[Gluster-devel] Default quorum for 2 way replication

[Atin Mukherjee]

I do not think a quorum feature does a fair job in a 2 way replication. The
minimum criteria to use quorum is 3 and it does not make any sense to me to
turn it on by default for 2 way replication  especially when HA is not
achieved if the 1st brick the pair goes down.

BTW, whats the history behind this limitation?

-Atin
Sent from one plus one
On 04-Mar-2016 6:00 pm, "Pranith Kumar Karampuri" <pkarampu at redhat.com>
wrote:

>
>
> On 03/04/2016 05:47 PM, Bipin Kunal wrote:
>
>> HI Pranith,
>>
>> Thanks for starting this mail thread.
>>
>> Looking from a user perspective most important is to get a "good copy"
>> of data.  I agree that people use replication for HA but having stale
>> data with HA will not have any value.
>> So I will suggest to make auto quorum as default configuration even
>> for 2-way replication.
>>
>> If user is willing to lose data at the cost of HA, he always have
>> option disable it. But default preference should be data and its
>> integrity.
>>
>
> That is the point. There is an illusion of choice between Data integrity
> and HA. But we are not *really* giving HA, are we? HA will be there only if
> second brick in the replica pair goes down. In your typical deployment, we
> can't really give any guarantees about what brick will go down when. So I
> am not sure if we can consider it as HA. But I would love to hear what
> others have to say about this as well. If majority of users say they need
> it to be auto, you will definitely see a patch :-).
>
> Pranith
>
>>
>> Thanks,
>> Bipin Kunal
>>
>> On Fri, Mar 4, 2016 at 5:43 PM, Ravishankar N <ravishankar at redhat.com>
>> wrote:
>>
>>> On 03/04/2016 05:26 PM, Pranith Kumar Karampuri wrote:
>>>
>>>> hi,
>>>>       So far default quorum for 2-way replication is 'none' (i.e.
>>>> files/directories may go into split-brain) and for 3-way replication and
>>>> arbiter based replication it is 'auto' (files/directories won't go into
>>>> split-brain). There are requests to make default as 'auto' for 2-way
>>>> replication as well. The line of reasoning is that people value data
>>>> integrity (files not going into split-brain) more than HA (operation of
>>>> mount even when bricks go down). And admins should explicitly change it
>>>> to
>>>> 'none' when they are fine with split-brains in 2-way replication. We
>>>> were
>>>> wondering if you have any inputs about what is a sane default for 2-way
>>>> replication.
>>>>
>>>> I like the default to be 'none'. Reason: If we have 'auto' as quorum for
>>>> 2-way replication and first brick dies, there is no HA.
>>>>
>>>
>>>
>>> +1.  Quorum does not make sense when there are only 2 parties. There is
>>> no
>>> majority voting. Arbiter volumes are a better option.
>>> If someone wants some background, please see 'Client quorum' and
>>> 'Replica 2
>>> and Replica 3 volumes' section of
>>>
>>> http://gluster.readthedocs.org/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/
>>>
>>> -Ravi
>>>
>>> If users are fine with it, it is better to use plain distribute volume
>>>> rather than replication with quorum as 'auto'. What are your thoughts
>>>> on the
>>>> matter? Please guide us in the right direction.
>>>>
>>>> Pranith
>>>>
>>>
>>>
>>>
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