## Add an Arbiter to Replica Set 

Arbiters are mongod instances that are part of a replica set but do not hold data. Arbiters participate in elections (page 580) in order to break ties. If a replica set has an even number of members, add an arbiter. 

Arbiters have minimal resource requirements and do not require dedicated hardware. You can deploy an arbiter on an application server or a monitoring host. 

Important: Do not run an arbiter on the same system as a member of the replica set. 

## Considerations 

An arbiter does not store data, but until the arbiter’s mongod process is added to the replica set, the arbiter will act like any other mongod process and start up with a set of data files and with a full-sized journal . 

To minimize the default creation of data, set the following in the arbiter’s configuration file : 

• journal.enabled to false 

Warning: Never set journal.enabled to false on a data-bearing node. 

• smallFiles to true 

These settings are specific to arbiters. Do not set journal.enabled to false on a data-bearing node. Similarly, do not set smallFiles unless specifically indicated. 

## Add an Arbiter 

1. Create a data directory (e.g. dbPath ) for the arbiter. The mongod instance uses the directory for configuration data. The directory will not hold the data set. For example, create the /data/arb directory: 

mkdir /data/arb 

2. Start the arbiter. Specify the data directory and the replica set name. The following, starts an arbiter using the /data/arb dbPath for the rs replica set: 

mongod --port 30000 --dbpath /data/arb --replSet rs 

3. Connect to the primary and add the arbiter to the replica set. Use the rs.addArb() method, as in the following example: 

rs.addArb("m1.example.net:30000") 

This operation adds the arbiter running on port 30000 on the m1.example.net host. 

## Convert a Standalone to a Replica Set 

This tutorial describes the process for converting a standalone mongod instance into a three-member replica set . Use standalone instances for testing and development, but always use replica sets in production. To install a standalone instance, see the installation tutorials (page 5). 

To deploy a replica set without using a pre-existing mongod instance, see Deploy a Replica Set (page 603). 

