oplog

--oplogSize Specifies a maximum size in megabytes for the replication operation log (i.e., the oplog). The mongod process creates an oplog based on the maximum amount of space available.
For 64-bit systems, the oplog is typically 5% of available disk space. Once the mongod has created the oplog for the first time, changing the --oplogSize option will not affect the size of the oplog. See replica-set-oplog-sizing for more information.