Are there any tools to estimate index size in MongoDB?

Question

I\'m looking for a tool to get a decent estimate of how large a MongoDB index will be based on a few signals like:

• How many documents in my collection

Answer 1

I just spoke with some of the 10gen engineers and there isn't a tool but you can do a back of the envelope calculation that is based on this formula:

2 * [ n * ( 18 bytes overhead + avg size of indexed field + 5 or so bytes of conversion fudge factor ) ]

Where n is the number of documents you have.

The overhead and conversion padding are mongo specific but the 2x comes from the b-tree data structure being roughly half full (but having allocated 100% of the space a full tree would require) in the worst case.

I'd explain more but I'm learning about it myself at the moment. This presentation will have more details: http://www.10gen.com/presentations/mongosp-2011/mongodb-internals