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

[john tillman]

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

> 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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