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21 lines
1.9 KiB
XML
21 lines
1.9 KiB
XML
<?xml version='1.0' encoding='utf-8' ?>
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<!DOCTYPE bookinfo PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "file:///C:/Program%20Files%20(x86)/Publican/DocBook_DTD/docbookx.dtd" [
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<!ENTITY % BOOK_ENTITIES SYSTEM "cloudstack.ent">
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%BOOK_ENTITIES;
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]>
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<section id="about-clusters">
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<title>About Clusters</title>
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<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>
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<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>
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<para>A cluster consists of one or more hosts and one or more primary storage servers.</para>
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<mediaobject>
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<imageobject>
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<imagedata fileref="./images/cluster-overview.png" />
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</imageobject>
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<textobject><phrase>cluster-overview.png: Structure of a simple cluster</phrase></textobject>
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</mediaobject>
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<para>&PRODUCT; allows multiple clusters in a cloud deployment.</para>
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<para>Even when local storage is used, clusters are still required. In this case, there is just one host per cluster.</para>
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<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>
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</section>
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