The Mongo approach is described in some internals documents:
http://www.mongodb.org/display/DOCS/Durability+Internals
https://docs.google.com/drawings/edit?id=1iXjXZGz_ru6n-ub3EWMPtXBS424rH623nsyMQx8x4mo&hl=en
https://docs.google.com/drawings/edit?id=1TklsmZzm7ohIZkwgeK6rMvsdaR13KjtJYMsfLr175Zc&hl=en