As part of an upgrade of a cluster from v2.2 to v2.4, the config database will need to be upgraded and epochs added to all collections (to support SERVER-939). v2.2 is backwards-compatible with these changes, however v2.0 is not. In order to migrate the cluster, a single v2.4 mongos must be started with --upgrade, and no collection metadata changes (adding shards, dropping databases, dropping collections) can happen while the upgrade is in progress.

When an upgrade fails in the critical section when adding epochs to all collections in the config server, we print a message with a documentation link - this page needs to be created and maintained.

Needed pre-2.4.