Every member in a replica size will have a copy of the oplog in db.oplog.rs(). The reason is that if the primary steps down, one of the secondaries will get elected and needs to have an up to date version of the oplog for the new secondaries to track.

The oplog size is configurable and we should set it as big as possible. Oplog size doesn't affect memory usage and can make or break the database in case of operational issues.

The reason is that if replication lag increases over time, we will eventually get to the point where secondaries will fall back so much behind the primary that they won't be able to read from the primary's oplog as the oldest entry in the primary's oplog will be later than the latest entry applied in our secondary server.

In general, oplog should be at least 1-2 days' worth of operations. Oplog should be longer than the time it takes for the initial sync for the same reason as detailed previously.