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A cluster provides a way to group hosts. To be precise, a cluster is a XenServer server pool, a set of KVM servers, a set of OVM hosts, 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 &PRODUCT; deployment. Clusters are contained within pods, and pods are contained within zones. Size of the cluster is limited by the underlying hypervisor, although the &PRODUCT; 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 &PRODUCT; allows multiple clusters in a cloud deployment. Even when local storage is used, clusters are still required. In this case, 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 &PRODUCT;. There may be multiple vCenter servers per zone. Each vCenter server may manage multiple VMware clusters.