Why Openshift 4.X only support 3 masters

Solution 1:

I think there is an important reason to do not use 5, 7 masters.

Could you guys tell me why?

Technically it is still possible to have 5 or 7 Master Nodes, there is nothing in place to stop you. It is just not supported, so if you have a problem with it you'll have to deal with it yourself.

However, you should understand why this recommendation is there. Many Kubernetes Master components (Controller Manager, Scheduler) are only active on one single Master Node anyway, while the same component on all other Masters are not active. API Server is active-active, so API Server scales nicely.

For etcd, Raft is leader-based; the leader handles all client requests which need cluster consensus. Any request that requires consensus sent to a follower is automatically forwarded to the leader. So for write operations or for consistent reads, there is an overhead if you have more than 3 etcd members. If you have more etcd members, this overhead gets larger, but YMMV.

So it comes down to additional overhead with 5 or 7 Master Nodes.

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