During Cloudera Manager installation, the first run of the Cloudera Manager wizard adds and configures the services you want to run on the hosts in your cluster. By default, Cloudera Manager autoconfigures the services based on the resources available on the hosts. After the first run of the wizard, you can use Cloudera Manager to reconfigure the existing services, and add and configure additional hosts and services.
When Cloudera Manager configures a service, it allocates one or more functions (called roles in Cloudera Manager) that are required for that service to the hosts in your cluster. The role determines which service daemons run on a given host. For example, when Cloudera Manager configures an HDFS service instance it configures one host to run the NameNode role, another host to run as the Secondary NameNode role, another host to run the Balancer role, and some or all of the remaining hosts as to run DataNode roles.
Configuration properties have hard-coded default values that are not dependent on host resources. Typically defaults align with whatever CDH uses by default for that configuration. Cloudera Manager generally only deviates when the CDH default is not a recommended configuration. Sometimes default values are illegal, such as for data directories (the default is no data directories). The complete catalog of properties and their default values are documented in Cloudera Manager Configuration Properties.
When a configuration property has been set to a value different from the default, a Reset to the default value link displays. You can use this link if you want to change a manually or autoconfigured value back to the default state. There is no mechanism for resetting to an autoconfigured value. However, you can use the configuration history and rollback feature to revert any configuration changes.