Berkeley DB Reference Guide:
  Berkeley DB Transactional Data Store Applications

Hot failover

For some applications, it may be useful to periodically snapshot the database environment for use as a hot failover should the primary system fail. The following steps can be taken to keep a backup environment in close synchrony with an active environment. The active environment is entirely unaffected by these procedures, and both read and write operations are allowed during all steps described here.

1. Run db_archive -s in the active environment to identify all of the active environment's database files, and copy them to the backup directory.

   If the database files are stored in a separate directory from the other Berkeley DB files, it will be simpler (and much faster!) to copy the directory itself instead of the individual files (see DB_ENV->set_data_dir for additional information). Note: if any of the database files did not have an open DB handle during the lifetime of the current log files, db_archive will not list them in its output! This is another reason it may be simpler to use a separate database file directory and copy the entire directory instead of archiving only the files listed by db_archive.

2. Remove all existing log files from the backup directory.

3. Run db_archive -l in the active environment to identify all of the active environment's log files, and copy them to the backup directory.

4. Run db_recover -c in the backup directory to catastrophically recover the copied environment.

Steps 2, 3 and 4 may be repeated as often as you like. If Step 1 (the initial copy of the database files) is repeated, then Steps 2, 3 and 4 must be performed at least once in order to ensure a consistent database environment snapshot.

These procedures must be integrated with your other archival procedures, of course. If you are periodically removing log files from your active environment, you must be sure to copy them to the backup directory before removing them from the active directory. Not copying a log file to the backup directory and subsequently running recovery with it present may leave the backup snapshot of the environment corrupted. A simple way to ensure this never happens is to archive the log files in Step 2 as you remove them from the backup directory, and move inactive log files from your active environment into your backup directory (rather than copying them), in Step 3. The following steps describe this procedure in more detail:

1. Run db_archive -s in the active environment to identify all of the active environment's database files, and copy them to the backup directory.

2. Archive all existing log files from the backup directory, moving them to a backup device such as CD-ROM, alternate disk, or tape.

3. Run db_archive (without any option) in the active environment to identify all of the log files in the active environment that are no longer in use, and move them to the backup directory.

4. Run db_archive -l in the active environment to identify all of the remaining log files in the active environment, and copy the log files to the backup directory.

5. Run db_recover -c in the backup directory to catastrophically recover the copied environment.

As before, steps 2, 3, 4 and 5 may be repeated as often as you like. If Step 1 (the initial copy of the database files) is repeated, then Steps 2 through 5 must be performed at least once in order to ensure a consistent database environment snapshot.

For an example of a hot backup implementation in the Berkeley DB distribution, see the source code for the db_hotbackup utility.

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