[ClusterLabs] Two node cluster without fencing and no split brain?

[Jehan-Guillaume de Rorthais]

On Thu, 22 Jul 2021 12:56:40 +0300
Andrei Borzenkov <arvidjaar at gmail.com> wrote:

> On Thu, Jul 22, 2021 at 12:43 PM Jehan-Guillaume de Rorthais
> <jgdr at dalibo.com> wrote:
> >
> > On Wed, 21 Jul 2021 12:45:40 -0400
> > Digimer <lists at alteeve.ca> wrote:
> >  
> > > On 2021-07-21 3:26 a.m., Jehan-Guillaume de Rorthais wrote:  
> > > > Hi,
> > > >
> > > > On Wed, 21 Jul 2021 04:28:30 +0000 (UTC)
> > > > Strahil Nikolov via Users <users at clusterlabs.org> wrote:
> > > >  
> > > >> Hi,
> > > >> consider using a 3rd system as a Q disk. Also, you can use iscsi from
> > > >> that node as a SBD device, so you will have proper fencing .If you
> > > >> don't have a hardware watchdog device, you can use softdog kernel
> > > >> module for that. Best  
> > > >
> > > > Having 3 nodes for quorum AND watchdog (using softdog in last resort) is
> > > > enough, isn't it?
> > > > But yes, having a shared storage to add a SBD device is even better.
> > > >
> > > > Regards,  
> > >
> > > The third node with storage-based death is a way of creating a fence
> > > configuration.  
> >
> > Yes, poison pill.
> >  
> > > It works because it's fencing, not because it's quorum.  
> >
> > That's not what I said. Two node + sbd is safe. OK.
> >
> > My consideration/question was: 3 nodes + watchdog, without storage-based
> > death, looks good enough to me. Do I miss something?
> >  
> 
> From an integrity point of view it is equivalent to SBD. SBD at the
> end relies on a watchdog as well. So if you have a reliable hardware
> watchdog it should be OK.
> 
> From an operational point of view pacemaker does not have the notion
> of "cluster wide shutdown". Which means - as soon as you stop
> pacemaker on two nodes the third node will commit suicide because it
> goes out of quorum. Last man standing may help here, I have not tried
> it in true three node cluster. This does not happen with SBD as long
> as storage remains accessible.

Ok, understood. Thanks for the details Andrei!