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2.3. About Clusters

A cluster provides a way to group hosts. To be precise, a cluster is a XenServer server pool, a set of KVM servers, , or a VMware cluster preconfigured in vCenter. The hosts in a cluster all have identical hardware, run the same hypervisor, are on the same subnet, and access the same shared primary storage. Virtual machine instances (VMs) can be live-migrated from one host to another within the same cluster, without interrupting service to the user.
A cluster is the third-largest organizational unit within a CloudStack deployment. Clusters are contained within pods, and pods are contained within zones. Size of the cluster is limited by the underlying hypervisor, although the CloudStack recommends less in most cases; see Best Practices.
A cluster consists of one or more hosts and one or more primary storage servers.
cluster-overview.png: Structure of a simple cluster
CloudStack allows multiple clusters in a cloud deployment.
Even when local storage is used exclusively, clusters are still required organizationally, even if there is just one host per cluster.
When VMware is used, every VMware cluster is managed by a vCenter server. Administrator must register the vCenter server with CloudStack. There may be multiple vCenter servers per zone. Each vCenter server may manage multiple VMware clusters.