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Import BSDDB 4.7.25 (as of svn r89086)

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Zachary Ware
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Configuring the Logging Subsystem</title>
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.62.4" />
<link rel="home" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
<link rel="up" href="filemanagement.html" title="Chapter 5. Managing DB Files" />
<link rel="previous" href="logfileremoval.html" title="Removing Log Files" />
<link rel="next" href="wrapup.html" title="Chapter 6. Summary and Examples" />
</head>
<body>
<div class="navheader">
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Configuring the Logging Subsystem</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="logfileremoval.html">Prev</a> </td>
<th width="60%" align="center">Chapter 5. Managing DB Files</th>
<td width="20%" align="right"> <a accesskey="n" href="wrapup.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="logconfig"></a>Configuring the Logging Subsystem</h2>
</div>
</div>
<div></div>
</div>
<p>
You can configure the following aspects of the logging
subsystem:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Size of the log files.
</p>
</li>
<li>
<p>
Size of the logging subsystem's region.
See <a href="logconfig.html#logregionsize">Configuring the Logging Region Size</a>.
</p>
</li>
<li>
<p>
Maintain logs entirely in-memory.
See <a href="logconfig.html#inmemorylogging">Configuring In-Memory Logging</a>
for more information.
</p>
</li>
<li>
<p>
Size of the log buffer in memory.
See <a href="logconfig.html#logbuffer">Setting the In-Memory Log Buffer Size</a>.
</p>
</li>
<li>
<p>
On-disk location of your log files. See
<a href="enabletxn.html#splittingdata">Identifying Specific File Locations</a>.
</p>
</li>
</ul>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="logfilesize"></a>Setting the Log File Size</h3>
</div>
</div>
<div></div>
</div>
<p>
Whenever a pre-defined amount of data is written to a
log file (10 MB by default), DB stops using the
current log file and starts writing to a new file.
You can change the maximum amount of data contained in each
log file by using the
<span>
<tt class="methodname">DB_ENV-&gt;set_lg_max()</tt>
method.
</span>
Note that this method can be used at any time
during an application's lifetime.
</p>
<p>
Setting the log file size to something larger than its
default value is largely a matter of
convenience and a reflection of the application's
preference in backup media and frequency. However, if
you set the log file size too low relative to your
application's traffic patterns, you can cause
yourself trouble.
</p>
<p>
From a performance perspective, setting the log file
size to a low value can cause your active transactions to
pause their writing activities more frequently than would
occur with larger log file sizes. Whenever a
transaction completes the log buffer is flushed to
disk. Normally other transactions can continue to
write to the log buffer while this flush is in
progress. However, when one log file is being closed
and another created, all transactions must cease
writing to the log buffer until the switch over is
completed.
</p>
<p>
Beyond performance concerns, using smaller log files
can cause you to use more physical files on disk.
As a result, your application could run out
of log sequence numbers, depending on how busy your
application is.
</p>
<p>
Every log file is identified with a 10 digit number.
Moreover, the maximum number of log files that your application
is allowed to create in its lifetime is 2,000,000,000.
</p>
<p>
For example, if your application performs 6,000 transactions per
second for 24 hours a day, and you are logging 500 bytes of
data per transaction into 10 MB log files, then you
will run out of log files in around 221 years:
</p>
<pre class="programlisting"> (10 * 2^20 * 2000000000) / (6000 * 500 * 365 * 60 *60 * 24) = 221 </pre>
<p>
However, if you were writing 2000 bytes of data per
transaction, and using 1 MB log files, then the same
formula shows you running out of log files in 5 years time.
</p>
<p>
All of these time frames are quite long, to be sure,
but if you do run out of log files after, say, 5 years
of continuous operations, then you must reset your log
sequence numbers. To do so:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Backup your databases as if to prepare for
catastrophic failure. See
<a href="backuprestore.html">Backup Procedures</a>
for more information.
</p>
</li>
<li>
<p>
Reset the log file's sequence number using the
<span><b class="command">db_load</b></span> utility's
<tt class="literal">-r</tt> option.
</p>
</li>
<li>
<p>
Remove all of the log files from your
environment. Note that this is the only
situation in which all of the log files are
removed from an environment; in all other
cases, at least a single log file is retained.
</p>
</li>
<li>
<p>
Restart your application.
</p>
</li>
</ol>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="logregionsize"></a>Configuring the Logging Region Size</h3>
</div>
</div>
<div></div>
</div>
<p>
The logging subsystem's default region size is 60 KB. The
logging region is used to store filenames, and so you may
need to increase its size if a large number of files (that
is, if you have a very large number of databases) will
be opened and registered with DB's log manager.
</p>
<p>
You can set the size of your logging region by using the
<span>
<tt class="methodname">DB_ENV-&gt;set_lg_regionmax()</tt>
</span>
method. Note that this method can only be called before the
first environment handle for your application is opened.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="inmemorylogging"></a>Configuring In-Memory Logging</h3>
</div>
</div>
<div></div>
</div>
<p>
It is possible to configure your logging subsystem such
that logs are maintained entirely in memory. When
you do this, you give up your transactional durability
guarantee. Without log files, you have no way to run
recovery so any system or software failures that you might
experience can corrupt your databases.
</p>
<p>
However, by giving up your durability guarantees, you can
greatly improve your application's throughput by avoiding
the disk I/O necessary to write logging information to
disk. In this case, you still retain your transactional
atomicity, consistency, and isolation guarantees.
</p>
<p>
To configure your logging subsystem to maintain your logs
entirely in-memory:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Make sure your log buffer is capable of holding all
log information that can accumulate during the longest
running transaction. See <a href="logconfig.html#logbuffer">Setting the In-Memory Log Buffer Size</a> for details.
</p>
</li>
<li>
<p>
Do not run normal recovery when you open your environment. In this configuration, there are no
log files available against which you can run recovery. As a result, if you specify recovery
when you open your environment, it is ignored.
</p>
</li>
<li>
<p>
Specify
<span>
<tt class="literal">DB_LOG_IN_MEMORY</tt> to the
<tt class="methodname">DB_ENV-&gt;log_set_config()</tt>
</span>
method. Note that you must specify this before
your application opens its first environment
handle.
</p>
</li>
</ul>
</div>
<p>
For example:
</p>
<pre class="programlisting">#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include "db.h"
int
main(void)
{
int ret, ret_c;
u_int32_t db_flags, env_flags;
DB_ENV *envp;
const char *db_home_dir = "/tmp/myEnvironment";
envp = NULL;
/* Create the environment */
ret = db_env_create(&amp;envp, 0);
if (ret != 0) {
fprintf(stderr, "Error creating environment handle: %s\n",
db_strerror(ret));
return (EXIT_FAILURE);
}
/*
* Indicate that logging is to be performed only in memory.
* Doing this means that we give up our transactional durability
* guarantee.
*/
envp-&gt;log_set_config(envp, DB_LOG_IN_MEMORY, 1);
/*
* Configure the size of our log memory buffer. This must be
* large enough to hold all the logging information likely
* to be created for our longest running transaction. The
* default size for the logging buffer is 1 MB when logging
* is performed in-memory. For this example, we arbitrarily
* set the logging buffer to 5 MB.
*/
ret = envp-&gt;set_lg_bsize(envp, 5 * 1024 * 1024);
if (ret != 0) {
fprintf(stderr, "Error setting log buffer size: %s\n",
db_strerror(ret));
goto err;
}
/*
* Set the normal flags for a transactional subsystem. Note that
* we DO NOT specify DB_RECOVER. Also, remember that the logging
* subsystem is automatically enabled when we initialize the
* transactional subsystem, so we do not explicitly enable
* logging here.
*/
env_flags = DB_CREATE | /* If the environment does not
* exist, create it. */
DB_INIT_LOCK | /* Initialize locking */
DB_INIT_LOG | /* Initialize logging */
DB_INIT_MPOOL | /* Initialize the cache */
DB_THREAD | /* Free-thread the env handle. */
DB_INIT_TXN; /* Initialize transactions */
/* Open the environment as normal */
ret = envp-&gt;open(envp, db_home_dir, env_flags, 0);
if (ret != 0) {
fprintf(stderr, "Error opening environment: %s\n",
db_strerror(ret));
goto err;
}
/*
* From here, you create transactions and perform database operations
* exactly as you would if you were logging to disk. This part is
* omitted for brevity.
*/
...
err:
/* Close the databases (omitted) */
...
/* Close the environment */
if (envp != NULL) {
ret_c = envp-&gt;close(envp, 0);
if (ret_c != 0) {
fprintf(stderr, "environment close failed: %s\n",
db_strerror(ret_c));
ret = ret_c;
}
}
return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="logbuffer"></a>Setting the In-Memory Log Buffer Size</h3>
</div>
</div>
<div></div>
</div>
<p>
When your application is configured for on-disk logging
(the default behavior for transactional applications), log
information is stored in-memory until the storage space
fills up, or a transaction commit forces the log
information to be flushed to disk.
</p>
<p>
It is possible to increase the amount of memory available
to your file log buffer. Doing so improves throughput for
long-running transactions, or for transactions that produce
a large amount of data.
</p>
<p>
When you have your logging subsystem configured to maintain
your log entirely in memory (see
<a href="logconfig.html#inmemorylogging">Configuring In-Memory Logging</a>), it is very important
to configure your log buffer size because the log buffer
must be capable of holding all log information that can
accumulate during the longest running transaction.
You must make sure that the in-memory log buffer size is
large enough that no transaction will ever span the entire
buffer. You must also avoid a state where the in-memory
buffer is full and no space can be freed because a
transaction that started the first log "file" is still
active.
</p>
<p>
When your logging subsystem is configured for on-disk
logging, the default log buffer space is 32 KB. When
in-memory logging is configured, the default log buffer
space is 1 MB.
</p>
<p>
You can increase your log buffer space using the
<span>
<tt class="methodname">DB_ENV-&gt;set_lg_bsize()</tt>
</span>
method. Note that this method can only be called before the
first environment handle for your application is opened.
</p>
</div>
</div>
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<hr />
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</td>
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</tr>
<tr>
<td width="40%" align="left" valign="top">Removing Log Files </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> Chapter 6. Summary and Examples</td>
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