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

Fri Jul 23 04:16:24 EDT 2021

On Fri, Jul 23, 2021 at 8:55 AM Ulrich Windl <
Ulrich.Windl at rz.uni-regensburg.de> wrote:

> >>> "john tillman" <johnt at panix.com> schrieb am 22.07.2021 um 16:48 in
> Nachricht
> <1175ffcec0033015e13d11d7821d5acb.squirrel at mail.panix.com>:
> > 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?
>
> Maybe explain how it should work:
> If the two nodes cannot rech each other, but each can reach the ping node,
> which node has the quorum then?
>

Guess both - which is what is played down as 'disadvantage' in the
description
below ;-)

>
> >
> > Best Regards,
> > ‑John
> >
> >> Interesting read.  Thank you for providing it!
> >>
> >> In this follow up post
> >>
> >
> https://techthoughts.typepad.com/managing_computers/2007/10/more
> ‑about‑quor.htm
>
> > l
> >> 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.htm
>
> > l
> >>>
> >>>> 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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