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The DB_MPOOLFILE Handle
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<h2 class="title"><a id="memp"></a>Chapter 8.
The DB_MPOOLFILE Handle
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<pre class="programlisting">#include <db.h>
typedef struct __db_mpoolfile DB_MPOOLFILE; </pre>
<p>
The memory pool interfaces for the Berkeley DB database environment are methods of the
<a class="link" href="env.html" title="Chapter 5. The DB_ENV Handle">DB_ENV</a> handle. The
<a class="link" href="env.html" title="Chapter 5. The DB_ENV Handle">DB_ENV</a> memory pool methods and the
<code class="classname">DB_MPOOLFILE</code> class provide general-purpose, page-oriented buffer
management of files. Although designed to work with the other
<a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>classes, they are also useful for more general
purposes. The memory pools are referred to in this document as simply
<span class="emphasis"><em>the cache</em></span>.
</p>
<p>
The cache may be shared between processes. The cache is usually filled by pages from one or
more files. Pages in the cache are replaced in LRU (least-recently-used) order, with each
new page replacing the page that has been unused the longest. Pages retrieved from the
cache using <a class="xref" href="mempfget.html" title="DB_MPOOLFILE->get()">DB_MPOOLFILE->get()</a> are
<span class="emphasis"><em>pinned</em></span> in the cache until they are returned to the control of the
cache using the <a class="xref" href="mempput.html" title="DB_MPOOLFILE->put()">DB_MPOOLFILE->put()</a> method.
</p>
<p>
The <code class="classname">DB_MPOOLFILE</code> object is the handle for a file in the
cache. The handle is not free-threaded. Once the
<a class="xref" href="mempfclose.html" title="DB_MPOOLFILE->close()">DB_MPOOLFILE->close()</a> method is called, the handle
may not be accessed again, regardless of that method's return.
</p>
<div class="sect1" lang="en" xml:lang="en">
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<h2 class="title" style="clear: both"><a id="memplist"></a>Memory Pools and Related Methods</h2>
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<div class="navtable">
<table border="1" width="80%">
<thead>
<tr>
<th>Memory Pools and Related Methods</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<a class="xref" href="dbget_mpf.html" title="DB->get_mpf()">DB->get_mpf()</a>
</td>
<td>Return the DB_MPOOLFILE for a DB</td>
</tr>
<tr>
<td>
<a class="xref" href="mempstat.html" title="DB_ENV->memp_stat()">DB_ENV->memp_stat()</a>
</td>
<td>Return cache statistics</td>
</tr>
<tr>
<td>
<a class="xref" href="mempstat_print.html" title="DB_ENV->memp_stat_print()">DB_ENV->memp_stat_print()</a>
</td>
<td>Print cache statistics</td>
</tr>
<tr>
<td>
<a class="xref" href="mempsync.html" title="DB_ENV->memp_sync()">DB_ENV->memp_sync()</a>
</td>
<td>Flush all pages from the cache</td>
</tr>
<tr>
<td>
<a class="xref" href="memptrickle.html" title="DB_ENV->memp_trickle()">DB_ENV->memp_trickle()</a>
</td>
<td>Flush some pages from the cache</td>
</tr>
<tr>
<td colspan="2">
<span class="bold">
<strong>Memory Pool Configuration</strong>
</span>
</td>
</tr>
<tr>
<td>
<a class="xref" href="mempregister.html" title="DB_ENV->memp_register()">DB_ENV->memp_register()</a>
</td>
<td>Register a custom file type</td>
</tr>
<tr>
<td><a class="xref" href="envset_cache_max.html" title="DB_ENV->set_cache_max()">DB_ENV->set_cache_max()</a>, <a class="xref" href="envget_cache_max.html" title="DB_ENV->get_cache_max()">DB_ENV->get_cache_max()</a></td>
<td>Set/get the maximum cache size</td>
</tr>
<tr>
<td><a class="xref" href="envset_cachesize.html" title="DB_ENV->set_cachesize()">DB_ENV->set_cachesize()</a>, <a class="xref" href="envget_cachesize.html" title="DB_ENV->get_cachesize()">DB_ENV->get_cachesize()</a></td>
<td>Set/get the environment cache size</td>
</tr>
<tr>
<td><a class="xref" href="mempset_mp_max_openfd.html" title="DB_ENV->set_mp_max_openfd()">DB_ENV->set_mp_max_openfd()</a>, <a class="xref" href="mempget_mp_max_openfd.html" title="DB_ENV->get_mp_max_openfd()">DB_ENV->get_mp_max_openfd()</a></td>
<td>Set/get the maximum number of open file descriptors</td>
</tr>
<tr>
<td><a class="xref" href="mempset_mp_max_write.html" title="DB_ENV->set_mp_max_write()">DB_ENV->set_mp_max_write()</a>, <a class="xref" href="mempget_mp_max_write.html" title="DB_ENV->get_mp_max_write()">DB_ENV->get_mp_max_write()</a></td>
<td>Set/get the maximum number of sequential disk writes</td>
</tr>
<tr>
<td><a class="xref" href="envset_mp_mmapsize.html" title="DB_ENV->set_mp_mmapsize()">DB_ENV->set_mp_mmapsize()</a>, <a class="xref" href="envget_mp_mmapsize.html" title="DB_ENV->get_mp_mmapsize()">DB_ENV->get_mp_mmapsize()</a></td>
<td>Set/get maximum file size to memory map when opened read-only</td>
</tr>
<tr>
<td colspan="2">
<span class="bold">
<strong>Memory Pool Files</strong>
</span>
</td>
</tr>
<tr>
<td>
<a class="xref" href="mempfcreate.html" title="DB_ENV->memp_fcreate()">DB_ENV->memp_fcreate()</a>
</td>
<td>Create a memory pool file handle</td>
</tr>
<tr>
<td>
<a class="xref" href="mempfclose.html" title="DB_MPOOLFILE->close()">DB_MPOOLFILE->close()</a>
</td>
<td>Close a file in the cache</td>
</tr>
<tr>
<td>
<a class="xref" href="mempfget.html" title="DB_MPOOLFILE->get()">DB_MPOOLFILE->get()</a>
</td>
<td>Get page from a file in the cache</td>
</tr>
<tr>
<td>
<a class="xref" href="mempfopen.html" title="DB_MPOOLFILE->open()">DB_MPOOLFILE->open()</a>
</td>
<td>Open a file in the cache</td>
</tr>
<tr>
<td>
<a class="xref" href="mempput.html" title="DB_MPOOLFILE->put()">DB_MPOOLFILE->put()</a>
</td>
<td>Return a page to the cache</td>
</tr>
<tr>
<td>
<a class="xref" href="mempfsync.html" title="DB_MPOOLFILE->sync()">DB_MPOOLFILE->sync()</a>
</td>
<td>Flush pages from a file from the cache</td>
</tr>
<tr>
<td colspan="2">
<span class="bold">
<strong>Memory Pool File Configuration</strong>
</span>
</td>
</tr>
<tr>
<td><a class="xref" href="mempset_clear_len.html" title="DB_MPOOLFILE->set_clear_len()">DB_MPOOLFILE->set_clear_len()</a>, <a class="xref" href="mempget_clear_len.html" title="DB_MPOOLFILE->get_clear_len()">DB_MPOOLFILE->get_clear_len()</a></td>
<td>Set/get number of bytes to clear when creating a new page</td>
</tr>
<tr>
<td><a class="xref" href="mempset_fileid.html" title="DB_MPOOLFILE->set_fileid()">DB_MPOOLFILE->set_fileid()</a>, <a class="xref" href="mempget_fileid.html" title="DB_MPOOLFILE->get_fileid()">DB_MPOOLFILE->get_fileid()</a></td>
<td>Set/get file unique identifier</td>
</tr>
<tr>
<td><a class="xref" href="mempset_flags.html" title="DB_MPOOLFILE->set_flags()">DB_MPOOLFILE->set_flags()</a>, <a class="xref" href="mempget_flags.html" title="DB_MPOOLFILE->get_flags()">DB_MPOOLFILE->get_flags()</a></td>
<td>Set/get file options</td>
</tr>
<tr>
<td><a class="xref" href="mempset_ftype.html" title="DB_MPOOLFILE->set_ftype()">DB_MPOOLFILE->set_ftype()</a>, <a class="xref" href="mempget_ftype.html" title="DB_MPOOLFILE->get_ftype()">DB_MPOOLFILE->get_ftype()</a></td>
<td>Set/get file type</td>
</tr>
<tr>
<td><a class="xref" href="mempset_lsn_offset.html" title="DB_MPOOLFILE->set_lsn_offset()">DB_MPOOLFILE->set_lsn_offset()</a>, <a class="xref" href="mempget_lsn_offset.html" title="DB_MPOOLFILE->get_lsn_offset()">DB_MPOOLFILE->get_lsn_offset()</a></td>
<td>Set/get file log-sequence-number offset</td>
</tr>
<tr>
<td><a class="xref" href="mempset_maxsize.html" title="DB_MPOOLFILE->set_maxsize()">DB_MPOOLFILE->set_maxsize()</a>, <a class="xref" href="mempget_maxsize.html" title="DB_MPOOLFILE->get_maxsize()">DB_MPOOLFILE->get_maxsize()</a></td>
<td>Set/get maximum file size</td>
</tr>
<tr>
<td><a class="xref" href="mempset_pgcookie.html" title="DB_MPOOLFILE->set_pgcookie()">DB_MPOOLFILE->set_pgcookie()</a>, <a class="xref" href="mempget_pgcookie.html" title="DB_MPOOLFILE->get_pgcookie()">DB_MPOOLFILE->get_pgcookie()</a></td>
<td>Set/get file cookie for pgin/pgout</td>
</tr>
<tr>
<td><a class="xref" href="mempset_priority.html" title="DB_MPOOLFILE->set_priority()">DB_MPOOLFILE->set_priority()</a>, <a class="xref" href="mempget_priority.html" title="DB_MPOOLFILE->get_priority()">DB_MPOOLFILE->get_priority()</a></td>
<td>Set/get cache file priority</td>
</tr>
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