Question

I just import my Mysql database (size 2,8 Mio) in my new Mongo database with very simple php script i build, import was ok without error but when i look my Mongo database (with RockMongo) i can see this : Data Size 8.01m, Storage Size 13.7m.

MongoDB is bigger than Mysql for the same amount of data, is this normal ?

Thanks for your help and sorry for my english.

Answer 1

Yes, it’s normal that the “same” data will take up more space in mongodb. There’s a few things you need to take into account:

1) the document _id that’s stored for each document (unless you are specifying your own value for that) is 12 bytes per doc
2) you’re storing the key for each key-value pair in each document, whereas in MySql the column name is not stored for every single row so you have that extra overhead in your mongodb documents too. One way to reduce this is to use shortened key names (“column names”) in your docs
3) mongodb automatically adds padding to allow documents to grow

In similar tests, loading data from SQL Server to MongoDB, with shortened 2 character document key names instead of the full names as per SQL Server, I see about 25-30% extra space being used in MongoDB.