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).
When protecting virtual machines in a cluster, if you are protecting a vApp, you must install a VRA on every ESX/ESXi host in the cluster on both the protected and recovery sites and ensure that DRS is enabled for these clusters. For other virtual machines, it is recommended to install a VRA on every ESX/ESXi host in the cluster, or to disable DRS on the machine with the virtual machines to be protected.
Note: When protecting virtual machines in vCloud Director and recovering to a vCenter Server, the virtual machines are recovered as a vApp and thus DRS must be enabled in the recovery site. When protecting virtual machines in vCenter Server and recovering to vCloud Director, after a recovery with reverse protection, from the vCD to the vCenter, the virtual machines are not replicated back to the original site as a vApp.