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Yasuhiro KIMURA
217 lines
11 KiB
HTML
217 lines
11 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>Creating a database environment</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.html" title="Chapter 9. The Berkeley DB Environment" />
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<link rel="next" href="env_size.html" title="Sizing a database environment" />
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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">Creating a database
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environment</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.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_size.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_create"></a>Creating a database
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environment</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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The Berkeley DB environment is created and described by the
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<a href="../api_reference/C/envcreate.html" class="olink">db_env_create()</a> and <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> interfaces. In situations where
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customization is desired, such as storing log files on a
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separate disk drive or selection of a particular cache size,
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applications must describe the customization by either
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creating an environment configuration file in the environment
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home directory or by arguments passed to other <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> handle
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methods.
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</p>
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<p>
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Once an environment has been created, database files
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specified using relative pathnames will be named relative to
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the home directory. Using pathnames relative to the home
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directory allows the entire environment to be easily moved,
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simplifying restoration and recovery of a database in a
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different directory or on a different system.
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</p>
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<p>
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Applications first obtain an environment handle using the
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<a href="../api_reference/C/envcreate.html" class="olink">db_env_create()</a> method, then call the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method which
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creates or joins the database environment. There are a number
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of options you can set to customize <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> for your
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environment. These options fall into four broad
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categories:
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</p>
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<div class="variablelist">
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<dl>
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<dt>
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<span class="term">Subsystem Initialization:</span>
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</dt>
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<dd>
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These flags indicate which Berkeley DB
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subsystems will be initialized for the environment,
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and what operations will happen automatically when
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databases are accessed within the environment. The
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flags include <a href="../api_reference/C/envopen.html#envopen_DB_INIT_CDB" class="olink">DB_INIT_CDB</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a>,
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<a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOG" class="olink">DB_INIT_LOG</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_MPOOL" class="olink">DB_INIT_MPOOL</a>, and <a href="../api_reference/C/envopen.html#envopen_DB_INIT_TXN" class="olink">DB_INIT_TXN</a>. The
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<a href="../api_reference/C/envopen.html#envopen_DB_INIT_CDB" class="olink">DB_INIT_CDB</a> flag does initialization for Berkeley DB
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Concurrent Data Store applications. (See <a class="xref" href="cam.html#cam_intro" title="Concurrent Data Store introduction">Concurrent Data Store
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introduction</a>
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for more information.) The rest of the flags
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initialize a single subsystem; that is, when
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<a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a> is specified, applications reading and
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writing databases opened in this environment will be
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using locking to ensure that they do not overwrite
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each other's changes.
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</dd>
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<dt>
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<span class="term">Recovery options:</span>
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</dt>
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<dd>
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These flags, which include <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> and
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<a href="../api_reference/C/envopen.html#envopen_DB_RECOVER_FATAL" class="olink">DB_RECOVER_FATAL</a>, indicate what recovery is to be
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performed on the environment before it is opened for
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normal use.
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</dd>
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<dt>
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<span class="term">Naming options:</span>
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</dt>
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<dd>
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These flags, which include <a href="../api_reference/C/envopen.html#envopen_DB_USE_ENVIRON" class="olink">DB_USE_ENVIRON</a> and
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<a href="../api_reference/C/envopen.html#envopen_DB_USE_ENVIRON_ROOT" class="olink">DB_USE_ENVIRON_ROOT</a>, modify how file naming happens
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in the environment.
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</dd>
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<dt>
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<span class="term">Miscellaneous:</span>
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</dt>
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<dd>
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Finally, there are a number of miscellaneous
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flags, for example, <a href="../api_reference/C/dbopen.html#open_DB_CREATE" class="olink">DB_CREATE</a> which causes
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underlying files to be created as necessary. See the
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<a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> manual pages for further
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information.
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</dd>
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</dl>
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</div>
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<p>
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Most applications either specify only the <a href="../api_reference/C/envopen.html#envopen_DB_INIT_MPOOL" class="olink">DB_INIT_MPOOL</a>
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flag or they specify all four subsystem initialization flags
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(<a href="../api_reference/C/envopen.html#envopen_DB_INIT_MPOOL" class="olink">DB_INIT_MPOOL</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOG" class="olink">DB_INIT_LOG</a>, and
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<a href="../api_reference/C/envopen.html#envopen_DB_INIT_TXN" class="olink">DB_INIT_TXN</a>). The former configuration is for applications
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that simply want to use the basic Access Method interfaces
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with a shared underlying buffer pool, but don't care about
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recoverability after application or system failure. The latter
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is for applications that need recoverability. There are
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situations in which other combinations of the initialization
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flags make sense, but they are rare.
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</p>
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<p>
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The <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> flag is specified by applications that want
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to perform any necessary database recovery when they start
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running. That is, if there was a system or application failure
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the last time they ran, they want the databases to be made
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consistent before they start running again. It is not an error
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to specify this flag when no recovery needs to be done.
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</p>
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<p>
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The <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER_FATAL" class="olink">DB_RECOVER_FATAL</a> flag is more special-purpose. It
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performs catastrophic database recovery, and normally requires
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that some initial arrangements be made; that is, archived log
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files be brought back into the filesystem. Applications should
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not normally specify this flag. Instead, under these rare
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conditions, the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility should be used.
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</p>
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<p>
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The following is a simple example of a function that opens a
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database environment for a transactional program.
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</p>
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<a id="prog_env27"></a>
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<pre class="programlisting">DB_ENV *
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db_setup(char *home, char *data_dir, FILE *errfp, char *progname)
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{
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DB_ENV *dbenv;
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int ret;
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/*
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* Create an environment and initialize it for additional error
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* reporting.
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*/
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if ((ret = db_env_create(&dbenv, 0)) != 0) {
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fprintf(errfp, "%s: %s\n", progname, db_strerror(ret));
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return (NULL);
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}
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dbenv->set_errfile(dbenv, errfp);
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dbenv->set_errpfx(dbenv, progname);
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/*
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* Specify the shared memory buffer pool cachesize: 5MB.
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* Databases are in a subdirectory of the environment home.
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*/
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if ((ret = dbenv->set_cachesize(dbenv, 0,
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5 * 1024 * 1024, 0)) != 0) {
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dbenv->err(dbenv, ret, "set_cachesize");
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goto err;
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}
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if ((ret = dbenv->add_data_dir(dbenv, data_dir)) != 0) {
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dbenv->err(dbenv, ret, "add_data_dir: %s", data_dir);
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goto err;
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}
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/* Open the environment with full transactional support. */
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if ((ret = dbenv->open(dbenv, home, DB_CREATE | DB_INIT_LOG |
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DB_INIT_LOCK | DB_INIT_MPOOL |
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DB_INIT_TXN, 0)) != 0) {
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dbenv->err(dbenv, ret, "environment open: %s", home);
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goto err;
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}
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return (dbenv);
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err: (void)dbenv->close(dbenv, 0);
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return (NULL);
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}</pre>
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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.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_size.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">Chapter 9. The Berkeley DB Environment </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"> Sizing a database environment</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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