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<title>Berkeley DB Reference Guide: Database and log file archival</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Transactional Data Store Applications</dl></h3></td>
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<td align=right><a href="../../ref/transapp/checkpoint.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../reftoc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/transapp/logfile.html"><img src="../../images/next.gif" alt="Next"></a>
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<h1 align=center>Database and log file archival</h1>
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<p>The third component of the administrative infrastructure, archival for
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catastrophic recovery, concerns the recoverability of the database in
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the face of catastrophic failure. Recovery after catastrophic failure
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is intended to minimize data loss when physical hardware has been
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destroyed -- for example, loss of a disk that contains databases or log
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files. Although the application may still experience data loss in this
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case, it is possible to minimize it.
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<p>First, you may want to periodically create snapshots (that is, backups)
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of your databases to make it possible to recover from catastrophic
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failure. These snapshots are either a standard backup, which creates a
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consistent picture of the databases as of a single instant in time; or
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an on-line backup (also known as a <i>hot</i> backup), which creates
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a consistent picture of the databases as of an unspecified instant
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during the period of time when the snapshot was made. The advantage of
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a hot backup is that applications may continue to read and write the
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databases while the snapshot is being taken. The disadvantage of a hot
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backup is that more information must be archived, and recovery based on
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a hot backup is to an unspecified time between the start of the backup
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and when the backup is completed.
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<p>Second, after taking a snapshot, you should periodically archive the
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log files being created in the environment. It is often helpful to
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think of database archival in terms of full and incremental filesystem
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backups. A snapshot is a full backup, whereas the periodic archival of
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the current log files is an incremental backup. For example, it might
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be reasonable to take a full snapshot of a database environment weekly
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or monthly, and archive additional log files daily. Using both the
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snapshot and the log files, a catastrophic crash at any time can be
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recovered to the time of the most recent log archival; a time long after
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the original snapshot.
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<p>To create a standard backup of your database that can be used to recover
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from catastrophic failure, take the following steps:
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<p><li>Commit or abort all ongoing transactions.
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<p><li>Stop writing your databases until the backup has completed. Read-only
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operations are permitted, but no write operations and no filesystem
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operations may be performed (for example, the <a href="../../api_c/env_remove.html">DB_ENV->remove</a> and
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<a href="../../api_c/db_open.html">DB->open</a> functions may not be called).
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<p><li>Force an environment checkpoint (see <a href="../../utility/db_checkpoint.html">db_checkpoint</a> for more
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<p><li>Run <a href="../../utility/db_archive.html">db_archive</a> <b>-s</b> to identify all the database data
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files, and copy them to a backup device such as CD-ROM, alternate disk,
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<p>If the database files are stored in a separate directory from the other
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Berkeley DB files, it may be simpler to archive the directory itself instead
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of the individual files (see <a href="../../api_c/env_set_data_dir.html">DB_ENV->set_data_dir</a> for additional
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information). <b>Note: if any of the database files did not have
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an open DB handle during the lifetime of the current log files,
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<a href="../../utility/db_archive.html">db_archive</a> will not list them in its output!</b> This is another
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reason it may be simpler to use a separate database file directory and
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archive the entire directory instead of archiving only the files listed
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by <a href="../../utility/db_archive.html">db_archive</a>.
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<p><li>Run <a href="../../utility/db_archive.html">db_archive</a> <b>-l</b> to identify all the log files,
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and copy the last one (that is, the one with the highest number) to a
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backup device such as CD-ROM, alternate disk, or tape.
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<p>To create a <i>hot</i> backup of your database that can be used to
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recover from catastrophic failure, take the following steps:
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<p><li>Archive your databases, as described in the previous step #4.
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You do not have to halt ongoing transactions or force a
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checkpoint. In the case of a hot backup, the utility you use to copy
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the databases must read database pages atomically (as described by
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<a href="../../ref/transapp/reclimit.html">Berkeley DB recoverability</a>).
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<p><li>When performing a hot backup, you must additionally archive all of the
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log files. Note that the order of these two operations is required,
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and the database files must be archived before the log files. This
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means that if the database files and log files are in the same
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directory, you cannot simply archive the directory; you must make sure
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that the correct order of archival is maintained.
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<p>To archive your log files, run the <a href="../../utility/db_archive.html">db_archive</a> utility using
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the <b>-l</b> option to identify all the database log files, and
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copy them to your backup media. If the database log files are stored
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in a separate directory from the other database files, it may be simpler
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to archive the directory itself instead of the individual files (see
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the <a href="../../api_c/env_set_lg_dir.html">DB_ENV->set_lg_dir</a> function for more information).
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<p>Once these steps are completed, your database can be recovered from
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catastrophic failure (see <a href="recovery.html">Recovery procedures</a> for
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<p>To update your snapshot so that recovery from catastrophic failure is
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possible up to a new point in time, repeat step 2 under the hot backup
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instructions -- copying all existing log files to a backup device. This
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is applicable to both standard and hot backups; that is, you can update
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snapshots made either way. Each time both the database and log files
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are copied to backup media, you may discard all previous database
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snapshots and saved log files. Archiving additional log files does not
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allow you to discard either previous database snapshots or log files.
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<p>The time to restore from catastrophic failure is a function of the
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number of log records that have been written since the snapshot was
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originally created. Perhaps more importantly, the more separate pieces
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of backup media you use, the more likely it is that you will have a
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problem reading from one of them. For these reasons, it is often best
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to make snapshots on a regular basis.
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<p><b>Obviously, the reliability of your archive media will affect the safety
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of your data. For archival safety, ensure that you have multiple copies
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of your database backups, verify that your archival media is error-free
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and readable, and that copies of your backups are stored offsite!</b>
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<p>The functionality provided by the <a href="../../utility/db_archive.html">db_archive</a> utility is also
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available directly from the Berkeley DB library. The following code fragment
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prints out a list of log and database files that need to be archived:
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<p><blockquote><pre>void
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log_archlist(DB_ENV *dbenv)
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char **begin, **list;
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/* Get the list of database files. */
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if ((ret = log_archive(dbenv,
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&list, DB_ARCH_ABS | DB_ARCH_DATA)) != 0) {
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dbenv->err(dbenv, ret, "log_archive: DB_ARCH_DATA");
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for (begin = list; *list != NULL; ++list)
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printf("database file: %s\n", *list);
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/* Get the list of log files. */
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if ((ret = log_archive(dbenv,
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&list, DB_ARCH_ABS | DB_ARCH_LOG)) != 0) {
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dbenv->err(dbenv, ret, "log_archive: DB_ARCH_LOG");
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for (begin = list; *list != NULL; ++list)
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printf("log file: %s\n", *list);
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