Hi Dave,
On Mon, Feb 26, 2018 at 4:45 PM, Dave Sherohman <dave@xxxxxxxxxxxxx> 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.
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)
(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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