[Gluster-users] Quorum in distributed-replicate volume

[Karthik Subrahmanya]

Hi Dave,

On Mon, Feb 26, 2018 at 4:45 PM, Dave Sherohman <dave at sherohman.org> wrote:

> I've configured 6 bricks as distributed-replicated with replica 2,
> expecting that all active bricks would be usable so long as a quorum of
> at least 4 live bricks is maintained.
>
The client quorum is configured per replica sub volume and not for the
entire volume.
Since you have a distributed-replicated volume with replica 2, the data
will have 2 copies,
and considering your scenario of quorum to be taken on the total number of
bricks will lead to split-brains.

>
> However, I have just found
>
> http://docs.gluster.org/en/latest/Administrator%20Guide/
> Split%20brain%20and%20ways%20to%20deal%20with%20it/
>
> Which states that "In a replica 2 volume... If we set the client-quorum
> option to auto, then the first brick must always be up, irrespective of
> the status of the second brick. If only the second brick is up, the
> subvolume becomes read-only."
>
By default client-quorum is "none" in replica 2 volume.

>
> Does this apply only to a two-brick replica 2 volume or does it apply to
> all replica 2 volumes, even if they have, say, 6 bricks total?
>
It applies to all the replica 2 volumes even if it has just 2 brick or more.
Total brick count in the volume doesn't matter for the quorum, what matters
is the number of bricks which are up in the particular replica subvol.

>
> If it does apply to distributed-replicated volumes with >2 bricks,
> what's the reasoning for it?  I would expect that, if the cluster splits
> into brick 1 by itself and bricks 2-3-4-5-6 still together, then brick 1
> will recognize that it doesn't have volume-wide quorum and reject
> writes, thus allowing brick 2 to remain authoritative and able to accept
> writes.
>
If I understood your configuration correctly it should look something like
this:
(Please correct me if I am wrong)
replica-1:  bricks 1 & 2
replica-2: bricks 3 & 4
replica-3: bricks 5 & 6
Since quorum is per replica, if it is set to auto then it needs the first
brick of the particular replica subvol to be up to perform the fop.

In replica 2 volumes you can end up in split-brains. It would be great if
you can consider configuring an arbiter or replica 3 volume.
You can find more details about their advantages over replica 2 volume in
the same document.

Regards,
Karthik

>
> --
> Dave Sherohman
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