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

Andrei Borzenkov arvidjaar at gmail.com
Wed Jul 21 05:18:23 EDT 2021

On Wed, Jul 21, 2021 at 11:50 AM Frank D. Engel, Jr. <fde101 at fjrhome.net> wrote:
> OpenVMS can do this sort of thing without a requirement for fencing (you
> still need a third disk as a quorum device in a 2-node cluster), but
> Linux (at least in its current form) cannot. From what I can tell the
> fencing requirements in the Linux solution are mainly due to limitations
> of how deeply the clustering solution is integrated into the kernel.

This is a common misunderstanding. *All* cluster solutions use
fencing. Maybe they do not call it "fencing" and many cluster
solutions use self-fencing (suicide), but it does not change the basic
fact - cluster needs fencing.

Pacemaker supports self-fencing. It does not support arbitration using
SCSI reservations that you mean here. It is not related to "kernel
integration" at all - you can do arbitration completely in user space.
It is just that someone needs to implement it. Given that in the case
of shared storage there is already a solution - SBD - and SCSI
arbitration cannot be used without shared storage it is not clear what
will be gained by implementing it. Better would be to spend the
efforts improving pacemaker/sbd integration.

Arbitration fits more in corosync than stonith. Arbitration does not
kill other node - it decides which node remains in quorum and node
that lost commits suicide.

> There is an overview here:
> https://sciinc.com/remotedba/techinfo/tech_presentations/Boot%20Camp%202013/Bootcamp_2013_Comparison%20of%20Red%20Hat%20Clusters%20with%20OpenVMS%20Clusters.pdf
> I am wondering how much of what OpenVMS does could be integrated into
> Linux in the future to simplify the HA clustering situation. This is one
> thing OpenVMS currently does FAR better than any other platform I've
> come across, so it is likely there is still much to be learned from it.
> On 7/20/21 6:45 PM, Digimer wrote:
> > On 2021-07-20 6:04 p.m., john tillman wrote:
> >> Greetings,
> >>
> >> Is it possible to configure a two node cluster (pacemaker 2.0) without
> >> fencing and avoid split brain?
> > No.
> >
> >> I was hoping there was a way to use a 3rd node's ip address, like from a
> >> network switch, as a tie breaker to provide quorum.  A simple successful
> >> ping would do it.
> > Quorum is a different concept and doesn't remove the need for fencing.
> >
> >> I realize that this 'ping' approach is not the bullet proof solution that
> >> fencing would provide.  However, it may be an improvement over two nodes
> >> alone.
> > It would be, at best, a false sense of security.
> >
> >> Is there a configuration like that already?  Any other ideas?
> >>
> >> Pointers to useful documents/discussions on avoiding split brain with two
> >> node clusters would be welcome.
> > https://www.alteeve.com/w/The_2-Node_Myth
> >
> > (note: currently throwing a cert error related to the let's encrypt
> > issue, should be cleared up soon).
> >
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