[ClusterLabs] Fencing on 2-node cluster

[Jehan-Guillaume de Rorthais]

On Thu, 21 Jun 2018 07:09:43 +0200
Klaus Wenninger <kwenning at redhat.com> wrote:

> On 06/21/2018 05:52 AM, Andrei Borzenkov wrote:
> > 21.06.2018 00:50, Digimer пишет:  
> >> On 2018-06-20 05:46 PM, Jehan-Guillaume de Rorthais wrote:  
> >>> On Wed, 20 Jun 2018 17:24:41 -0400
> >>> Digimer <lists at alteeve.ca> wrote:
> >>>  
> >>>> Make sure quorum is disabled. Quorum doesn't work on 2-node clusters.  
> >>> It does with the "two_node" parameter enabled in corosync.conf...as far
> >>> as I understand it anyway...  
> >> It doesn't, that option disables quorum in corosync.
> >>  
> > This option does not disable quorum - this option fakes quorum so
> > corosync continues to report "in quorum" even when one node is lost. it
> > is quite possible that pacemaker quorum does not map one-to-one to
> > corosync quorum though.
> >  
> >> Quorum is floor(($nodes / 2) + 1). So in a 3-node, that is 3 -> 1.5 ->
> >> 2.5 -> 2 votes needed for quorum. In a 2-node, that is 2 -> 1 -> 2 -> 2
> >> votes needed for quorum, meaning you can't lose a node to operate (which
> >> is kinda not HA :) ).
> >>  
> 
> Let me try to shed some light on this:
> With Corosync 2.x.x and up Pacemaker gets quorum from Corosync so
> setting two_node there directly gives you the behavior desired.
> It was with Corosync 1.x.x when Pacemaker just used the messaging
> from there and had it's own quorum logic built in.

This was my comprehension of the manpage.

https://github.com/corosync/corosync/blob/master/man/votequorum.5#L94

Thanks for the confirmation Andrei and Klaus.