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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Transaction Protected Applications</dl></h3></td>
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<h1 align=center>Recovery and filesystem operations</h1>
<p>When running in a transaction-protected environment, database creation
and deletion are logged as stand-alone transactions internal to Berkeley DB.
That is, for each such operation a new transaction is begun and aborted
or committed internally, so that they will be recovered during recovery.
<p>The Berkeley DB API supports removing and renaming files. Renaming files is
supported by the <a href="../../api_c/db_rename.html">DB->rename</a> method, and removing files by the
<a href="../../api_c/db_remove.html">DB->remove</a> method. Berkeley DB does not permit specifying the
<a href="../../api_c/db_open.html#DB_TRUNCATE">DB_TRUNCATE</a> flag when opening a file in a transaction protected
environment. This is an implicit file deletion, but one that does not
always require the same operating system file permissions as does deleting
and creating a file.
<p>If you have changed the name of a file or deleted it outside of the Berkeley DB
library (e.g., you explicitly removed a file using your normal operating
system utilities), then it is possible that recovery will not be able to
find a database referenced in the log. In this case, <a href="../../utility/db_recover.html">db_recover</a>
will produce a warning message saying it was unable to locate a file it
expected to find. This message is only a warning, as the file may have
been subsequently deleted as part of normal database operations before
the failure occurred, and so is not necessarily a problem.
<p>Generally, any filesystem operations that are performed outside the Berkeley DB
interface should be performed at the same time as making a snapshot of
the database. To perform filesystem operations correctly:
<p><ol>
<p><li>Cleanly shutdown database operations.
<p>To shutdown database operations cleanly, all applications accessing the
database environment must be shutdown and a transaction checkpoint must
be taken. If the applications are not implemented such that they can be
shutdown gracefully (i.e., closing all references to the database
environment), recovery must be performed after all applications have been
killed to ensure that the underlying databases are consistent on disk.
<p><li>Perform the filesystem operations, e.g., remove or rename one
or more files.
<p><li>Make an archival snapshot of the database.
<p>While this step is not strictly necessary, it is strongly recommended.
If this step is not performed, recovery from catastrophic failure will
require that recovery first be performed up to the time of the
filesystem operations, the filesystem operations be redone, and then
recovery be performed from the filesystem operations forward.
<p><li>Restart the database applications.
</ol>
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