[Gluster-devel] Default quorum for 2 way replication

[Pranith Kumar Karampuri]

On 03/04/2016 09:10 PM, Jeff Darcy wrote:
>> 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. If users are
>> fine with it, it is better to use plain distribute volume
> "Availability" is a tricky word.  Does it mean access to data now, or
> later despite failure?  Taking a volume down due to loss of quorum might
> be equivalent to having no replication in the first sense, but certainly
> not in the second.  When the possibility (likelihood?) of split brain is
> considered, enforcing quorum actually does a *better* job of preserving
> availability in the second sense.  I believe this second sense is most
> often what users care about, and therefore quorum enforcement should be
> the default.
>
> I think we all agree that quorum is a bit slippery when N=2.  That's
> where there really is a tradeoff between (immediate) availability and
> (highest levels of) data integrity.  That's why arbiters showed up first
> in the NSR specs, and later in AFR.  We should definitely try to push
> people toward N>=3 as much as we can.  However, the ability to "scale
> down" is one of the things that differentiate us vs. both our Ceph
> cousins and our true competitors.  Many of our users will stop at N=2 no
> matter what we say.  However unwise that might be, we must still do what
> we can to minimize harm when things go awry.
I always felt 2-way replication, 3-way replication analogy is similar to 
2-wheeler(motor-bikes) and 4-wheeler vehicles(cars). You have more fatal 
accidents with 2-wheelers than 4-wheelers. But it has its place. Arbiter 
volumes is like a 3-wheeler(auto rickshaw) :-). I feel users should be 
given the power to choose what they want based on what they are looking 
for and how much hardware they want to buy (affordability). We should 
educate them about the risks but the final decision should be theirs. So 
in that sense I don't like to *push* them to N>=3.

        "Many of our users will stop at N=2 no matter what we say". That 
right there is what I had to realize, some years back. I naively thought 
that people will rush to replica-3 with client quorum, but it didn't 
happen. That is the reason for investing time in arbiter volumes as a 
solution. Because we wanted to reduce the cost. People didn't want to 
spend so much money for consistency(based on what we are still seeing). 
Fact of the matter is, even after arbiter volumes I am sure some people 
will stick with replica-2 with unsplit-brain patch from facebook (For 
people who don't know: it resolves split-brain based on policies 
automatically without human intervention, it will be available soon in 
gluster). You do have a very good point though. I think it makes sense 
to make more people aware of what they are getting into with 2-way 
replication. So may be an interactive question at the time of 2-way 
replica volume creation about the possibility of split-brains and 
availability of other options(like arbiter/unsplit-brain in 2-way 
replication) could be helpful, keeping the default still as 'none'. I 
think it would be better if we educate users about value of arbiter 
volumes, so that users naturally progress towards that and embrace it. 
We are seeing more and more questions on the IRC and mailing list about 
arbiter volumes, so there is a +ve trend.

Pranith