[Linux-cluster] quorum

[Jakov Sosic]

On 03/22/2013 02:42 AM, Prathyush wrote:
> HI,
> 
> 
> I have 3 node  cluster with 1 vote each ,and  decided to shut down the 2
> nodes for  maintenance .
> When a proper power off /init 0 given to two of the nodes ,i never loses
> the quorum  
> When i am making  force power off (switching off the power/pulling
> the power cable) quorum  was lost and services failed in the third node.
> why this is happening... any idea 

That's the exactly as should be.

When you have 3 node cluster, and first node goes down - it's irrelevant
if it went down by standard shutdown or by pulling the cord, other two
nodes still have enough votes to hold the quorum.

But now you're on just 2 out of 3 votes and things get a little bit more
complicated.

If you shut down one of the remaining two nodes by standard shutdown
procedure, cman service will announce that the node is leaving cluster
(ccs_tool leave).

Other node will now know that he's the only one left, and although it
won't have quorum, it will continue to provide services. It does not
have a reason not to, cluster was disassembled nicely.


But, if you pull out power out of one of the two remaining nodes,
simulating it's crash, it's OK for the remaining node to stop providing
services. He just doesn't know what's happening to other node, other
node could think the same and you would get split-brain scenario.

One thing that could solve your problem is introduction of quorum disk,
which would hold 2 votes, so in the latter scenario, one node that
survives would make scsi reservation on quorum disk and would thus hold
3 out of 5 votes.

What kind of fencing are you using? Is it IPMI?