[CentOS] two 2-node clusters or one 4-node cluster?

Thu Jul 5 17:10:02 UTC 2018
Digimer <lists at alteeve.ca>

On 2018-07-05 11:27 AM, Gianluca Cecchi wrote:
> Hello,
> I'm planning migration of current two clusters based on CentOS 6.x with
> Cman/Rgmanager going to CentOS 7.x and Corosync/Pacemaker.
> As the clusters and their services are on the same subnet, and there no
> particular security concerns differentiating them, I'm also evaluating the
> option to transform the two clusters into a unique 4-node one during the
> upgrade.
> Currently I'm testing a virtual 4-node CentOS 7.4 cluster inside oVirt 4.2
> and things seem to behave well.
> Before going further in deep with tests and so on, I'd like to check with
> the community about how many CentOS 7.x clusters composed by more than two
> nodes are in place and what are the feedbacks on them in terms of
> incremented latency/communication, ecc scaling out.
> Also general feedback related to CentOS 6 and scalability of cluster nodes
> number is welcome.
> Thanks in advance,
> Gianluca

I always prioritize simplicity and isolation, so I vote fore 2x 2-nodes.
There is no effective benefit to 3+ nodes (quorum is arguably helpful,
but proper stonith, which you need anyway, makes it mostly a moot point).

Keep in mind; If your services are critical enough to justify an HA
cluster, they're probably important enough that adding the
complexity/overhead of larger clusters doesn't offset any hardware
efficiency savings. Lastly, with 2x 2-node, you could lose two nodes
(one per cluster) and still be operational. If you lose 2 nodes of a
four node cluster, you're offline.

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