Replication - MongoDB: The Definitive Guide - page 142

--oplogSize . To change the oplog size to size , we shut down the master and run the
following:
$ rm /data/db/local.*
$ ./mongod --master --oplogSize size
size is specified in megabytes.
Preallocating space for a large oplog can be time-consuming and might
cause too much downtime for the master node. It is possible to manually
preallocate data files for MongoDB if that is the case; see the MongoDB
documentation on halted replication for more detailed information.
After restarting the master, any slaves should either be restarted with the
--autoresync or have a manual resync performed.
Replication with Authentication
If you are using replication in tandem with MongoDB's support for authentication (see
“Authentication Basics” on page 118 ), there is some additional configuration that needs
to be performed to allow the slave to access the data on the master. On both the master
and the slave, a user needs to be added to the local database, with the same username
and password on each node. Users on the local database are similar to users on ad-
min ; they have full read and write permissions on the server.
When the slave attempts to connect to the master, it will authenticate using a user
stored in local.system.users . The first username it will try is “repl,” but if no such user
is found, it will just use the first available user in local.system.users . So, to set up
replication with authentication, run the following code on both the master and any
slaves, replacing password with a secure password:
> use local
switched to db local
> db.add User("repl", password );
{
"user" : "repl",
"readOnly" : false,
"pwd" : "..."
}
The slave will then be able to replicate from the master.