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

[Ulrich Windl]

>>> "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?

> 
> 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).
>>>> >
>>>> >
>>>> > _______________________________________________
>>>> > 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/