MongoDB Memory Management

Nice Article: http://www.briancarpio.com/2012/05/03/mongodb-memory-management/

Credit goes to the Brian Carpio

How MongoDB's Journaling Works

MongoDB journal provides durability in the event of a failure, MongoDB uses write-ahead logging to on-disk journal files.

It has explained in nicely in below article.  Credit should go to the original author 

https://www.mongodb.com/blog/post/how-mongodbs-journaling-works