166 lines
11 KiB
HTML
166 lines
11 KiB
HTML
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<title>How geographic mirroring works</title>
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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">How geographic mirroring works</h1>
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<div><div class="section"><h4 class="sectiontitle">Configure</h4><p>The nodes participating in geographic
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mirroring must be in the same cluster, the same device domain, and the same
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cluster resource group. Before configuring geographic mirroring, you must
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specify a site name and the TCP/IP address(es) for each node in the recovery
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domain. If you have more than one node at a site, then the hardware (disk
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units) you select for the disk pool must be switchable between the nodes at
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the site. If you only have one node at a site, the hardware does not have
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to be switchable and should be non-switchable (private). </p>
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<p>See <a href="rzalyconfiguregeographic.htm">Configure geographic mirroring with dedicated independent disk pools</a> and <a href="rzalyconfigureswitchgeographic.htm">Configure geographic mirroring with switchable independent disk pools</a> for more information. </p>
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<p>When geographic mirroring
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is configured, the mirror copy has the same disk pool number and name as
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the original disk pool, the production copy. Geographic mirroring is logical
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mirroring, not physical mirroring. The two disk pools must have similar disk
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capacities, but the mirror copy may have different numbers and types of disk
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units as well as different types of disk protection.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Manage</h4><div class="p">After geographic mirroring is configured,
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the production copy and mirror copy function as a unit. When the production
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copy is made available, the mirror copy is brought to a state that allows
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geographic mirroring to be performed. Synchronization occurs when you make
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the disk pool available after you configure geographic mirroring. When geographic
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mirroring is active, changes to the production copy data are transmitted to
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the mirror copy across TCP/IP connections. Changes can be transmitted either
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synchronously or asynchronously. <ul><li><span class="uicontrol">Synchronous mode</span>: The client waits until the operation
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is complete to disk on both the source and target systems. The mirror copy
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is always eligible to become the production copy, because the order of writes
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is preserved on the mirror copy. It is recommended to try synchronous mode
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first. If your performance remains acceptable, continue to use synchronous
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mode. </li>
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<li><span class="uicontrol">Asynchronous mode</span>: The client must wait only until
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the operation is complete to disk on the source system and is received for
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processing on the target system. However, synchronous mode is safer because
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if the primary node fails or the production copy fails, the mirror copy can
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become the production copy. In asynchronous mode, the pending updates must
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be completed before the mirror copy can become the production copy.</li>
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</ul>
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</div>
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<p>To maintain the data integrity of the mirror copy, the user cannot
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access the mirror copy while geographic mirroring is being performed. The
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user can detach the mirror copy to perform save operations, to create reports,
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and to perform data mining. However, the mirror copy must be synchronized
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with the production copy after it is reattached. </p>
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</div>
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<div class="section"><h4 class="sectiontitle">Tracking space </h4>To suspend geographic mirroring with
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tracking, you set the tracking space when you configure geographic mirroring
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or change geographic mirroring attributes. Tracking space is allocated in
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the independent ASPs. The more tracking space you specify, the more changes
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the system can track. The maximum tracking space allowed is approximately
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1% of the independent ASPs capacity. </div>
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<div class="section"><h4 class="sectiontitle">Suspend geographic mirroring with tracking</h4>If you suspend
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with tracking, the system will attempt to track changes made to those disk
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pools. This may reduce the synchronization process by performing partial synchronization
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when you resume geographic mirroring. If tracking space is exhausted, then
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when you resume geographic mirroring, complete synchronization is required.
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<div class="note"><span class="notetitle">Note:</span> When you resume geographic mirroring, a complete synchronization can
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be a lengthy process, anywhere from several hours to even longer. </div>
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</div>
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<div class="section"><h4 class="sectiontitle">Suspend without tracking </h4>If you suspend geographic
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mirroring without tracking changes, then when you resume geographic mirroring,
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a complete synchronization is required between the production and mirror copies.
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If you suspend geographic mirroring and you do track changes, then only a
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partial synchronization is required. Complete synchronization can be a very
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lengthy process, anywhere from one hour, to several hours, to even longer.
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The length of time it takes to synchronize is dependent on the number and
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type of disk units as well as how many TCP/IP communication interfaces are
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dedicated to geographic mirroring. </div>
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<div class="section"><h4 class="sectiontitle">Synchronization</h4><p>The production copy can function
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normally during synchronization, but performance might be negatively affected.
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During synchronization, the contents of the mirror copy are unusable, and
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it cannot become the production copy. If the independent disk pool is made
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unavailable during the synchronization process, synchronization resumes where
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it left off when the independent disk pool is made available again. Note that
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the first % complete message (CP1095D), after resuming an interrupted synchronization,
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shows 0%.</p>
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</div>
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<div class="section"><img src="./delta.gif" alt="Start of change" /><h4 class="sectiontitle">Synchronization type</h4>These are two types
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of synchronization: <p>Full synchronization</p>
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<ul><li>Indicates that a complete synchronization takes place. Changes to the
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production copy are not tracked to apply to the synchronization.</li>
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<li>Deletes all of the data on the mirror copy and copies all of the latest
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data from the production copy to the mirror copy. </li>
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</ul>
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<p>Partial synchronization</p>
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<ul><li>Indicates that changes to the production copy are tracked to apply to
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the synchronization. This may shorten the synchronization time because a complete
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synchronization is unnecessary.</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="section"><h4 class="sectiontitle">Synchronization priority</h4><p>When
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you set the attributes for geographic mirroring, you can set the synchronization
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priority. If synchronization priority is set high, the system uses more resources
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for synchronization, which results in a sooner completion time. The mirror
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copy is eligible to become a production copy faster, so you are protected
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sooner. However, high priority can cause degradation to your application.
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It is recommended that you try high priority first, so you are protected as
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soon as possible. If the degradation to your application performance is not
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tolerable, then lower the priority.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Recovery timeout</h4><p>In addition to synchronization
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priority, you can also set recovery time out. The recovery timeout specifies
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how long your application can wait when geographic mirroring cannot be performed.
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When an error, such as IP failure, prevents geographic mirroring, the source
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system waits and retries for the specified recovery timeout before suspending
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geographic mirroring which allows your application to continue. The trade-off
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is between blocking your application or requiring synchronization after suspending
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geographic mirroring. When your application is blocked for an extended time,
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other jobs might also be blocked waiting for resources and locks owned by
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the applications using the geographic mirrored disk pool. When geographic
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mirroring is suspended, you no longer have the protection of
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the mirror copy. If your application can tolerate a delay, it is recommended
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to set recovery timeout from 2 to 5 minutes. If the volume of your data is
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large (over a terabyte), consider a longer recovery timeout value to reduce
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the possibility of suspending geographic mirroring. If mirroring is suspended
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without tracking, the system performs a full synchronization. If geographic
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mirroring is suspended with tracking, the system performs a partial synchronization. </p>
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</div>
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<div class="section"><h4 class="sectiontitle">System roles</h4><p>When
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you <a href="rzalyconfiguregeographic.htm">configure</a> the
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cluster for geographic mirroring, you have many options for defining the availability
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and protection of the independent disk pool. When you create the switchable
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hardware group, you list the order of the backup systems to which the independent
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disk pool will failover or switch over. If the primary node switches to a
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backup node at the same site, a hardware switch will occur. If the primary
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node switches to the other site, the mirror copy on the backup node changes
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roles to become the production copy. The old primary node becomes the new
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backup node, and the production copy becomes the mirror copy. The new production
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copy is now accessible for updates on the remote system. If the independent
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disk pools are part of a disk pool group, all of the disk pools in the group
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will switchover together. See <a href="rzalyexamplegeomirror.htm">Example: Independent disk pools with geographic mirroring</a>. </p>
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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="rzalygeographicmirror.htm">Geographic mirroring</a></div>
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</div>
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</div>
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</body>
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