Memory-Mapped Storage Engine

Question

From MongoDb's site, the current only support storage engine is Memory-Mapped Storage Engine. I am wondering how it supports atomicity in the presence of system crash(i.e power failure)? Does it depend on the OS? Is there any reference about this topic?

What are the pros and cons of this design comparing with:

1. couchdb's crash only design which depends on append only b+tree.
2. sqlite's rollback journal file

Thanks

Answer 1

This is the page you are looking for:

http://www.mongodb.org/display/DOCS/Durability+and+Repair

Answer 2

Either you use a replica set for ensure that the data arrived on at least N replica nodes or for the standalone case you'll wait for MongoDB 1.8 having a dedicated journal to ensure better durability across crashes (should be available with the latest 1.7.X unstable release). And apart from that: Google - this issue has been discussed and documented in depth in various postings and blogs. Not sure why don#t research first yourself.