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HTML
194 lines
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<meta name="DC.Title" content="Comparison of logical replication, switched disks, and cross-site mirroring" />
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<meta name="abstract" content="This topic provides an overview of different data resilience technologies that can be used with clusters to enhance high availability." />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<title>Comparison of logical replication, switched disks, and cross-site mirroring</title>
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<body id="rzaigconceptsreplicationvsswitchable"><a name="rzaigconceptsreplicationvsswitchable"><!-- --></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">Comparison of logical replication, switched disks, and cross-site mirroring</h1>
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<div><p><span><img src="./delta.gif" alt="Start of change" />This topic provides an overview of different
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data resilience technologies that can be used with clusters to enhance high
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availability.<img src="./deltaend.gif" alt="End of change" /></span></p>
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<p><img src="./delta.gif" alt="Start of change" /><dfn class="term">Data resiliency</dfn> allows data to remain available
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to applications and users even though the system that originally hosted the
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data fails. Choosing the correct set of data resiliency technologies in the
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context of your overall business continuity strategy can be complex and difficult.
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It's important to understand the different data resilience solutions that
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can be used to enhance availability in multiple system environments. You can
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either choose a single solution or use a combination of these technologies
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to meet your needs.<img src="./deltaend.gif" alt="End of change" /></p>
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<p>For more details on these solutions, see <a href="http://publib-b.boulder.ibm.com/abstracts/redp0888.html?Open" target="_blank">Data Resilience Solutions for IBM<sup>®</sup> i5/OS™ High Availability Clusters</a>.
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The section called "Data Resilience Solutions for IBM i5/OS High Availability Clusters" contains
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a detailed comparison of the attributes for each of these technologies. </p>
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<div class="section"><h4 class="sectiontitle">Logical replication</h4><p><dfn class="term">Logical replication</dfn> is
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the process of copying objects from one node in a cluster to one or more other
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nodes in the cluster, which makes the objects on all the systems identical. </p>
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<p>A
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replicated resource allows for objects, such as an application and its data,
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to be copied from one node in the cluster to one or more other nodes in the
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cluster. This process keeps the objects on all servers in the resource's recovery
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domain identical. If you make a change to an object on one node in a cluster,
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the change is replicated to other nodes in the cluster. Then, should a failover
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or switchover occur, the backup node can seamlessly take on the role of the
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primary node. The server or servers that act as backups are defined in the
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recovery domain. When an outage occurs on the server defined as the primary
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node in the recovery domain and a switchover or failover is initiated, the
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node designated as the backup in the recovery domain becomes the primary access
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point for the resource.</p>
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<p>Replication requires the use of either a custom-written
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application or a software application written by a cluster middleware business
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partner. See <a href="rzaigplanreplication.htm">Plan
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for logical replication</a> for details.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Switchable disks</h4><p><dfn class="term">Switchable disks</dfn> enable
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the resources, such as data and applications, residing on an expansion unit
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or on an input-output processor (IOP) on a shared bus or in an I/O Pool for
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a logical partition, to be switched between a cluster's primary node and backup
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node. This allows for a set of disk units to be accessed from a second server,
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a server defined as a backup node in the cluster resource group's recovery
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domain, when the server currently using those disk units experiences an outage
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and a failover or switchover occurs. </p>
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<p>Taking advantage of switchable
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resources in your cluster requires the use of independent disk pools. See <a href="../rzaly/rzalyplan.htm">Plan for independent
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disk pools</a> for more information.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Cross-site mirroring</h4><p><dfn class="term">Cross-site mirroring</dfn>,
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combined with the geographic mirroring function, enables you to mirror data
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on disks at sites that can be separated by a significant geographic distance.
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This technology can be used to extend the functionality of a device cluster
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resource group (CRG) beyond the limits of physical component connection. Geographic
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mirroring provides the ability to replicate changes made to the production
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copy of an independent disk pool to a mirror copy of that independent disk
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pool. As data is written to the production copy of an independent disk pool,
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the operating system mirrors that data to a second copy of the independent
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disk pool through another system. This process keeps multiple identical copies
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of the data. </p>
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<p>Through the device CRG, should a failover or switchover
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occur, the backup node can seamlessly take on the role of the primary node.
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The server or servers that act as backups are defined in the recovery domain.
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The backup nodes can be at the same or different physical location as the
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primary. When an outage occurs on the server defined as the primary node in
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the recovery domain and a switchover or failover is initiated, the node designated
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as the backup in the recovery domain becomes the primary access point for
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the resource and will then own the production copy of the independent disk
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pool. Thus, you can gain protection from the single point of failure associated
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with switchable resources.</p>
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</div>
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<div class="section">
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<div class="tablenoborder"><img src="./delta.gif" alt="Start of change" /><a name="rzaigconceptsreplicationvsswitchable__table1"><!-- --></a><table cellpadding="4" cellspacing="0" summary="" id="rzaigconceptsreplicationvsswitchable__table1" frame="border" border="1" rules="all"><caption>Table 1. Comparison
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of data resilient technologies that can be used with clusters. Learn
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about characteristics of different data resiliency technologies to help you
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determine the best solution for your cluster.</caption><thead align="left"><tr><th valign="top" class="firstcol" id="d0e88">Factor</th>
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<th valign="top" id="d0e90">Replication</th>
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<th valign="top" id="d0e92">Switchable disks</th>
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<th valign="top" id="d0e94">Cross-site mirroring</th>
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</tr>
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</thead>
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<tbody><tr><th valign="top" class="firstcol" id="d0e98" headers="d0e88 ">Flexibility</th>
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<td valign="top" headers="d0e98 d0e90 ">10s of servers</td>
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<td valign="top" headers="d0e98 d0e92 ">2 servers</td>
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<td valign="top" headers="d0e98 d0e94 ">4 servers</td>
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</tr>
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<tr><th valign="top" class="firstcol" id="d0e107" headers="d0e88 ">Single point of failure</th>
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<td valign="top" headers="d0e107 d0e90 ">None</td>
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<td valign="top" headers="d0e107 d0e92 ">Disk subsystem</td>
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<td valign="top" headers="d0e107 d0e94 ">None</td>
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</tr>
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<tr><th valign="top" class="firstcol" id="d0e116" headers="d0e88 ">Cost</th>
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<td valign="top" headers="d0e116 d0e90 "><ul><li>Additional disk capacity required.</li>
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<li>Business partner replication software.</li>
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<li>Duplicate disks</li>
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</ul>
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</td>
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<td valign="top" headers="d0e116 d0e92 "><ul><li>Switchable I/O expansion tower or IOP </li>
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<li>Option 41</li>
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</ul>
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</td>
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<td valign="top" headers="d0e116 d0e94 "><ul><li>Additional disk for mirror copy of independent disk pool </li>
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<li>Optionally switchable I/O expansion unit </li>
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<li>Option 41</li>
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</ul>
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</td>
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</tr>
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<tr><th valign="top" class="firstcol" id="d0e141" headers="d0e88 ">Performance</th>
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<td valign="top" headers="d0e141 d0e90 ">Replication overhead</td>
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<td valign="top" headers="d0e141 d0e92 ">Little impact</td>
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<td valign="top" headers="d0e141 d0e94 ">Geographic mirroring overhead</td>
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</tr>
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<tr><th valign="top" class="firstcol" id="d0e150" headers="d0e88 ">Real time coverage</th>
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<td valign="top" headers="d0e150 d0e90 ">Journaled objects</td>
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<td valign="top" headers="d0e150 d0e92 ">Objects contained in independent disk pool</td>
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<td valign="top" headers="d0e150 d0e94 ">Objects contained in independent disk pool</td>
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</tr>
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<tr><th valign="top" class="firstcol" id="d0e159" headers="d0e88 ">Geographic dispersion</th>
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<td valign="top" headers="d0e159 d0e90 ">Limited by performance considerations</td>
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<td valign="top" headers="d0e159 d0e92 ">Limited attach distance as servers and expansion units must be attached
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to HSL OptiConnect loop (250 meters maximum)</td>
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<td valign="top" headers="d0e159 d0e94 ">Limited by performance considerations (No limits are imposed by the
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system. However, response time and throughput over selected communications
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lines can dictate some practical limit.)</td>
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</tr>
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<tr><th valign="top" class="firstcol" id="d0e168" headers="d0e88 ">Disaster recovery protection</th>
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<td valign="top" headers="d0e168 d0e90 ">Yes</td>
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<td valign="top" headers="d0e168 d0e92 ">No</td>
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<td valign="top" headers="d0e168 d0e94 ">Yes</td>
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</tr>
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<tr><th valign="top" class="firstcol" id="d0e177" headers="d0e88 ">Concurrent backup</th>
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<td valign="top" headers="d0e177 d0e90 ">Yes</td>
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<td valign="top" headers="d0e177 d0e92 ">No</td>
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<td valign="top" headers="d0e177 d0e94 ">No</td>
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</tr>
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<tr><th valign="top" class="firstcol" id="d0e186" headers="d0e88 ">Setup</th>
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<td valign="top" headers="d0e186 d0e90 "><ul><li>Replication environment. </li>
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<li>Determining what to replicate.</li>
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</ul>
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</td>
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<td valign="top" headers="d0e186 d0e92 "> <ul><li>Independent disk pool environment.</li>
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<li>Populate independent disk pool</li>
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</ul>
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</td>
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<td valign="top" headers="d0e186 d0e94 "> <ul><li>Independent disk pool environment (include setting up geographic mirroring) </li>
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<li>Populate independent disk pool</li>
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</ul>
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</td>
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</tr>
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</tbody>
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</table>
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<img src="./deltaend.gif" alt="End of change" /></div>
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</div>
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</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="rzaigplandataresilience.htm" title="Data resilience is achieved when data is always available to an user or application. You can achieve data resilience through the use of logical replication or switchable independent disk pools.">Plan for data resilience</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="rzaigplanreplication.htm" title="Multiple copies of the data are maintained with logical replication. Data is replicated, or copied, from the primary node in the cluster to the backup nodes designated in the recovery domain. When an outage occurs on the primary node, the data remains available as a designated backup node takes over as the primary point of access.">Plan for logical replication</a></div>
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<div><a href="rzaigplanswitcheddisks.htm" title="A single copy of the data is maintained on switchable hardware; either an expansion unit (tower) or an IOP in a logical partition environment.">Plan for switchable independent disk pools and cross-site mirroring (XSM)</a></div>
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</div>
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</div>
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</body>
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