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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<title>Administering Log Files</title>
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<th colspan="3" align="center">Administering Log Files</th>
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<td width="20%" align="left"><a accesskey="p" href="configcache.html">Prev</a> </td>
<th width="60%" align="center">Chapter 3. Configuring the Berkeley DB SQL interface</th>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="admin-logfile"></a>Administering Log Files</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="admin-logfile.html#logfilesize">Setting the Log File Size</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="admin-logfile.html#logregionsize">Configuring the Logging Region Size</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="admin-logfile.html#logbuffer">Setting the In-Memory Log Buffer Size</a>
</span>
</dt>
</dl>
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<p>
Your environment directory contains log
files. Berkeley DB log files are used to record all the
transactional activity performed against the Berkeley DB database
files. This information is used after an application or
system failure to automatically restore the database to an up-to-date
consistent point.
</p>
<p>
Your log files are maintained by Berkeley DB's logging subsystem.
There are some aspects of the Berkeley DB logging subsystem that
you can configure using <code class="literal">DB_CONFIG</code>
parameters, and (sometimes) by using PRAGMAs.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
For most users of the BDB SQL interface, there should not normally
be any reason to manage your log files or otherwise worry
about them. However, it is important to realize that they
can not simply be deleted. Note that when using the
Berkeley DB SQL interface, your log files will be automatically deleted by
Berkeley DB when they are no longer needed.
</p>
</div>
<p>
The things you can manage for your logging subsystem are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Size of the log files. See <a class="xref" href="admin-logfile.html#logfilesize" title="Setting the Log File Size">Setting the Log File Size</a>.
</p>
</li>
<li>
<p>
Size of the logging subsystem's region. See <a class="xref" href="admin-logfile.html#logregionsize" title="Configuring the Logging Region Size">Configuring the Logging Region Size</a>.
</p>
</li>
<li>
<p>
Size of the log buffer in memory.
<a class="xref" href="admin-logfile.html#logbuffer" title="Setting the In-Memory Log Buffer Size">Setting the In-Memory Log Buffer Size</a>.
</p>
</li>
</ul>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="logfilesize"></a>Setting the Log File Size</h3>
</div>
</div>
</div>
<p>
Whenever a pre-defined amount of data is written to a
log file (10 MB by default), the BDB SQL interface stops using the
current log file and starts writing to a new file.
You can change the maximum amount of data contained in each
log file by using either <code class="literal">PRAGMA journal_size_limit</code> or
the <code class="literal">set_lg_max</code> DB_CONFIG file parameter.
</p>
<p>
If you use <code class="literal">PRAGMA journal_size_limit</code>,
then using this PRAGMA with no value simply returns the
current journal size limit. Using:
</p>
<pre class="programlisting">PRAGMA journal_size_limit=N</pre>
<p>
sets the log size to <span class="emphasis"><em>N</em></span> bytes. If
the PRAGMA is successful, <span class="emphasis"><em>N</em></span> is
returned. If it fails, the previous log file size is
returned. Failures can occur if you specify a log file
size that is less than 4K bytes, or if you specify a
log file size larger than the permitted file size on
the system.
</p>
<p>
If you use the DB_CONFIG file to manage this value,
<code class="literal">set_lg_max</code> may be changed without
re-creating the environment. You will, however, have to
restart your application in order for the DB_CONFIG
file to be re-read.
</p>
<p>
The DB_CONFIG file is described in <a class="xref" href="using_dbconfig.html" title="The DB_CONFIG File">The DB_CONFIG File</a>.
The <code class="literal">set_lg_max</code> parameter is
described in the
<a class="ulink" href="http://download.oracle.com/docs/cd/E17076_02/html/api_reference/C/BDB-C_APIReference.pdf" target="_top">
Berkeley DB C API
</a>.
</p>
<p>
For a description of how, when and why you should
change the size of your log files, see
the <a href="../programmer_reference/log_config.html" class="olink">Selecting a page size</a> section in the
<em class="citetitle">Berkeley DB Programmer's Reference Guide</em>.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="logregionsize"></a>Configuring the Logging Region Size</h3>
</div>
</div>
</div>
<p>
The logging subsystem's default region size is 512 KB. The
logging region is used to store database and table names, and so you may
need to increase its size if you will be using a large
number of tables.
</p>
<p>
You can set the size of your logging region by using the
<code class="literal">set_lg_regionmax</code> DB_CONFIG parameter. Note that
to manage this value you must set it before you create your
environment, or you must re-create your environment. See <a class="xref" href="using_dbconfig.html" title="The DB_CONFIG File">The DB_CONFIG File</a>
for more information.
</p>
<p>
The <code class="literal">set_lg_regionmax</code> parameter is described in the
<a class="ulink" href="http://download.oracle.com/docs/cd/E17076_02/html/api_reference/C/BDB-C_APIReference.pdf" target="_top">
Berkeley DB C API
</a>.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="logbuffer"></a>Setting the In-Memory Log Buffer Size</h3>
</div>
</div>
</div>
<p>
When using named (persistent) databases, log information is stored
in-memory until the storage space fills up, or a
transaction commit forces the log information to be flushed
to disk.
</p>
<p>
It is possible to increase the amount of memory available
to your file log buffer. Doing so improves throughput for
long-running transactions, or for transactions that produce
a large amount of data. Note that for named (persistent) databases, the
default log buffer space is 32 KB.
</p>
<p>
You can increase your log buffer space by using the
<code class="literal">set_lg_bsize</code> DB_CONFIG parameter.
For the BDB SQL interface, when the logging subsystem is configured for on-disk logging, the
default size of the in-memory log buffer is approximately 64KB.
Note that this
method can only be called before the environment
is first opened, so you will have to set this by creating
your <code class="literal">-journal</code> directory, and then
creating your database. See
<a class="xref" href="using_dbconfig.html" title="The DB_CONFIG File">The DB_CONFIG File</a>
for more information.
</p>
<p>
The <code class="literal">set_lg_bsize</code> parameter is described
in the <a class="ulink" href="http://download.oracle.com/docs/cd/E17076_02/html/api_reference/C/BDB-C_APIReference.pdf" target="_top">
Berkeley DB C API
</a>.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
When working with in-memory databases, the environment is
configured to perform logging in-memory. The log buffer is
set to 64 * 1024, and the maximum log size is set to 32 *
1024.
</p>
</div>
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