2011-09-13 17:44:24 +00:00
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<title>Chapter 5. Managing DB Files</title>
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Chapter 5. Managing DB Files</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="reversesplit.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="backuprestore.html">Next</a></td>
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<hr />
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<div class="chapter" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title"><a id="filemanagement"></a>Chapter 5. Managing DB Files</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="filemanagement.html#checkpoints">Checkpoints</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="backuprestore.html">Backup Procedures</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="backuprestore.html#copyutilities">About Unix Copy Utilities</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="backuprestore.html#standardbackup">Offline Backups</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="backuprestore.html#hotbackup">Hot Backup</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="backuprestore.html#incrementalbackups">Incremental Backups</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="recovery.html">Recovery Procedures</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="recovery.html#normalrecovery">Normal Recovery</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="recovery.html#catastrophicrecovery">Catastrophic Recovery</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="architectrecovery.html">Designing Your Application for Recovery</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="architectrecovery.html#multithreadrecovery">Recovery for Multi-Threaded Applications</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="architectrecovery.html#multiprocessrecovery">Recovery in Multi-Process Applications</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="hotfailover.html">Using Hot Failovers</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="logfileremoval.html">Removing Log Files</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="logconfig.html">Configuring the Logging Subsystem</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="logconfig.html#logfilesize">Setting the Log File Size</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="logconfig.html#logregionsize">Configuring the Logging Region Size</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="logconfig.html#inmemorylogging">Configuring In-Memory Logging</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="logconfig.html#logbuffer">Setting the In-Memory Log Buffer Size</a>
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</span>
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</dt>
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</dl>
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</dd>
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</dl>
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</div>
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<p>
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DB is capable of storing several types of files on disk:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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Data files, which contain the actual data in your database.
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</p>
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</li>
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<li>
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<p>
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Log files, which contain information required to recover your
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database in the event of a system or application failure.
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</p>
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</li>
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<li>
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<p>
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Region files, which contain information necessary for the
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overall operation of your application.
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</p>
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</li>
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<li>
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<p>
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Temporary files, which are created only under certain special circumstances. These files never need to
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be backed up or otherwise managed and so they are not a consideration for the topics described in this
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chapter. See <a class="xref" href="enabletxn.html#security" title="Security Considerations">Security Considerations</a>
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for more information on temporary files.
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</p>
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</li>
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</ul>
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</div>
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<p>
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Of these, you must manage your data and log files by ensuring that they
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are backed up. You should also pay attention to the amount of disk space
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your log files are consuming, and periodically remove any unneeded
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files. Finally, you can optionally tune your logging subsystem to best
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suit your application's needs and requirements.
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These topics are discussed in this chapter.
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</p>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="checkpoints"></a>Checkpoints</h2>
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</div>
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</div>
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</div>
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<p>
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Before we can discuss DB file management, we need to
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describe checkpoints. When databases are modified (that is, a
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transaction is committed), the modifications are recorded in
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DB's logs, but they are <span class="emphasis"><em>not</em></span>
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necessarily reflected in the actual database files on disk.
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</p>
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<p>
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This means that as time goes on, increasingly
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more data is contained in your log files that is not
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contained in your data files. As a result, you must keep more
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log files around than you might actually need. Also, any
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recovery run from your log files will take increasingly longer
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amounts of time, because there is more data in the log files
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that must be reflected back into the data files during the
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recovery process.
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</p>
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<p>
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You can reduce these problems by periodically
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running a checkpoint against your environment. The checkpoint:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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Flushes dirty pages from the in-memory cache. This means that data modifications found in your
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in-memory cache are written to the database files on disk. Note that a checkpoint also causes data
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dirtied by an uncommitted transaction to also be written to your database files on disk. In this latter
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case, DB's normal recovery is used to remove any such modifications that were subsequently
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abandoned by your application using a transaction abort.
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</p>
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<p>
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Normal recovery is describe in <a class="xref" href="recovery.html" title="Recovery Procedures">Recovery Procedures</a>.
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</p>
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</li>
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<li>
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<p>
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Writes a checkpoint record.
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</p>
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</li>
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<li>
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<p>
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Flushes the log. This causes all log data that has not yet been written to disk to be written.
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</p>
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</li>
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<li>
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<p>
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Writes a list of open databases.
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</p>
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</li>
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</ul>
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</div>
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<p>
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There are several ways to run a checkpoint. One way is to use
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the <span class="command"><strong>db_checkpoint</strong></span> command line utility. (Note, however, that this command line utility
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cannot be used if your environment was opened using
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<span>
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<code class="literal">DB_PRIVATE</code>.)
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</span>
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</p>
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<p>
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You can also run a thread that periodically checkpoints your
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environment for you by calling the
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<code class="methodname">DbEnv::txn_checkpoint()</code>
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method.
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</p>
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<p>
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Note that you can prevent a checkpoint from occurring unless more
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than a specified amount of log data has been written since the
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last checkpoint. You can also prevent the checkpoint from
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running unless more than a specified amount of time has
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occurred since the last checkpoint. These conditions are
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particularly interesting if you have multiple threads
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<span>or processes</span>
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running checkpoints.
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</p>
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<p>
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For configuration information, see the
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<a class="ulink" href="http://download.oracle.com/docs/cd/E17076_02/html/api_reference/CXX/txncheckpoint.html" target="_top">
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DbEnv::txn_checkpoint() API reference page.
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</a>
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</p>
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<p>
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Note that running checkpoints can be quite expensive. DB must
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flush every dirty page to the backing database files. On the
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other hand, if you do not run checkpoints often enough, your
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recovery time can be unnecessarily long and you may be using more
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disk space than you really need. Also, you cannot remove log files
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until a checkpoint is run. Therefore, deciding how frequently
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to run a checkpoint is one of the most
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common tuning activity for DB applications.
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</p>
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<p>
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For example, to run a checkpoint from a separate thread of control:
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</p>
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<pre class="programlisting">#include <pthread.h>
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#include "db_cxx.h"
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...
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void *checkpoint_thread(void *);
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int main(void)
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{
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u_int32_t env_flags = DB_CREATE | // If the environment does not
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// exist, create it.
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DB_INIT_LOCK | // Initialize locking
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DB_INIT_LOG | // Initialize logging
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DB_INIT_MPOOL | // Initialize the cache
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DB_THREAD | // Free-thread the env handle
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DB_INIT_TXN; // Initialize transactions
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std::string envHome("/export1/testEnv");
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DbEnv myEnv(0);
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try {
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myEnv.open(envHome.c_str(), env_flags, 0);
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// Start a checkpoint thread.
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pthread_t ptid;
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int ret;
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if ((ret = pthread_create(
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&ptid, NULL, checkpoint_thread, (void *)&myEnv)) != 0) {
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fprintf(stderr,
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"txnapp: failed spawning checkpoint thread: %s\n",
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strerror(errno));
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myEnv.close(0);
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exit (1);
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}
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// All other threads and application shutdown code
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// omitted for brevity.
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...
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}
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void *
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checkpoint_thread(void *arg) {
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DbEnv *dbenv = arg;
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// Checkpoint once a minute.
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for (;; sleep(60)) {
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try {
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dbenv->txn_checkpoint(0, 0, 0));
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} catch(DbException &e) {
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dbenv->err(e.get_errno(), "checkpoint thread");
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exit (e.get_errno());
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}
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}
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// NOTREACHED
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} </pre>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="reversesplit.html">Prev</a> </td>
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<td width="20%" align="center"> </td>
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<td width="40%" align="right"> <a accesskey="n" href="backuprestore.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Reverse BTree Splits </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Backup Procedures</td>
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</tr>
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</table>
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