ibm-information-center/dist/eclipse/plugins/i5OS.ic.rzahq_5.4.0.1/rzahqhighavailabilityconcepts.htm

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<title>High availability concepts</title>
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<h2 id="rzahqhighavailabilityconcepts">High availability concepts</h2>
<p>iSeries&trade; and xSeries&reg; integration and storage virtualization provide
innovative options that can enable you to enhance the reliability and recoverability
of your Windows server environment. Hosted systems can provide increased availability
with one or more of the following technologies.</p>
<p><span class="bold">Hot spare hardware</span></p>
<p>Hot spare hardware provides a way to quickly recover from certain types
of hardware failures. This can reduce the server downtime from hours or days
to minutes. With hosted systems, there are two ways to use hot spare hardware
to minimize downtime that is caused by hardware failures:</p>
<ol type="1">
<li>Hosted system hardware, including Integrated xSeries Servers, xSeries servers that are attached via an Integrated xSeries Adapter, and xSeries or IBM&reg; BladeCenter&trade; servers that are attached via an
iSCSI host bus adapter, can be hot spared. If the hardware that is used to
run the hosted system fails, you can quickly switch the hosted system's
disk images to compatible spare hardware and restart the hosted system. For
more information, see <a href="rzahqhotsparebetw.htm#rzahqhotsparebetw">Hot spare between server hardware</a>.</li>
<li>For iSCSI attached servers, the iSeries target iSCSI host bus adapters (iSCSI
HBA) can be hot spared. If an iSCSI HBA that a hosted system is using fails,
you can quickly switch the hosted system to use a spare iSCSI HBA and restart
the hosted system. For more information, see <a href="rzahqhotsparebetweeniscsi.htm#rzahqhotsparebetweeniscsi">Hot spare between iSCSI local host adapters</a>.</li></ol>
<p><span class="bold">iSCSI multi-path</span></p>
<p>A hosted system can use redundant iSCSI data paths to access virtual disks
hosted by i5/OS&trade;. This is accomplished by defining a group of two or more iSCSI
HBAs and then specifying that a given virtual disk should be accessed using
a group, rather than a single iSCSI HBA. With this configuration, the data
on the virtual disk can be accessed using any of the iSCSI HBAs in the group.</p>
<p>One advantage of the multi-path configuration is that if one of the iSCSI
HBAs in the multi-path group fails, the hosted system can continue to access
the disks that are configured to use the multi-path group uninterrupted, using
any of the other iSCSI HBAs that are configured in the multi-path group.
For more information, see <a href="rzahqadvancediscsihardware.htm#rzahqadvancediscsihardware">Advanced iSCSI support</a>.</p>
<p><span class="bold">Microsoft&reg; Windows Cluster Service (MSCS)</span></p>
<p>Hosted servers can use MSCS to provide real-time application failover in
the case of hosted system hardware or software failures. User-initiated failovers
can be used to take a server offline so that maintenance or backups can be
performed while the application continues to run on the other server(s) in
the cluster. For more information, see <a href="rzahqclusterintro.htm#rzahqclusterintro">Windows Cluster Service</a>.</p>
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