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<title>Berkeley DB Reference Guide: Access method FAQ</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></h3></td>
<td align=right><a href="../am_misc/tune.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../java/conf.html"><img src="../../images/next.gif" alt="Next"></a>
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<p>
<h3 align=center>Access method FAQ</h3>
<ol>
<p><li><b>Is a Berkeley DB database the same as a "table"?</b>
<p>Yes; "tables" are databases, "rows" are key/data pairs, and "columns"
are application-encapsulated fields within a data item (to which Berkeley DB
does not directly provide access).</p>
<p><li><b>I'm getting an error return in my application, but I can't
figure out what the library is complaining about.</b>
<p>See <a href="../../api_c/env_set_errcall.html">DB_ENV->set_errcall</a>, <a href="../../api_c/env_set_errfile.html">DB_ENV->set_errfile</a> and
<a href="../../api_c/db_set_errfile.html">DB->set_errfile</a> for ways to get additional information about
error returns from Berkeley DB.</p>
<p><li><b>Are Berkeley DB databases portable between architectures?</b>
<p>Yes. See <a href="../../ref/am_conf/byteorder.html">Selecting a byte
order</a> for more information.</p>
<p><li><b>I'm seeing database corruption when creating multiple databases
in a single physical file.</b>
<p>This problem is usually the result of <a href="../../api_c/db_class.html">DB</a> handles not sharing an
underlying database environment. See <a href="../../ref/am/opensub.html">Opening multiple databases in a single file</a> for more information.</p>
<a name="3"><!--meow--></a><a name="4"><!--meow--></a>
<p><li><b>Is there any way to compact databases, or return unused
database pages to the filesystem?</b>
<p>When Berkeley DB database pages are emptied, they are made available for other
uses, that is, new pages will not be allocated from the underlying
filesystem as long as there are unused pages available. However, the
pages cannot be returned to the filesystem without dumping the database,
removing the physical file, and reloading the database. The one
exception to this rule is Queue access method extent files. Queue
extent files are removed when they are emptied, and their pages returned
to the underlying filesystem.</p>
<p><li><b>I'm using integers as keys for a Btree database, and even
though the key/data pairs are entered in sorted order, the page-fill
factor is low.</b>
<p>This is usually the result of using integer keys on little-endian
architectures such as the x86. Berkeley DB sorts keys as byte strings, and
little-endian integers don't sort well when viewed as byte strings.
For example, take the numbers 254 through 257. Their byte patterns on
a little-endian system are:</p>
<blockquote><pre>254 fe 0 0 0
255 ff 0 0 0
256 0 1 0 0
257 1 1 0 0</pre></blockquote>
<p>If you treat them as strings, then they sort badly:</p>
<blockquote><pre>256
257
254
255</pre></blockquote>
<p>On a big-endian system, their byte patterns are:</p>
<blockquote><pre>254 0 0 0 fe
255 0 0 0 ff
256 0 0 1 1
257 0 0 1 1</pre></blockquote>
<p>and so, if you treat them as strings they sort nicely. Which means, if
you use steadily increasing integers as keys on a big-endian system
Berkeley DB behaves well and you get compact trees, but on a little-endian
system Berkeley DB produces much less compact trees. To avoid this problem,
you may want to convert the keys to flat text or big-endian
representations, or provide your own
<a href="../../ref/am_conf/bt_compare.html">Btree comparison function.</a></p>
<a name="5"><!--meow--></a>
<p><li><b>Is there any way to avoid double buffering in the Berkeley DB system?</b>
<p>While you cannot avoid double buffering entirely, there are a few things
you can do to address this issue:</p>
<p>First, the Berkeley DB cache size can be explicitly set. Rather than allocate
additional space in the Berkeley DB cache to cover unexpectedly heavy load or
large table sizes, double buffering may suggest you size the cache to
function well under normal conditions, and then depend on the file
buffer cache to cover abnormal conditions. Obviously, this is a
trade-off, as Berkeley DB may not then perform as well as usual under abnormal
conditions.</p>
<p>Second, depending on the underlying operating system you're using, you
may be able to alter the amount of physical memory devoted to the file
buffer cache. Running as the system super-user makes a difference for
some UNIX or UNIX-like operating systems as well.</p>
<p>Third, changing the size of the Berkeley DB environment regions can change
the amount of space the operating system makes available for the file
buffer cache, and it's often worth considering exactly how the operating
system is dividing up its available memory. Further, moving the Berkeley DB
database environment regions from filesystem backed memory into system
memory (or heap memory), can often make additional system memory
available for the file buffer cache, especially on systems without a
unified buffer cache and VM system.</p>
<p>Finally, for operating systems that allow buffering to be turned off,
specifying the <a href="../../api_c/env_set_flags.html#DB_DIRECT_DB">DB_DIRECT_DB</a> and <a href="../../api_c/env_set_flags.html#DB_DIRECT_LOG">DB_DIRECT_LOG</a> flags
will attempt to do so.</p>
<p><li><b>I'm seeing database corruption when I run out of disk space.</b>
<p>Berkeley DB can continue to run when when out-of-disk-space errors occur, but
it requires the application to be transaction protected. Applications
which do not enclose update operations in transactions cannot recover
from out-of-disk-space errors, and the result of running out of disk
space may be database corruption.</p>
<p><li><b>How can I associate application information with a <a href="../../api_c/db_class.html">DB</a>
or <a href="../../api_c/env_class.html">DB_ENV</a> handle?</b>
<p>In the C API, the <a href="../../api_c/db_class.html">DB</a> and <a href="../../api_c/env_class.html">DB_ENV</a> structures each contain
an "app_private" field intended to be used to reference
application-specific information. See the <a href="../../api_c/db_class.html">db_create</a> and
<a href="../../api_c/env_class.html">db_env_create</a> documentation for more information.</p>
<p>In the C++ or Java APIs, the easiest way to associate
application-specific data with a handle is to subclass the <a href="../../api_cxx/db_class.html">Db</a>
or <a href="../../api_cxx/env_class.html">DbEnv</a>, for example subclassing <a href="../../api_cxx/db_class.html">Db</a> to get MyDb.
Objects of type MyDb will still have the Berkeley DB API methods available on
them, and you can put any extra data or methods you want into the MyDb
class. If you are using "callback" APIs that take <a href="../../api_cxx/db_class.html">Db</a> or
<a href="../../api_cxx/env_class.html">DbEnv</a> arguments (for example, <a href="../../api_cxx/db_set_bt_compare.html">Db::set_bt_compare</a>)
these will always be called with the <a href="../../api_cxx/db_class.html">Db</a> or <a href="../../api_cxx/env_class.html">DbEnv</a>
objects you create. So if you always use MyDb objects, you will be able
to take the first argument to the callback function and cast it to a
MyDb (in C++, cast it to (MyDb*)). That will allow you to access your
data members or methods.</p>
</ol>
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