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<section id="about-clusters">
    <title>About Clusters</title>
    <para>
       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.
    </para>
    <para>
       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.
    </para>
    <para>
       A cluster consists of one or more hosts and one or more primary storage
       servers.
    </para>
    <mediaobject>
        <imageobject>
            <imagedata fileref="./images/cluster-overview.png" />
        </imageobject>
        <textobject><phrase>cluster-overview.png: Structure of a simple cluster</phrase></textobject>
    </mediaobject>
    <para>&PRODUCT; allows multiple clusters in a cloud deployment.</para>
    <para>
       Even when local storage is used exclusively, clusters are still required
       organizationally, even if there is just one host per cluster.
    </para>
    <para>
       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.
    </para>
</section>