[Gluster-users] Conflict resolution

[Ravishankar N]

Hi David,

On Wed, Oct 20, 2021 at 6:23 AM David Cunningham <dcunningham at voisonics.com>
wrote:

> Hello,
>
> I've a few questions about conflict resolution in a net-split scenario:
>
> 1. What are the default values for cluster.server-quorum-type and
> cluster.server-quorum-ratio? (at the moment "gluster volume info gvol0"
> does not report either)
>
> $gluster volume get gvol0 cluster.server-quorum-type
$gluster volume get all cluster.server-quorum-ratio



> 2. If there are 3 mirrored nodes and cluster.server-quorum-ratio is 50,
> and node1 and node2 are net-split from node3, then am I right in thinking
> that the volume on node3 will automatically shut down and prevent access
> thus preventing a conflict?
>
> Yes, the glusterd on node 3 will kill the brick processes on that node.


> 3. If there are 2 mirrored nodes and a net-split happens and
> cluster.server-quorum-ratio is 50 then:
> a) If existing file A is changed on node1 and node2 then the file will
> enter net-split state, right?
>

Yes, for 2 node setups, you must set the ratio to 51% to avoid this. I'm
assuming that the 'changing' that you refer to is happening via fuse mounts
on each of these nodes.


> b) If existing file B is changed on node1 but not node2 then will the file
> enter a net-split state?
>
No.


> c) If new file C is written on node1 but not node2 then will the file
> enter a net-split state?
>
> No.


> 4. Is the outcome of conflict resolution at a file level the same whether
> node3 is a full replica or just an arbiter?
>
> server quorum feature is not related to replication. It really is a
glusterd thing and doesn't really help prevent split-brains, because
split-brains are caused by failed I/Os from clients and the clients can be
mounted even outside the storage pool. What you need to ensure is
that cluster.quorum-type (which is 'client' quorum) is set to auto for
replica 3 and arbiter. It already is by default. See
https://docs.gluster.org/en/latest/Administrator-Guide/arbiter-volumes-and-quorum/#split-brains-in-replica-volumes
for more info.

-Ravi

> Thank you very much for any advice,
>
> --
> David Cunningham, Voisonics Limited
> http://voisonics.com/
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