[Pacemaker] split brain situation

Philip Pinto pinto at dpcomputersolutions.com
Mon Feb 9 22:19:43 EST 2009

Well one other reason you may want a two node cluster would be running under
a virtualization engine - VMware or z/VM - just to refrain from wasting
hypervisor resources - all of the clusters we run are two node clusters for
this reason - and HA is the only solution we have to allow for monitoring
and restarting failed resources.  We do automatically restart fenced nodes
in the event of a failure.  Last time I checked I was sane - at least that's
what I keep telling myself ;-) 

- Phil

-----Original Message-----
From: Andrew Beekhof [mailto:beekhof at gmail.com] 
Sent: Friday, February 06, 2009 6:52 AM
To: The Pacemaker cluster resource manager
Subject: Re: [Pacemaker] spilit brain situation

On Feb 6, 2009, at 12:11 PM, Lars Marowsky-Bree wrote:

> On 2009-02-06T10:39:42, Andrew Beekhof <beekhof at gmail.com> wrote:
>> which is why two node clusters are good for demonstrations and  
>> thats about
>> it.
>> there are some limited uses for them, but in general you need 3  
>> nodes for a
>> sane cluster.
> Andrew, this is not true. The majority of all clusters out ther - 90%+
> would be my guess - are 2 nodes only, and _work perfectly fine_ with

I'm not going to rehash the entire IRC conversation I just had with  
Lars, suffice to say that there are traps to setting up a two node  
cluster that I consider non-obvious to people new to clustering...  
resulting in clusters that appear to work right up until they implode.

I will concede though that they can be made to work safely :-)

However I maintain that adding an old/underpowered box to act as a tie- 
breaker (it doesn't even need to run resources to be useful) is still  
the saner choice as it means the cluster behaves more predictably and  
allows it to respond better to transient failures (because its no  
longer necessary to prevent stonith'd nodes from rejoining the cluster).

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