RMAN backup keeps the backup metadata information in the reusable section of the controlfile. Its 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. Therfor it is recommended that the CONTROL_FILE_RECORD_KEEP_TIME should set to a higher value than the retention policy.
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 45 days then in this
case the CONTROL_FILE_RECORD_KEEP_TIME should be 45+7+1=53
SQL> SHOW PARAMETER KEEP TIME;
SQL> ALTER SYSTEM SET CONTROL_FILE_RECORD_KEEP_TIME=53 SCOPE=BOTH;