So let me first start the question with my understanding of how MongoDb stores data on disk: So when you create a database in mongodb, it allocates a large file named
<databasename>.0 and within that file allocates extents that are contiguous regions that correspond to data for a particular collection or particular index.
At such point as this datafile is filled, it creates a new file called
<databasename>.1 and populates it in a similar way. Therefore it seems sensible to assume that the most recently inserted data into a particular database will be in the highest numbered file (and my performance tests confirm this).
However, I can't see how this could be true for indices... since we're talking about a bTree, it doesn't seem possible/sensible to have this bTree scattered across files in the same way. As Mongo is doing the maintenance for an index, does the whole index live in one extent until it outgrows it at which point it is relocated to the current (highest numbered datafile)?.
This has become important to me as when starting a database from Amazon EBS snapshot, it seems there's a huge overhead for hitting these datafiles until the volume warms up. I am only interested in a subset of the most recent N documents from a collection. If I could be sure I was only going to need the most recent couple of datafiles I could prewarm these files by reading sequentially before starting mongod.