mongod currently creates/uses subdirectories under the dbpath:
- _tmp - used by external sorter
- moveChunk - used by sharding
- rollback - used for rollbacks
Specifying journalPath is already covered by SERVER-13455.
For a number of reasons it would be desirable to make these paths user defined. One case would be when applying security and/or encryption policies for these directories.
Note that currently the _tmp directory will be removed when the server starts. This can also be problematic when attempting to apply such a policy.