimer
E-Mail: digimer alteeve com
AN!Whitepapers: http://alteeve.com
Node Assassin:  http://nodeassassin.org


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