Put your MongoDB journal files and data files on the same disk
It is a common recommendation to separate components onto different storage devices. However, persistent disk already stripes data across a very large number of volumes, so there is no need to do it yourself.
MongoDB journal data is small and putting it on its own disk means either creating a small disk with insufficient performance or creating a large disk that goes mostly unused. Put your MongoDB journal files on the same disk as your data. Otherwise putting your MongoDB journal files on a small persistent disk will dramatically decrease performance of database writes.