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<title>Berkeley DB Reference Guide: Copying databases</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Programmer Notes</dl></h3></td>
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<h3 align=center>Copying databases</h3>
<p>Because file identification cookies (for example, filenames, device and
inode numbers, volume and file IDs, and so on) are not necessarily
unique or maintained across system reboots, each Berkeley DB database file
contains a 20-byte file identification bytestring that is stored in the
first page of the database, starting with the 53rd byte on the page.
When multiple processes or threads open the same database file in Berkeley DB,
it is this bytestring that is used to ensure that the same underlying
pages are updated in the shared memory buffer pool, no matter which
Berkeley DB handle is used for the operation.</p>
<p>It is usually a bad idea to physically copy a database to a new name.  In
the few cases in which copying is the best solution for your application,
you must guarantee that there are never two different databases with
the same file identification bytestring in the memory pool at the same
time.  Copying databases is further complicated by the fact that the
shared memory buffer pool does not discard all cached copies of pages
for a database when the database is logically closed; that is, when
<a href="../../api_c/db_close.html">DB-&gt;close</a> is called.  Nor is there a Berkeley DB interface to
explicitly discard pages from the shared memory buffer pool for any
particular database.</p>
<p>Before copying a database, you must ensure that all modified pages have
been written from the memory pool cache to the backing database file.
This is done using the <a href="../../api_c/db_sync.html">DB-&gt;sync</a> or <a href="../../api_c/db_close.html">DB-&gt;close</a> methods.</p>
<p>Before using a copy of a database from Berkeley DB, you must ensure that all
pages from any database with the same bytestring have been removed from
the memory pool cache.  If the environment in which you intend to open
the copy of the database potentially has pages from files with identical
bytestrings to the copied database (which is likely to be the case), there
are a few possible solutions:</p>
<ol>
<p><li>Remove the environment, either explicitly or by calling <a href="../../api_c/env_remove.html">DB_ENV-&gt;remove</a>.
Note that this will not allow you to access both the original and copy
of the database at the same time.
<p><li>Create a new file that will have a new bytestring.  The simplest way to
create a new file that will have a new bytestring is to call the
<a href="../../utility/db_dump.html">db_dump</a> utility to dump out the contents of the database and
then use the <a href="../../utility/db_load.html">db_load</a> utility to load the dumped output into a
new file.  This allows you to access both the original and copy of
the database at the same time.
<p><li>If your database is too large to be copied, overwrite the bytestring in
the copied database with a new bytestring.  This allows you to access
both the original and copy of the database at the same time.
If there are multiple databases in a single physical file,
the bytestring found in the first page of each database needs to
be overwritten, not just the first page of the physical file.
</ol>
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