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Asked: 2026-05-28T16:08:58+00:00

I use the below to create a new database in mongo sudo mkdir -p

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I use the below to create a new database in mongo

sudo mkdir -p /data/db2/

When I start mongo e.g. 

mongod --port 27019 --dbpath /data/db2 --replSet rtb/test:27017 --rest

Mongo creates a 3 gig file. I am in dev and to reduce the size to e.g. 100 Megs. How to I do that?

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  1. Editorial Team

    MongoDB will pre-allocate data files. when it starts running. By default a new database will contain the following files (where test is the name of the DB)

    • test.ns (16MB)
    • test.0 (16MB)
    • test.1 (32MB)

    As more files are needed, the pattern continues: 64MB, 128MB, 256MB, 1024MB, 2048MB. After 2GB, each new file will be 2GB.

    In your case, you are running a replica set. Replica Sets require a database called local. This database contains the oplog which is used replication. The oplog is a capped collection.

    The capped collection is a special collection which must be pre-allocated. In your case it is likely defaulting to about 3GB.

    You can control the size of the local database using the --oplogSize parameter.

    If you want to keep your local copy small you can also use the --smallfiles option which will slow down the speed of pre-allocation.

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