RMAN backup keeps the backup metadata information in the reusable section of the controlfile. It depends on the parameter CONTROL_FILE_RECORD_KEEP_TIME. CONTROL_FILE_RECORD_KEEP_TIME specifies the minimum number of days before a reusable record in the control file can be reused. In the event a new record needs to be added to a reusable section and there is not enough space then it will delete the oldest record, which are aged enough.
Backup retention policy is the rule to set regarding which backups must be retained (whether on disk or other backup media) to meet the recovery and other requirements.
If the CONTROL_FILE_RECORD_KEEP_TIME is less than the retention policy then it may overwrite reusable records prior to obsoleting them in the RMAN metadata. Therefor it is recommended that the CONTROL_FILE_RECORD_KEEP_TIME should set to a higher value than the retention policy.
NOTE: Best practice is to NOT set control_file_record_keep_time to a value greater than 10. If you need retention greater than this in the controlfile, you should use an RMAN catalog.
CONTROL_FILE_RECORD_KEEP_TIME = retention period + level 0 backup interval + 1
e.q. level 0 backup once a week with retention policy of a recovery windows of 14 days then in this case the CONTROL_FILE_RECORD_KEEP_TIME should be 14+7+1=22
Please note to set the retention policy of “14” days in this RMAN configuration format “CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF 14 days;” and NOT “CONFIGURE RETENTION POLICY TO REDUNDANCY 14;” (Redundancy keeps no of backup pieces includes level-0/level-1/archives and NOT number of days)