Yeah, sorry for using mirrored instead of replica. I've worked with 3+ replica sets where I needed read scaling, and that worked well enough. But there are cases where I want read scaling and don't want to have a full copy of all data on all servers, but do want redundancy.

I really like, by comparison, how RethinkDB, Cassandra and others do it with clustering that has sharding + replication. It seems to be a better model in a lot of ways when you want to spread things out (read/write) but also want some replication of data. It's just a different way of working with things.

Thanks for pointing out Atlas.