Overview of Using Cloudera Manager for Configuring Services
During Cloudera Manager installation, the first run of the Cloudera Manager Wizard will add and configure the Hadoop services you want to run on the hosts in your cluster. By default, Cloudera Manager will determine what it considers the optimal configuration of role instances on the available hosts, though you can make these determinations yourself if you want. 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 configures host machines in your cluster with one or more functions (called roles in Cloudera Manager) that are required for that service. The role determines which Hadoop 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. Services are named by default based on their function (e.g. the HDFS service may be named hdfs1) though you can optionally provide your own display names.
The associated role instances are named based on a combination of the role name and the host on which that role runs. So, if you have a set of hosts (myhost1, myhost2,... etc.) the roles associated with the HDFS service would be namenode (myhost1), secondarynamenode (myhost2), datanode (myhost3), datanode (myhost4), and so on, which all run under the hdfs1 service instance on those same hosts.
Whenever you add and configure a service, you are creating an instance of that service on your cluster; that is, you can uniquely configure and run multiple instances of some services and roles. For most purposes, configuring and running one instance of the services may be sufficient.