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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:
Has anyone stumbled across something like this? I can imagine it would be extremely useful given Mongo's performance degradation once it hits the memory wall and documents start getting paged out to disk. If I have a functioning database and want to add another index, the only way I'll know if it will be too big is to actually add it.
It wouldn't need to be accurate down to the bit, but with some assumptions about B-Trees and the index implementation I'm sure it could be reasonable enough to be helpful.
If this doesn't exist already I'd like to build and open source it, so if I've missed any required parameters for this calculation please include in your answer.
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Get size of index and data in MongoDB collection with scaling factor. db. user. stats(1024) , is used to get size of data and size of index on user collection in MongoDB.
collection. totalSize() method is used to reports the total size of a collection, including the size of all documents and all indexes on a collection. Returns: The total size in bytes of the data in the collection plus the size of every index on the collection.
A collection cannot have more than 64 indexes. The length of the index name cannot be longer than 125 characters. A compound index can have maximum 31 fields indexed.
Finding indexes You can find all the available indexes in a MongoDB collection by using the getIndexes method. This will return all the indexes in a specific collection. Result: The output contains the default _id index and the user-created index student name index.
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
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You can check the sizes of the indexes on a collection by using command:
db.collection.stats()
More details here: http://docs.mongodb.org/manual/reference/method/db.collection.stats/#db.collection.stats
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