3.15.3 Database Maintenance

There are two principal operations of database management: expiration and compaction. Expiration consists in removing expired entries from the database. In fact, it is rarely needed, since the expired entries are removed in the process of normal mailfromd work. Nevertheless, a special option is provided in case an explicit expiration is needed (for example, before dumping the database to another format, to avoid transferring useless information).

The command line option --expire instructs mfdbtool to delete expired entries from the specified database. As usual, the database is specified using --format option. If it is not given explicitly, ‘cache’ is assumed.

While removing expired entries the space they occupied is marked as free, so it can be used by subsequent inserts. The database does not shrink after expiration is finished. To actually return the unused space to the file system you should compact your database.

This is done by running mfdbtool --compact (and, optionally, specifying the database to operate upon with --format option). Notice, that compacting a database needs roughly as much disk space on the partition where the database resides as is currently used by the database. Database compaction runs in three phases. First, the database is scanned and all non-expired records are stored in the memory. Secondly, a temporary database is created in the state directory and all the cached entries are flushed into it. This database is named after the PID of the running mfdbtool process. Finally, the temporary database is renamed to the source database.

Both --compact and --expire can be applied to all databases by combining them with --all. It is useful, for example, in crontab files. For example, I have the following monthly job in my crontab:

0 1 1 * * /usr/bin/mfdbtool --compact --all