[Gluster-users] [Gluster-devel] Default quorum for 2 way replication

[Pranith Kumar Karampuri]

On 03/04/2016 08:36 PM, Shyam wrote:
> On 03/04/2016 07:30 AM, Pranith Kumar Karampuri 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.
>
> I think we need to consider *maintenance* activities on the volume, 
> like replacing a brick in a replica pair, or upgrading one half of the 
> replica and then the other, at which time the replica group would 
> function read-only, if we choose 'auto' in a 2-way replicated state, 
> is this correct?

Yes.

>
> Having said the above, we already have the option in place, right? I.e 
> admins can already choose 'auto', it is just the default that we are 
> discussing. This could also be tackled via documentation/best 
> practices ("yeah right! who reads those again?" is a valid comment here).

Yes. I just sent a reply to Jeff, where I told it is better to have 
interactive question at the time of creating 2-way replica volume which 
gives this information :-).

>
> I guess we need to be clear (in documentation or otherwise) what they 
> get when they choose one over the other (like the HA point below and 
> also upgrade concerns etc.), irrespective of how this discussion ends 
> (just my 2 c's).

Totally agree. We will give an interactive question above, a link which 
gives detailed explanation.

>
>>
>> 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
>
> @Pranith, can you elaborate on this? I am not so AFR savvy, so unable 
> to comprehend why HA is available if only when the second brick goes 
> down and is not when the first does. Just helps in understanding the 
> issue at hand.

Because it is client side replication there is a fixed *leader* i.e. 1st 
brick.

As a side note. We recently had a discussion with NSR team (Jeff, avra). 
We will be using some infra for NSR to implement server side afr as well 
with leader election etc.

Pranith
>
>> 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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