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<meta name="DC.Title" content="Cluster basics" />
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<meta name="abstract" content="Understand the basic clustering concepts before you begin to design and customize a cluster to satisfy your needs." />
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<title>Cluster basics</title>
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<body id="rzaigconceptsbasiccluster"><a name="rzaigconceptsbasiccluster"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Cluster basics</h1>
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<div><p>Understand the basic clustering concepts before you begin to design
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and customize a cluster to satisfy your needs.</p>
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<p><img src="./delta.gif" alt="Start of change" />There are two basic concepts related to cluster: cluster
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nodes and cluster resource group. A <dfn class="term"> cluster node</dfn> is either an <span class="keyword">iSeries™</span> system or logical partition
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that is a member of the cluster. When you create a cluster, you specify the
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systems or logical partitions that you want to include in the cluster as nodes.
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A <dfn class="term">cluster resource group (CRG)</dfn> serves as the control object for
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a collection of resilient resources. A CRG may contain a subset or all of
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nodes within the cluster. An iSeries cluster supports four types of CRGs: application,
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data, device, and peer. Within these types of CRGs there are two common elements:
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a recovery domain and an exit program.<img src="./deltaend.gif" alt="End of change" /></p>
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<p>A <dfn class="term">recovery domain</dfn> defines the role of each node in the CRG.
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When you create a CRG in a cluster, the CRG object is created on all nodes
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specified to be included in the recovery domain. However, a single system
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image of the CRG object, which you can access from any active node in the
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CRG's recovery domain, is provided. That is, any changes made to the CRG will
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be made on all nodes in the recovery domain. </p>
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<p>An <dfn class="term">exit program</dfn> is called during cluster-related
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events for the CRG. One such event is moving an access point from one node
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to another node. </p>
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<div class="p"><img src="./delta.gif" alt="Start of change" />There are two models of CRGs that can be created in a cluster:
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primary-backup model and peer model. In the primary-backup model, the nodes
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in the recovery domain of the CRG can be defined as follows:<ul><li>The <dfn class="term">primary node</dfn> is the cluster node that is the primary
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point of access for the resilient cluster resource. </li>
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<li>A <dfn class="term">backup node</dfn> is a cluster node that will take over the role
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of primary access if the present primary node fails or a manual switchover
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is initiated.</li>
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<li>A <dfn class="term">replicate node</dfn> is a cluster node that has copies of cluster
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resources, but is unable to assume the role of primary or backup.</li>
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</ul>
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<img src="./deltaend.gif" alt="End of change" /></div>
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<div class="p"><img src="./delta.gif" alt="Start of change" />In a peer model, the recovery domain of a peer CRG defines
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an peer relationship between nodes. The nodes in the recovery domain of the
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peer CRG can be defined as follows:<ul><li>A <dfn class="term">peer node</dfn> is a cluster node can be an active access point
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for cluster resources. </li>
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<li>A <dfn class="term">replicate node</dfn> is a cluster node that has copies of cluster
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resources. Nodes defined as replicate in a peer CRG represent the inactive
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access point for cluster resources.</li>
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</ul>
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With a peer CRG, the nodes in the recovery domain are equivalent with
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respect to the role the nodes plays in recovery. Because each node in this
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peer CRG has essentially the same role, the concepts of a failover and switchover
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do not apply. The nodes have a peer relationship, and when one of the nodes
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fails, others peer nodes will continue operating. <img src="./deltaend.gif" alt="End of change" /></div>
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<p><img src="./delta.gif" alt="Start of change" />You can also create a cluster administrative domain which
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is represented by a peer CRG. The nodes in a cluster administrative domain
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are all peer nodes in the CRG's recovery domain. There are no replicate nodes. <img src="./deltaend.gif" alt="End of change" /></p>
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<p> In the example below, one CRG of each type is present:</p>
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<br /><img src="./delta.gif" alt="Start of change" /><img src="rzaig506.gif" alt="A basic cluster consisting of five nodes, an application CRG, a data CRG and a device CRG. " /><img src="./deltaend.gif" alt="End of change" /><br /><dl><dt class="dlterm">Data CRG</dt>
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<dd>The data CRG is present on Node 1, Node 2 and Node 3. This means that
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the recovery domain for the data CRG has specified a role for Node 1 (primary),
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Node 2 (first backup) and Node 3 (second backup). In the example, Node 1 is
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currently serving as the primary point of access. Node 2 is defined as the
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first backup in the recovery domain. This means that Node 2 contains a copy
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of the resource which is kept current through logical replication. Should
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a failover or switchover occur, Node 2 becomes the primary point of access.</dd>
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<dt class="dlterm">Application CRG</dt>
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<dd>The application CRG is present on Node 4 and Node 5. This means that the
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recovery domain for the application CRG has specified Node 4 and Node 5. In
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the example, Node 4 is currently serving as the primary point of access. Should
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a failover or switchover occur, Node 5 becomes the primary point of access
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for the application. Requires a takeover IP address.</dd>
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<dt class="dlterm"><img src="./delta.gif" alt="Start of change" />Peer CRG<img src="./deltaend.gif" alt="End of change" /></dt>
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<dd><img src="./delta.gif" alt="Start of change" />The peer CRG is present on Node 6 and Node 7. This means
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that the recovery domain for the peer CRG has specified Node 6 and Node 7.
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In this example, nodes 6 and 7 can be either peer or replicate nodes. If this
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is a cluster administrative domain that is represented by peer CRG, resources
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that are monitored by the cluster administrative domain will have any changes
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synchronized across the domain represented by node 6 and node 7, regardless
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on which node the change originated.<img src="./deltaend.gif" alt="End of change" /></dd>
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<dt class="dlterm">Device CRG</dt>
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<dd>The device CRG is present on Node 2 and Node 3. This means that the recovery
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domain for the device CRG has specified Node 2 and Node 3. In the example,
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Node 2 is currently serving as the primary point of access. This means that
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the resilient device owned by the device CRG can currently be accessed from
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Node 2. Should a failover or switchover occur, Node 3 becomes the primary
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point of access for the device. <p>A device CRG requires a resilient device
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called an independent disk pool (also called an independent auxiliary storage
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pool or independent ASP) to be configured on an external device, an expansion
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unit (tower) or IOP in a logical partition.</p>
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<p>The nodes in the recovery
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domain of a device CRG must also be members of the same device domain. The
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example below illustrates a device CRG with Node L and Node R in its recovery
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domain. Both nodes are also members of the same device domain.</p>
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</dd>
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</dl>
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<br /><img src="rzaig502.gif" alt="A data CRG featuring a device domain and an external expansion unit" /><br /></div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzaigconcepts.htm" title="Get a complete understanding of how clusters work. Read about the benefits of clusters and how they can be important to you, as well as information on important clustering concepts and how they fit together.">Cluster concepts</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="rzaigconceptsclusternode.htm" title="A cluster node is an iSeries system or logical partition that is a member of a cluster.">Cluster node</a></div>
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<div><a href="rzaigconceptscrg.htm" title="A cluster resource group (CRG) is an i5/OS system object that is a set or grouping of cluster resources that are used to manage events that occur in a clustered environment. The cluster resource group describes a recovery domain and supplies the name of the cluster resource group exit program which gets called when certain cluster events occur.">Cluster resource group</a></div>
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<div><a href="rzaigconceptsrecoverydomain.htm" title="A recovery domain is a subset of cluster nodes that are grouped together in a cluster resource group (CRG) for a common purpose such as performing a recovery action or synchronizing events.">Recovery domain</a></div>
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<div><a href="rzaigconceptsexitprograms.htm" title="A cluster resource group exit program is called after a cluster-related event for a CRG occurs.">Cluster resource group exit programs</a></div>
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<div><a href="../rzaly/rzalyoverview.htm">Independent disk pools</a></div>
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<div><a href="rzaigconceptsdevicedomain.htm" title="A device domain is a subset of nodes in a cluster that share device resources. More specifically, nodes in a device domain can participate in a switching action for some collection of resilient device resources.">Device domains</a></div>
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</div>
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</div>
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