What is difference between "size" and "storageSize" displayed by Mongo stats() function

Question

What is difference between "size" and "storageSize" displayed by Mongo stats() function? Which one shows the actual size on disk for a particular collection? Refer sample data returned by command

db.getCollection('temp_collection').stats()


{"ns" : "DB1.temp_collection",
    "count" : 1035219,
    "size" : 1186,
    "avgObjSize" : 1202,
    "storageSize" : 177,
    "capped" : false,
    "wiredTiger" : 
}

Answer 1

in simple words:

• size refers for logical allocation (this is seen by database engine)
• storageSize refers for physical file space allocation

As WT (WiredTiger) is used, with enabled data compression, then physical allocation is smaller because of compression

db.collection.storageSize

db.collection.stats