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

[john tillman]

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/
>
>