Currently log components (introduced in SERVER-5092) are laid out in a flat namespace with a default:

• Debug
  • Access Control
  • Commands
  • Indexing
  • Networking
  • Query
  • Replication
  • Sharding
    **Storage

It would be desirable to support a hierarchy of log components:

• Default
  • Access Control
  • ...
  • Storage
    • Journalling

In the example above, if the log level for Default is 2 and Storage is set to 3 and Journalling is not configured, Journalling would inherit Storage's log level so that MONGO_LOG(3, Journalling) messages would be written to the log destination.