There is a secondary problem with using mmap. You don't know if an access will take a page fault. When thrashing starts happening MongoDB doesn't throttle new

There is a secondary problem with using mmap. You don't know if an access will take a page fault. When thrashing starts happening MongoDB doesn't throttle new queries. New queries coming in then add fuel to the fire making existing queries take longer and longer. This causes a huge and rapid performance dropoff. Of course not all new queries will cause paging so they could be left unthrottled. There is a system call mincore that will tell you if pages will take faults but it doesn't support scatter/gather and has race conditions especially when there is lots of paging! I did report this at the beginning of 2010 - currently marked as major priority, planned but not scheduled: https://jira.mongodb.org/browse/SERVER-574 https://jira.mongodb.org/browse/SERVER-574 That said MongoDB is still my first database of choice. Nothing beats arbitrary JSON in, the same JSON back out.