VMware Clusters
A cluster is a group of tightly coupled hosts that work closely together so that in many respects they can be viewed as though they are a single computer. Clusters are used for high availability and load balancing. With a cluster, you define two or more physical machines that will provide resources for the hosts that are assigned to that cluster. By using clusters, you can achieve high availability and load balancing of virtual machines. Load balancing is referred to as DRS (Distributed Resource Scheduler) by VMware.
Thus, you use clusters for the following:
■ If one of the physical hosts goes down, the other physical host starts up the VMs that the original host was running (high availability).
■ If one physical host is over utilized by a VM, that VM is moved to the other physical host (DRS).
Both of these features use vMotion to move these virtual guests from one system to another.
You cannot apply high availability nor DRS to a Virtual Replication Appliance (VRA).