How to separate MongoDB journal
You have to create a symbolic link that points to the new mountpoint:


  If your server runs out of disk space for journal files, the server
  process will exit. By default, mongod creates journal files in a
  sub-directory of dbpath named journal. You may elect to put the
  journal files on another storage device using a filesystem mount or a
  symlink.
  
  Note
  
  If you place the journal files on a separate storage device you will
  not be able to use a file system snapshot tool to capture a valid
  snapshot of your data files and journal files.


Updated link about: Journal files