How to know MongoDB collection size?

Question

db.collection.stats()

Response :

    "count" : 20696445,
    "size" : NumberLong("1478263842661"),
    "storageSize" : 334732324864,
    "totalIndexSize" : 676327424,
    "indexSizes" : {
            "_id_" : 377094144,
            "leadID_1" : 128049152,
            "leadID_hashed" : 171184128
    },
    "avgObjSize" : 71425.97884134208

My actual disk size is matched with storageSize. So what is the size and other keys.

Answer 1

You haven't mentioned the version of MongoDB server you are using but given the size of your data is much larger than the storageSize on disk, I'm assuming you are using the WiredTiger storage engine which compresses data and indexes by default. The WiredTiger storage engine was first available as an option in the MongoDB 3.0 production series and became the default storage engine for new deployments in MongoDB 3.2+.

In your example output it looks like you have 1.4TB of uncompressed data which is currently occupying 334GB on disk (the storageSize value). Storage space used by indexes for this collection is reported separately under indexSizes and summed up as totalIndexSize.

The output of collection.stats() will vary depending on your MongoDB server version and configured storage engine, but is generally described in the MongoDB manual as part of the output of the collStats command which is called by the db.collection.stats() shell helper.

Note: MongoDB documentation is versioned so you should always make sure you are referencing documentation that matches your release series of MongoDB (i.e. 3.2, 3.4, ...). Default documentation links will point to the current production release.