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134 lines
7.2 KiB
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<title>Six Things Everyone Should Know about JE Log Files</title>
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<link rel="up" href="introduction.html" title="Chapter 1. Introduction to Berkeley DB Java Edition" />
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<tr>
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<th colspan="3" align="center">Six Things Everyone Should Know about JE Log Files</th>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="jeexceptions.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 1. Introduction to Berkeley DB Java Edition </th>
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<td width="20%" align="right"> <a accesskey="n" href="env.html">Next</a></td>
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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="logfilesrevealed"></a>Six Things Everyone Should Know about JE Log Files</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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JE log files are not like the log files of other database systems. Nor are they like the log files or database
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files created by Berkeley DB C Edition. In this guide you will learn more about log files as you go along, but it is good
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to keep the following points in mind as you begin using JE.
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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JE log files are "append only". Record insertions, deletions, and updates are always added at the end
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of the current file. The first file is named <code class="literal">00000000.jdb</code>. When that file grows to a certain size
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(10 MB by default) a new file named <code class="literal">00000001.jdb</code> becomes the current file, and so on.
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</p>
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</li>
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<li>
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<p>
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There are no separate database files. Unlike Berkeley DB C Edition, databases are not stored in files
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that are separate from the transaction log. The transaction log and the database records are stored
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together in a single sequential log consisting of multiple log files.
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</p>
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</li>
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<li>
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<p>
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The JE cleaner is responsible for reclaiming unused disk space. When the records in a log file are
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superseded by deletions or updates recorded in a later log file, the older log file is no longer fully
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utilized. The cleaner, which operates by default as a separate thread, determines the least utilized
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log files, copies any still utilized records in those files to the end of the current log file, and
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finally deletes the now completely un-utilized log file.
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</p>
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<p>
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See <a class="xref" href="backgroundthreads.html#cleaner" title="The Cleaner Thread">The Cleaner Thread</a> for more information on the cleaner.
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</p>
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</li>
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<li>
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<p>
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Cleaning does not start immediately and never produces 100% utilization. Until you have written enough
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data to create several log files, and some of that data is obsoleted through deletions and updates, you
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will not notice any log files being deleted by the cleaner. By default cleaning occurs in the background and
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maintains the log files at 50% utilization. You can configure a higher utilization value, but
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configuring too high a utilization value will reduce overall performance.
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</p>
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</li>
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<li>
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<p>
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Cleaning is not automatically performed when closing the environment. If you wish to reduce unused disk
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space to a minimum at a particular point in time, you must explicitly call a method to perform log
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cleaning. See the <a class="xref" href="envclose.html" title="Closing Database Environments">Closing Database Environments</a> for more information.
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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 file deletion only occurs after a checkpoint. The cleaner prepares log files to be deleted, but
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file deletion must be performed after a checkpoint to ensure that the files are no longer referenced.
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Checkpoints occur on their own schedule, which is every 20 MB of log written, by default. This is part
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of the reason that you will not see log files being deleted until after several files have been created.
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</p>
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<p>
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When using JE's replication (high availability) feature, a checkpoint does not delete
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the cleaned log file but instead instead changes the status of the file to <span class="emphasis"><em>reserved</em></span>.
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Reserved files are only deleted when disk utilization exceeds the values set for either
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<a class="ulink" href="../java/com/sleepycat/je/EnvironmentConfig.html#MAX_DISK" target="_top">EnvironmentConfig.MAX_DISK</a> or <a class="ulink" href="../java/com/sleepycat/je/EnvironmentConfig.html#FREE_DISK" target="_top">EnvironmentConfig.FREE_DISK</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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Log files can be spread across multiple directories, and therefore across multiple
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disks through the use of links or mount points. See
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<a class="xref" href="env.html#multisubdir" title="Multiple Environment Subdirectories">Multiple Environment Subdirectories</a>
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for more information.
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</p>
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</li>
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</ol>
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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="jeexceptions.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="introduction.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="env.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">JE Exceptions </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"> Chapter 2. Database Environments</td>
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</tr>
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</table>
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</body>
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</html>
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