en-US/about-clusters.xml

<?xml version='1.0' encoding='utf-8' ?>
<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY % BOOK_ENTITIES SYSTEM "cloudstack.ent">
%BOOK_ENTITIES;
]>

<!-- Licensed to the Apache Software Foundation (ASF) under one
 or more contributor license agreements.  See the NOTICE file
 distributed with this work for additional information
 regarding copyright ownership.  The ASF licenses this file
 to you under the Apache License, Version 2.0 (the
 "License"); you may not use this file except in compliance
 with the License.  You may obtain a copy of the License at
 
   http://www.apache.org/licenses/LICENSE-2.0
 
 Unless required by applicable law or agreed to in writing,
 software distributed under the License is distributed on an
 "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 KIND, either express or implied.  See the License for the
 specific language governing permissions and limitations
 under the License.
-->

<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 fourth-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.
       An 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>