MongoDB Sharding

A city phone directory that grows too thick to bind as one book gets divided into volumes: A to K in one, and L to Z in another. A reader looking for "Khan" opens only the first volume. MongoDB divides a large collection in a similar way, and placing the parts on different servers is called sharding.

A replica set cannot do this for you. Each member of a replica set stores all the data, and every write goes to one primary. So when the data outgrows one server's disk, or the writes outgrow one primary, adding secondaries does not help