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

[kgaillot at redhat.com]

On Thu, 2021-07-22 at 10:48 -0400, john tillman wrote:
> There was a lot of discussion on this topic which might have
> overshadowed
> this question so I will ask it again in case someone missed it.
> 
> It comes from a post (see below) that we were pointed to here by
> Andrei:
> 
> Is there something like the described "ping tiebreaker" in the
> current
> world of pacemaker/corosync?
> 
> Best Regards,
> -John

corosync-qdevice is the closest ... you run qdevice on the third node
and the cluster nodes talk to it

there's also fence_heuristics_ping to allow a node to fence another
node only if it can ping an address (to ensure that a node with
connectivity survives a split-brain). this requires some fencing device
to be configured.

> > Interesting read.  Thank you for providing it!
> > 
> > In this follow up post
> > https://techthoughts.typepad.com/managing_computers/2007/10/more-about-quor.html
> > the author mentions the following:
> > 
> > Ping tiebreaker
> > 
> > Some HA systems provide  a ping tiebreaker.  To make this work, you
> > pick a
> > address outside the cluster to ping, and any partition that can
> > ping that
> > address has quorum.  The obvious advantage is that it's very simple
> > to set
> > up - doesn't require any additional servers or shared disk.  The
> > disadvantage (and it's a big one) is that it's very possible for
> > multiple
> > partitions to think they have quorum.  In the case of split-site
> > (disaster
> > recovery) type clusters, it's going to happen fairly often.  If you
> > can
> > use this method for a single site in conjunction with fencing, then
> > it
> > will likely work out quite well.  It's a lot better than no
> > tiebreaker, or
> > one that always says "you have quorum".  Having said that, it's
> > significantly inferior to any of the other methods.
> > 
> > The quote "It's a lot better than no tiebreaker..." is what I am
> > looking
> > for.  Is there something like a "ping tiebreaker" in the current
> > world of
> > pacemaker/corosync?
> > 
> > Thanks to all those who have already commented on my question.  I
> > appreciate the input/education.
> > 
> > Best Regards,
> > -John
> > 
> > 
> > 
> > > On Wed, Jul 21, 2021 at 3:55 PM Ulrich Windl
> > > <Ulrich.Windl at rz.uni-regensburg.de> wrote:
> > > > 
> > > > Hi!
> > > > 
> > > > Maybe someone feels motivated to write some article comparing
> > > > the
> > > > concepts
> > > > * split brain
> > > > * quorum
> > > > * fencing
> > > > 
> > > 
> > > Yet another one? Using your own reply "search is free".
> > > 
> > > https://techthoughts.typepad.com/managing_computers/2007/10/split-brain-quo.html
> > > 
> > > > There are eight possible states that I tried to illustrate on
> > > > the
> > > > attached sketch (S="Split Brain", "Q=Quorum, F=Fencing).
> > > > 
> > > > ;-)
> > > > 
> > > > Regards,
> > > > Ulrich
> > > > 
> > > > 
> > > > > > > Andrei Borzenkov 21.07.2021, 07:52 >>>
> > > > 
> > > > On 21.07.2021 07:28, Strahil Nikolov via Users wrote:
> > > > > Hi,
> > > > > consider using a 3rd system as a Q disk.
> > > > 
> > > > What was not clear in "Quorum is a different concept and
> > > > doesn't remove
> > > > the need for fencing"?
> > > > 
> > > > > 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 Regards,Strahil Nikolov
> > > > > 
> > > > > 
> > > > > On Wed, Jul 21, 2021 at 1:45, Digimer<lists at alteeve.ca>
> > > > > 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).
> > > > > 
> > > > > 
> > > > > _______________________________________________
> > > > > Manage your subscription:
> > > > > https://lists.clusterlabs.org/mailman/listinfo/users
> > > > > 
> > > > > ClusterLabs home: https://www.clusterlabs.org/
> > > > > 
> > > > 
> > > > _______________________________________________
> > > > Manage your subscription:
> > > > https://lists.clusterlabs.org/mailman/listinfo/users
> > > > 
> > > > ClusterLabs home: https://www.clusterlabs.org/
> > > > 
> > > > _______________________________________________
> > > > Manage your subscription:
> > > > https://lists.clusterlabs.org/mailman/listinfo/users
> > > > 
> > > > ClusterLabs home: https://www.clusterlabs.org/
> > > 
> > > _______________________________________________
> > > Manage your subscription:
> > > https://lists.clusterlabs.org/mailman/listinfo/users
> > > 
> > > ClusterLabs home: https://www.clusterlabs.org/
> > > 
> > > 
> > 
> > 
> > _______________________________________________
> > Manage your subscription:
> > https://lists.clusterlabs.org/mailman/listinfo/users
> > 
> > ClusterLabs home: https://www.clusterlabs.org/
> > 
> > 
> 
> 
> _______________________________________________
> Manage your subscription:
> https://lists.clusterlabs.org/mailman/listinfo/users
> 
> ClusterLabs home: https://www.clusterlabs.org/
> 
-- 
Ken Gaillot <kgaillot at redhat.com>