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<div class="chapter" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="filemanagement"></a>Chapter 5. Managing DB Files</h2>
</div>
</div>
<div></div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="filemanagement.html#checkpoints">Checkpoints</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="backuprestore.html">Backup Procedures</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="backuprestore.html#copyutilities">About Unix Copy Utilities</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="backuprestore.html#standardbackup">Offline Backups</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="backuprestore.html#hotbackup">Hot Backup</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="backuprestore.html#incrementalbackups">Incremental Backups</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="recovery.html">Recovery Procedures</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="recovery.html#normalrecovery">Normal Recovery</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="recovery.html#catastrophicrecovery">Catastrophic Recovery</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="architectrecovery.html">Designing Your Application for Recovery</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="architectrecovery.html#multithreadrecovery">Recovery for Multi-Threaded Applications</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="architectrecovery.html#multiprocessrecovery">Recovery in Multi-Process Applications</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="hotfailover.html">Using Hot Failovers</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="logfileremoval.html">Removing Log Files</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="logconfig.html">Configuring the Logging Subsystem</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="logconfig.html#logfilesize">Setting the Log File Size</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="logconfig.html#logregionsize">Configuring the Logging Region Size</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="logconfig.html#inmemorylogging">Configuring In-Memory Logging</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="logconfig.html#logbuffer">Setting the In-Memory Log Buffer Size</a>
</span>
</dt>
</dl>
</dd>
</dl>
</div>
<p>
DB is capable of storing several types of files on disk:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Data files, which contain the actual data in your database.
</p>
</li>
<li>
<p>
Log files, which contain information required to recover your
database in the event of a system or application failure.
</p>
</li>
<li>
<p>
Region files, which contain information necessary for the
overall operation of your application.
</p>
</li>
<li>
<p>
Temporary files, which are created only under certain special circumstances. These files never need to
be backed up or otherwise managed and so they are not a consideration for the topics described in this
chapter. See <a href="enabletxn.html#security">Security Considerations</a>
for more information on temporary files.
</p>
</li>
</ul>
</div>
<p>
Of these, you must manage your data and log files by ensuring that they
are backed up. You should also pay attention to the amount of disk space
your log files are consuming, and periodically remove any unneeded
files. Finally, you can optionally tune your logging subsystem to best
suit your application's needs and requirements.
These topics are discussed in this chapter.
</p>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="checkpoints"></a>Checkpoints</h2>
</div>
</div>
<div></div>
</div>
<p>
Before we can discuss DB file management, we need to
describe checkpoints. When databases are modified (that is, a
transaction is committed), the modifications are recorded in
DB's logs, but they are <span class="emphasis"><em>not</em></span>
necessarily reflected in the actual database files on disk.
</p>
<p>
This means that as time goes on, increasingly
more data is contained in your log files that is not
contained in your data files. As a result, you must keep more
log files around than you might actually need. Also, any
recovery run from your log files will take increasingly longer
amounts of time, because there is more data in the log files
that must be reflected back into the data files during the
recovery process.
</p>
<p>
You can reduce these problems by periodically
running a checkpoint against your environment. The checkpoint:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Flushes dirty pages from the in-memory cache. This means that data modifications found in your
in-memory cache are written to the database files on disk. Note that a checkpoint also causes data
dirtied by an uncommitted transaction to also be written to your database files on disk. In this latter
case, DB's normal recovery is used to remove any such modifications that were subsequently
abandoned by your application using a transaction abort.
</p>
<p>
Normal recovery is describe in <a href="recovery.html">Recovery Procedures</a>.
</p>
</li>
<li>
<p>
Writes a checkpoint record.
</p>
</li>
<li>
<p>
Flushes the log. This causes all log data that has not yet been written to disk to be written.
</p>
</li>
<li>
<p>
Writes a list of open databases.
</p>
</li>
</ul>
</div>
<p>
There are several ways to run a checkpoint. One way is to use
the <span><b class="command">db_checkpoint</b></span> command line utility. (Note, however, that this command line utility
cannot be used if your environment was opened using
<span>
<tt class="literal">DB_PRIVATE</tt>.)
</span>
</p>
<p>
You can also run a thread that periodically checkpoints your
environment for you by calling the
<tt class="methodname">DbEnv::txn_checkpoint()</tt>
method.
</p>
<p>
Note that you can prevent a checkpoint from occurring unless more
than a specified amount of log data has been written since the
last checkpoint. You can also prevent the checkpoint from
running unless more than a specified amount of time has
occurred since the last checkpoint. These conditions are
particularly interesting if you have multiple threads
<span>or processes</span>
running checkpoints.
</p>
<p>
For configuration information, see the
<a href="http://www.oracle.com/technology/documentation/berkeley-db/db/api_cxx/txn_checkpoint.html" target="_top">
DbEnv::txn_checkpoint() API reference page.
</a>
</p>
<p>
Note that running checkpoints can be quite expensive. DB must
flush every dirty page to the backing database files. On the
other hand, if you do not run checkpoints often enough, your
recovery time can be unnecessarily long and you may be using more
disk space than you really need. Also, you cannot remove log files
until a checkpoint is run. Therefore, deciding how frequently
to run a checkpoint is one of the most
common tuning activity for DB applications.
</p>
<p>
For example, to run a checkpoint from a separate thread of control:
</p>
<pre class="programlisting">#include <pthread.h>
#include "db_cxx.h"
...
void *checkpoint_thread(void *);
int main(void)
{
u_int32_t 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
std::string envHome("/export1/testEnv");
DbEnv myEnv(0);
try {
myEnv.open(envHome.c_str(), env_flags, 0);
// Start a checkpoint thread.
pthread_t ptid;
int ret;
if ((ret = pthread_create(
&ptid, NULL, checkpoint_thread, (void *)&myEnv)) != 0) {
fprintf(stderr,
"txnapp: failed spawning checkpoint thread: %s\n",
strerror(errno));
myEnv.close(0);
exit (1);
}
// All other threads and application shutdown code
// omitted for brevity.
...
}
void *
checkpoint_thread(void *arg) {
DbEnv *dbenv = arg;
// Checkpoint once a minute.
for (;; sleep(60)) {
try {
dbenv->txn_checkpoint(0, 0, 0));
} catch(DbException &e) {
dbenv->err(e.get_errno(), "checkpoint thread");
exit (e.get_errno());
}
}
// NOTREACHED
} </pre>
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