For local tests of replica sets (repliation cluster in MongoDB) it is useful to have the server instances to be lightweight with short ramp-up times. This can be achieved by using a specific combinations of these command line options:
$ mongod --smallfiles --oplogSIze <MB> --noprealloc --nohttpinterface --nojournal ...
With --smallfiles MongoDB uses a smaller size for the database files. The option --oplogSize <MB> determines the size of the oplog (which is by default 5% of the available disk space on 64bit systems). Use a value of 50 (MB). With --noprealloc you disable the preallocation of the database files. The options --nohttpinterface and --nojournal disable the HTTP admin interface and the journaling.
This is for testing only - do not use in production!