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Clustering

"A cluster is a group of hosts. When a host is added to a cluster, the host's resources become part of the cluster's resources. The cluster manages the resources of all hosts within it." - VMware vSphere v5.1 Docs


Clustering is a grouping of servers that put their resources into a pool. Using vMotion they can move VMs within the cluster to balance load across all the machines in the cluster. In this way no one member of the cluster is overwhelmed by one or more VMs that need resources (CPU, RAM, etc.).

Limitations:
  • vCenter must be in use to federate all the hosts together in one management infrastructure
  • All disk storage must be on a SAN device. Local storage (storage on the host itself) is not supported.
  • Hosts must be of the same CPU type and/or family. This gets a little tricky and could be an entire other tutorial/post.
  • These are the same limitations that vMotion has for the same reasons.
Adding a host to a cluster:
  1. Login to web client and enter the "Hosts and Clusters" view.
  2. Right-click on the host you wish to add to your cluster, from the context menu select "Enter Maintence mode". Hosts cannot be added or removed from a cluster unless they are in this mode.