A typical Hadoop cluster comprises of machines based on the various machine roles (master, slaves, and clients). For more details, see: Machine Roles In A Typical Hadoop Cluster. The following illustration provides information about a typical Hadoop cluster. A smallest cluster can be deployed using a minimum of four machines (for evaluation purpose only). One machine can be used to deploy all the master processes (NameNode, Secondary NameNode, JobTracker, HBase Master), Hive Metastore, WebHCat Server, and ZooKeeper nodes. The other three machines can be used to co-deploy the slave nodes (YARN NodeManagers, DataNodes, and RegionServers).
A minimum of three machines is required for the slave nodes in order to maintain the replication factor of three. For more details, see: Data Replication in Apache Hadoop
