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

Pranith Kumar Karampuri pkarampu at redhat.com
Sat Mar 5 07:15:53 UTC 2016

On 03/04/2016 10:05 PM, Diego Remolina wrote:
> I run a few two node glusterfs instances, but always have a third
> machine acting as an arbiter. I am with Jeff on this one, better safe
> than sorry.
> Setting up a 3rd system without bricks to achieve quorum is very easy.

This is server side quorum. This is good. But what we are discussing 
here is for just 2 nodes, what should be the default.

> Diego
> On Fri, Mar 4, 2016 at 10:40 AM, Jeff Darcy <jdarcy at redhat.com> 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.
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