CODEWITHSUNDEEP
mongodbreplication
Tiberiu Savin
Tiberiu Savin

Reputation: 221

Mongodb replica set odd member count vs even member count + an arbiter

I've read quite a bit about mongodb replica set and how the elections work on a failover. My curiosity is assuming the client will use readPreference set to primary only, is there any advantage to having odd number of members against having even number of members + 1 arbiter?

For example if you have a 3 member replica set you can set all 3 members to be replicas or you can have only 2 replicas and an arbiter (which you can install on a smaller machine). The safety is basically the same, any machine can go down and the replica set is still ok, but if two of them go down then the replica set is in stalemate (it cannot elect a new primary). The only difference is that in the second case you can use a way smaller machine for the arbiter.

Upvotes: 0

Views: 660

Answers (1)

Asya Kamsky
Asya Kamsky

Reputation: 42352

It's actually not true that three data holding nodes provide the same "safety" net as two data holding nodes plus an arbiter.

Consider these cases:

1) one of your nodes loses its disk and you need to fully resync it. If you had three data holding nodes you can resync off of the other secondary, instead of the primary (which would reduce the load on the primary).

2) one of your nodes loses its disk and it takes you a while to located a new one. While that secondary is down you are running with ZERO safety net if you had two nodes and an arbiter since you only have one node with data left, if anything happens to it, you are toast.

Upvotes: 1

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