mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
178 lines
6.4 KiB
HTML
178 lines
6.4 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>Checkpoints</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="transapp.html" title="Chapter 11. Berkeley DB Transactional Data Store Applications" />
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<link rel="prev" href="transapp_deadlock.html" title="Deadlock detection" />
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<link rel="next" href="transapp_archival.html" title="Database and log file archival" />
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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">Checkpoints</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="transapp_deadlock.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 11. Berkeley DB Transactional Data Store Applications </th>
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<td width="20%" align="right"> <a accesskey="n" href="transapp_archival.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="transapp_checkpoint"></a>Checkpoints</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 second component of the infrastructure is performing
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checkpoints of the log files. Performing checkpoints is
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necessary for two reasons.
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</p>
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<p>
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First, you may be able to remove Berkeley DB log files from
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your database environment after a checkpoint. Change records
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are written into the log files when databases are modified,
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but the actual changes to the database are not necessarily
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written to disk. When a checkpoint is performed, changes to
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the database are written into the backing database file. Once
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the database pages are written, log files can be archived and
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removed from the database environment because they will never
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be needed for anything other than catastrophic failure. (Log
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files which are involved in active transactions may not be
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removed, and there must always be at least one log file in the
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database environment.)
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</p>
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<p>
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The second reason to perform checkpoints is because
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checkpoint frequency is inversely proportional to the amount
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of time it takes to run database recovery after a system or
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application failure. This is because recovery after failure
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has to redo or undo changes only since the last checkpoint, as
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changes before the checkpoint have all been flushed to the
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databases.
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</p>
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<p>
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Berkeley DB provides the <a href="../api_reference/C/db_checkpoint.html" class="olink">db_checkpoint</a> utility, which can be used
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to perform checkpoints. Alternatively, applications can write
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their own checkpoint thread using the underlying
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<a href="../api_reference/C/txncheckpoint.html" class="olink">DB_ENV->txn_checkpoint()</a> function. The following code fragment
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checkpoints the database environment every 60 seconds:
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</p>
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<pre class="programlisting">int
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main(int argc, char *argv)
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{
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extern int optind;
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DB *db_cats, *db_color, *db_fruit;
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DB_ENV *dbenv;
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pthread_t ptid;
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int ch;
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while ((ch = getopt(argc, argv, "")) != EOF)
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switch (ch) {
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case '?':
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default:
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usage();
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}
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argc -= optind;
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argv += optind;
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env_dir_create();
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env_open(&dbenv);
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<span class="bold"><strong> /* Start a checkpoint thread. */
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if ((errno = pthread_create(
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&ptid, NULL, checkpoint_thread, (void *)dbenv)) != 0) {
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fprintf(stderr,
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"txnapp: failed spawning checkpoint thread: %s\n",
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strerror(errno));
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exit (1);
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}</strong></span>
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/* Open database: Key is fruit class; Data is specific type. */
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db_open(dbenv, &db_fruit, "fruit", 0);
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/* Open database: Key is a color; Data is an integer. */
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db_open(dbenv, &db_color, "color", 0);
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/*
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* Open database:
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* Key is a name; Data is: company name, cat breeds.
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*/
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db_open(dbenv, &db_cats, "cats", 1);
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add_fruit(dbenv, db_fruit, "apple", "yellow delicious");
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add_color(dbenv, db_color, "blue", 0);
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add_color(dbenv, db_color, "blue", 3);
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add_cat(dbenv, db_cats,
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"Amy Adams",
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"Oracle",
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"abyssinian",
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"bengal",
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"chartreaux",
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NULL);
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return (0);
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}
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<span class="bold"><strong>void *
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checkpoint_thread(void *arg)
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{
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DB_ENV *dbenv;
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int ret;
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dbenv = arg;
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dbenv->errx(dbenv, "Checkpoint thread: %lu", (u_long)pthread_self());
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/* Checkpoint once a minute. */
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for (;; sleep(60))
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if ((ret = dbenv->txn_checkpoint(dbenv, 0, 0, 0)) != 0) {
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dbenv->err(dbenv, ret, "checkpoint thread");
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exit (1);
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}
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/* NOTREACHED */
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}</strong></span></pre>
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<p>
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Because checkpoints can be quite expensive, choosing how
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often to perform a checkpoint is a common tuning parameter for
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Berkeley DB applications.
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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="transapp_deadlock.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="transapp.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="transapp_archival.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">Deadlock detection </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"> Database and log file
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archival</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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