This database configuration parameter specifies the number of database backups to retain for a database. This parameter is configurable online.
After the specified number of backups is reached, old backups are marked as expired in the recovery history file. Recovery history file entries for the table space backups and load copy backups that are related to the expired database backup are also marked as expired. When a backup is marked as expired, the physical backups can be removed from where they are stored (for example, disk, tape, TSM). The next database backup prunes the expired entries from the recovery history file.
The rec_his_retentn configuration parameter should be set to a value compatible with the value of num_db_backups. For example, if num_db_backups is set to a large value, the value for rec_his_retentn should be large enough to support the number of backups set as num_db_backups.
If both the num_db_backups and rec_his_retentn configuration parameters are set, backups are not removed unless both the num_db_backups and rec_his_retentn conditions are satisfied.