I have a MongoDB replica set consisting of two database nodes and one arbiter. I want to upgrade the replica set by launching new, larger instances (Google compute engine), installing MongoDB 3, then adding these to the existing 2.6 replica set, then once the two new 3.0 members are online, I'd like to promote one of the new DBs to Primary, then remove the 2.6 members from the replica set. Reading the MongoDB docs, MongoDB 3 binaries are 'drop-in replacement', so does this mean that I should be OK using this upgrade path? I'm also allowing the new members to build their own databases vs. seeding them from existing members, to take advantage of the new wiredTiger storage engine.

1 Answer 1


MongoDB's operating documentation is usually rather thin, but they do cover a similar case. See https://docs.mongodb.org/master/release-notes/3.0-upgrade/#upgrade-a-replica-set-to-3-0.

In short, yes, you can do this. You should keep a backup, though.

Also, there is little reason to do it this way, since you can convert to the new storage engine after the upgrade.

  • The goal of doing it this way is to also scale my db servers to larger instances at the same time. You have to allow MongoDB to sync with an empty DB directory in order to 'convert' to wired tiger, so I thought this way would accomplish both goals. Aug 29, 2015 at 20:37

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