See SERVER-13182 for background and http://dom.as/2014/03/31/mongo-io. We want an option to use a different directory for the journal and prefer not to use symbolic links or mount $dbpath/journal as a separate filesystem.
Config option to support custom journal location
Include "WiredTigerTmplog" as a valid file to expect in the journal directory for disk/journalpath.js
Explicit control over directories created under dbpath by mongod
Add API in BackupCursorService for new-style backup cursors
How to write journal directory in a seperate hard disk drive