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<title>Berkeley DB Reference Guide: Copying databases</title>
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<table width="100%"><tr valign=top>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Programmer Notes</dl></h3></td>
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<h1 align=center>Copying databases</h1>
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<p>Because file identification cookies (for example, filenames, device and
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inode numbers, volume and file IDs, and so on) are not necessarily
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unique or maintained across system reboots, each Berkeley DB database file
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contains a 20-byte file identification bytestring that is stored in the
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first page of the database, starting with the 53rd byte on the page.
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When multiple processes or threads open the same database file in Berkeley DB,
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it is this bytestring that is used to ensure that the same underlying
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pages are updated in the shared memory buffer pool, no matter which
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Berkeley DB handle is used for the operation.
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<p>It is usually a bad idea to physically copy a database to a new name. In
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the few cases in which copying is the best solution for your application,
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you must guarantee that there are never two different databases with
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the same file identification bytestring in the memory pool at the same
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time. Copying databases is further complicated by the fact that the
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shared memory buffer pool does not discard all cached copies of pages
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for a database when the database is logically closed; that is, when
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<a href="../../api_c/db_close.html">DB->close</a> is called. Nor is there a Berkeley DB interface to
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explicitly discard pages from the shared memory buffer pool for any
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<p>Before copying a database, you must ensure that all modified pages have
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been written from the memory pool cache to the backing database file.
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This is done using the <a href="../../api_c/db_sync.html">DB->sync</a> or <a href="../../api_c/db_close.html">DB->close</a> interfaces.
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<p>Before using a copy of a database from Berkeley DB, you must ensure that all
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pages from any database with the same bytestring have been removed from
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the memory pool cache. If the environment in which you intend to open
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the copy of the database potentially has pages from files with identical
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bytestrings to the copied database (which is likely to be the case), there
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are a few possible solutions:
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<p><li>Remove the environment, either explicitly or by calling <a href="../../api_c/env_remove.html">DB_ENV->remove</a>.
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Note that this will not allow you to access both the original and copy
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of the database at the same time.
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<p><li>Create a new file that will have a new bytestring. The simplest way to
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create a new file that will have a new bytestring is to call the
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<a href="../../utility/db_dump.html">db_dump</a> utility to dump out the contents of the database and
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then use the <a href="../../utility/db_load.html">db_load</a> utility to load the dumped output into a
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new file. This allows you to access both the original and copy of
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the database at the same time.
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<p><li>If your database is too large to be copied, overwrite the bytestring in
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the copied database with a new bytestring. This allows you to access
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both the original and copy of the database at the same time.
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