mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
164 lines
8.7 KiB
HTML
164 lines
8.7 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Shared memory regions</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="env.html" title="Chapter 9. The Berkeley DB Environment" />
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<link rel="prev" href="env_naming.html" title="File naming" />
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<link rel="next" href="env_security.html" title="Security" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 18.1.40</p>
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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">Shared memory regions</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="env_naming.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 9. The Berkeley DB Environment </th>
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<td width="20%" align="right"> <a accesskey="n" href="env_security.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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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="env_region"></a>Shared memory regions</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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Each of the Berkeley DB subsystems within an environment is
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described by one or more regions, or chunks of memory. The
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regions contain all of the per-process and per-thread shared
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information (including mutexes), that comprise a Berkeley DB
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environment. These regions are created in one of three types
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of memory, depending on the flags specified to the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a>
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method:
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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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If the <a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a> flag is specified to the
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<a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method, regions are created in per-process
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heap memory; that is, memory returned by
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<code class="literal">malloc</code>(3).
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</p>
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<p>
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If this flag is specified, then you cannot open
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more than a single handle for the environment. For
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example, if both a server application and Berkeley DB
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utilities (for example, the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility, the
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<a href="../api_reference/C/db_checkpoint.html" class="olink">db_checkpoint</a> utility or the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility) are expected to
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access the environment, the <a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a> flag should
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not be specified because the second attempt to open
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the environment will fail.
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</p>
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</li>
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<li>
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<p>
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If the <a href="../api_reference/C/envopen.html#envopen_DB_SYSTEM_MEM" class="olink">DB_SYSTEM_MEM</a> flag is specified to
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<a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a>, shared regions are created in system memory
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rather than files. This is an alternative mechanism
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for sharing the Berkeley DB environment among multiple
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processes and multiple threads within processes.
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</p>
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<p>
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The system memory used by Berkeley DB is
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potentially useful past the lifetime of any particular
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process. Therefore, additional cleanup may be
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necessary after an application fails because there may
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be no way for Berkeley DB to ensure that system
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resources backing the shared memory regions are
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returned to the system. <a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a> can be called to
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free up system resources when the shared memory regions
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are not used any more.
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</p>
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<p>
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The system memory that is used is
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architecture-dependent. For example, on systems
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supporting X/Open-style shared memory interfaces, such
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as UNIX systems, the <code class="literal">shmget</code>(2) and
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related System V IPC interfaces are used.
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Additionally, VxWorks systems use system memory. In
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these cases, an initial segment ID must be specified
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by the application to ensure that applications do not
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overwrite each other's database environments, so that
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the number of segments created does not grow without
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bounds. See the <a href="../api_reference/C/envset_shm_key.html" class="olink">DB_ENV->set_shm_key()</a> method for more
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information.
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</p>
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<p>
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On Windows platforms, the use of the
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<a href="../api_reference/C/envopen.html#envopen_DB_SYSTEM_MEM" class="olink">DB_SYSTEM_MEM</a> flag is problematic because the
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operating system uses reference counting to clean up
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shared objects in the paging file automatically. In
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addition, the default access permissions for shared
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objects are different from files, which may cause
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problems when an environment is accessed by multiple
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processes running as different users. See the
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<a href="../installation/build_win_notes.html" class="olink">Windows Notes</a> section in the Berkeley DB Installation and Build Guide for
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more information.
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</p>
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</li>
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<li>
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If no memory-related flags are specified to
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<a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a>, memory backed by the filesystem is used to
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store the regions. On UNIX systems, the Berkeley DB
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library will use the POSIX mmap interface. If mmap is not
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available, the UNIX shmget interfaces may be used instead,
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if they are available.
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</li>
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</ol>
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</div>
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<p>
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Any files created in the filesystem to back the regions are
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created in the directory set by calling <a href="../api_reference/C/envset_region_dir.html" class="olink">DB_ENV->set_region_dir()</a>.
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If no region directory is set then the files are
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created in the environment home directory specified to the
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<a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> call. These files are named __db.### (for example,
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__db.001, __db.002 and so on). When region files are backed by
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the filesystem, one file per region is created. When region
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files are backed by system memory, a single file will still be
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created because there must be a well-known name in the
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filesystem so that multiple processes can locate the system
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shared memory that is being used by the environment.
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</p>
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<p>
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Statistics about the shared memory regions in the
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environment can be displayed using the <span class="bold"><strong>
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-e</strong></span> option to the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility.
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</p>
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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="env_naming.html">Prev</a> </td>
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||
<td width="20%" align="center">
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<a accesskey="u" href="env.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="env_security.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">File naming </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"> Security</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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