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

[john tillman]

> 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.
>

Thank you.  I'll check into "corosync-qdevice".

I had already stumbled across the "heuristics ping" and noted the fence
device requirement.  Thank you for confirming that.

>> > 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).
>> > > > >
>> > > > >
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> --
> Ken Gaillot <kgaillot at redhat.com>
>
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