[davisp]

That’ll just depend on whatever code was deployed to the cluster. For the clusters I used to operate, the answer would be absolutely all nodes talk to all nodes all the time.

I personally never operated anything above roughly 250 nodes, but that limit was mostly due to following the OP’s advice about paying attention to the configuration of each node in the cluster. In my case, fewer nodes with fancier and larger raid arrays ended up being a better scaling strategy.