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160 lines
6.3 KiB
160 lines
6.3 KiB
<?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>Reverse BTree Splits</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="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="up" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
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<link rel="prev" href="txnnowait.html" title="No Wait on Blocks" />
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<link rel="next" href="filemanagement.html" title="Chapter 5. Managing DB Files" />
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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 12.1.6.2</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">Reverse BTree Splits</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="txnnowait.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Concurrency</th>
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<td width="20%" align="right"> <a accesskey="n" href="filemanagement.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="reversesplit"></a>Reverse BTree Splits</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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If your application is using the Btree access method, and your
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application is repeatedly deleting then adding records to your
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database, then you might be able to reduce lock contention by
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turning off reverse Btree splits.
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</p>
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<p>
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As pages are emptied in a database, DB attempts to
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delete empty pages in order to keep
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the database as small as possible and minimize search time.
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Moreover, when a page in the database fills
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up, DB, of course, adds additional pages
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to make room for more data.
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</p>
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<p>
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Adding and deleting pages in the database requires that the
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writing thread lock the parent page. Consequently, as the
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number of pages in your database diminishes, your application
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will see increasingly more lock contention; the maximum level
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of concurrency in a database of two pages is far smaller than
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that in a database of 100 pages, because there are fewer pages
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that can be locked.
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</p>
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<p>
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Therefore, if you prevent the database from being reduced to a
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minimum number of pages, you can improve your application's
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concurrency throughput. Note, however, that you should do so
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only if your application tends to delete and then add the same
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data. If this is not the case, then preventing reverse Btree
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splits can harm your database search time.
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</p>
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<p>
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To turn off reverse Btree splits,
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<span>
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provide the
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<code class="literal">DB_REVSPLITOFF</code> flag to the
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<code class="methodname">Db::set_flags()</code>
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method.
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</span>
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</p>
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<p>
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For example:
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</p>
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<pre class="programlisting">#include "db_cxx.h"
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...
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int main(void)
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{
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u_int32_t env_flags = DB_CREATE | // If the environment does not
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// exist, create it.
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DB_INIT_LOCK | // Initialize locking
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DB_INIT_LOG | // Initialize locking
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DB_INIT_MPOOL | // Initialize the cache
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DB_THREAD | // Free-thread the env handle
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DB_INIT_TXN; // Initialize transactions
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u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
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Db *dbp = NULL;
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const char *file_name = "mydb.db";
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std::string envHome("/export1/testEnv");
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DbEnv myEnv(0);
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try {
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myEnv.open(envHome.c_str(), env_flags, 0);
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dbp = new Db(&myEnv, 0);
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// Turn off BTree reverse split.
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dbp=>set_flags(DB_REVSPLITOFF);
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dbp->open(dbp, // Pointer to the database
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NULL, // Txn pointer
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file_name, // File name
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NULL, // Logical db name
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DB_BTREE, // Database type (using btree)
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db_flags, // Open flags
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0); // File mode. Using defaults
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} catch(DbException &e) {
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std::cerr << "Error opening database and environment: "
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<< file_name << ", " << envHome << std::endl;
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std::cerr << e.what() << std::endl;
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}
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try {
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dbp->close(dbp, 0);
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myEnv.close(0);
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} catch(DbException &e) {
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std::cerr << "Error closing database and environment: "
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<< file_name << ", " << envHome << std::endl;
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std::cerr << e.what() << std::endl;
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return (EXIT_FAILURE);
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}
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return (EXIT_SUCCESS);
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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="txnnowait.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="txnconcurrency.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="filemanagement.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">No Wait on Blocks </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"> Chapter 5. Managing DB Files</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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