mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
274 lines
11 KiB
HTML
274 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>Chapter 13. External Files</title>
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<link rel="stylesheet" href="apiReference.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 C API Reference" />
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<link rel="up" href="index.html" title="Berkeley DB C API Reference" />
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<link rel="prev" href="txnset_timeout.html" title="DB_TXN->set_timeout()" />
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<link rel="next" href="get_blob_dir.html" title="DB->get_ext_file_dir()" />
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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">Chapter 13.
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External Files
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="txnset_timeout.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="get_blob_dir.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="chapter" 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"><a id="blob"></a>Chapter 13.
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External Files
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</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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External file support is designed for efficient
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storage of large objects. An object is considered to be large
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if it is more than a third of the size of a page. Without external
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file support, large objects must be broken up into smaller pieces,
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and then reassembled and/or disassembled every time the record
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is read or updated. Berkeley DB external file support avoids this
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assembly/disassembly process by storing the large object in a
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special directory set aside for the purpose. The data itself
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is not kept in the database, nor is it placed into the
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in-memory cache.
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</p>
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<p>
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External files can only be stored using the data portion of a
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key/data pair. They are supported only for Btree, Hash, and Heap
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databases, and only so long as the database is not configured
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for duplicate records, or duplicate
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sorted records. In addition, the DBT that you use to
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access the external file data cannot be configured as a partial
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DBT if you want to access the data using the external
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file's streaming interface (introduced below).
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</p>
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<p>
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Note that if the environment is transactionally-protected, then
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all access to the external file is also transactionally protected.
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</p>
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<p>
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External files cannot be used with in-memory-only databases.
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</p>
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<p>
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In order to use Berkeley DB's external file support, you must set
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the external file threshold to a non-zero positive value. (The
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default value is <code class="literal">0</code>, which means that external
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files are not enabled). You set the external file threshold using
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either <a class="xref" href="set_blob_threshold.html" title="DB->set_ext_file_threshold()">
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<span>DB->set_ext_file_threshold()</span>
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</a> or <a class="xref" href="envset_blob_threshold.html" title="DB_ENV->set_ext_file_threshold()">
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<span>DB_ENV->set_ext_file_threshold()</span>
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</a>.
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</p>
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<p>
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Once external files are enabled, there are two ways to create an
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external file record:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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Configure the DBT used to access the external file
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data (that is, the DBT used for the data portion of
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the record) with the
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<a class="link" href="dbt.html#dbt_DB_DBT_BLOB">DB_DBT_EXT_FILE</a>
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flag. This causes the data to be stored as an external file
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regardless of its size, so long as the database
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otherwise supports external 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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Alternatively, creating a data item with a size greater
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than the external file threshold will cause that data item
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to be stored as an external file. Of course, for this
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method to work, the external file threshold must be greater
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than <code class="literal">0</code>.
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</p>
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</li>
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</ul>
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</div>
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<p>
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external files may be accessed in the same way as other DBT
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data, so long as the data itself will fit into memory. More
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likely, you will find it necessary to use the external file
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streaming API to read and write external file data. You open an
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external file stream using the <a class="xref" href="dbstream.html" title="DBC->db_stream()">
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<span>DBC->db_stream()</span>
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</a> method, close it with
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the <a class="xref" href="dbstream_close.html" title="DB_STREAM->close()">
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<span>DB_STREAM->close()</span>
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</a> method, write to it using the the
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<a class="xref" href="dbstream_write.html" title="DB_STREAM->write()">
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<span>DB_STREAM->write()</span>
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</a> method, and read it using the <a class="xref" href="dbstream_read.html" title="DB_STREAM->read()">
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<span>DB_STREAM->read()</span>
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</a>
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method.
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</p>
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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="bloblist"></a>External Files and Related Methods</h2>
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</div>
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</div>
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</div>
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<div class="navtable">
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<table border="1" width="80%">
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<thead>
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<tr>
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<th>External File Operations</th>
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<th>Description</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>
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<a class="xref" href="dbstream.html" title="DBC->db_stream()">
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<span>DBC->db_stream()</span>
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</a>
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</td>
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<td>Create an external file stream</td>
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</tr>
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<tr>
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<td>
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<a class="xref" href="dbstream_close.html" title="DB_STREAM->close()">
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<span>DB_STREAM->close()</span>
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</a>
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</td>
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<td>Close an external file stream</td>
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</tr>
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<tr>
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<td>
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<a class="xref" href="dbstream_read.html" title="DB_STREAM->read()">
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<span>DB_STREAM->read()</span>
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</a>
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</td>
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<td>Read an external file stream</td>
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</tr>
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<tr>
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<td>
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<a class="xref" href="dbstream_size.html" title="DB_STREAM->size()">
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<span>DB_STREAM->size()</span>
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</a>
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</td>
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<td>Returns the size of an external file</td>
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</tr>
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<tr>
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<td>
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<a class="xref" href="dbstream_write.html" title="DB_STREAM->write()">
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<span>DB_STREAM->write()</span>
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</a>
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</td>
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<td>Write an external file stream</td>
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</tr>
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<tr>
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<td colspan="2">
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<span class="bold">
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<strong>External File Configuration</strong>
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</span>
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</td>
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</tr>
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<tr>
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<td><a class="xref" href="set_blob_dir.html" title="DB->set_ext_file_dir()">
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<span>DB->set_ext_file_dir()</span>
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</a>, <a class="xref" href="get_blob_dir.html" title="DB->get_ext_file_dir()">
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<span>DB->get_ext_file_dir()</span>
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</a></td>
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<td>Sets/gets the location where external file data is stored</td>
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</tr>
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<tr>
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<td><a class="xref" href="set_blob_threshold.html" title="DB->set_ext_file_threshold()">
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<span>DB->set_ext_file_threshold()</span>
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</a>, <a class="xref" href="get_blob_threshold.html" title="DB->get_ext_file_threshold()">
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<span>DB->get_ext_file_threshold()</span>
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</a></td>
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<td>Sets/gets the size when data records are stored as external files</td>
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</tr>
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<tr>
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<td><a class="xref" href="envset_blob_dir.html" title="DB_ENV->set_ext_file_dir()">
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<span>DB_ENV->set_ext_file_dir()</span>
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</a>, <a class="xref" href="envget_blob_dir.html" title="DB_ENV->get_ext_file_dir()">
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<span>DB_ENV->get_ext_file_dir()</span>
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</a></td>
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<td>Sets/gets the location where external file data is stored</td>
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</tr>
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<tr>
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<td><a class="xref" href="envset_blob_threshold.html" title="DB_ENV->set_ext_file_threshold()">
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<span>DB_ENV->set_ext_file_threshold()</span>
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</a>, <a class="xref" href="envget_blob_threshold.html" title="DB_ENV->get_ext_file_threshold()">
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<span>DB_ENV->get_ext_file_threshold()</span>
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</a></td>
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<td>Sets/gets the default size for the environment when external files are used</td>
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</tr>
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</tbody>
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</table>
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</div>
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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="txnset_timeout.html">Prev</a> </td>
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<td width="20%" align="center"> </td>
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<td width="40%" align="right"> <a accesskey="n" href="get_blob_dir.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">
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<span>DB_TXN->set_timeout()</span>
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</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">
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<span>DB->get_ext_file_dir()</span>
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</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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