control_file_record_keep_time设置与RMAN保留策略的关系

The default value of CONTROL_FILE_RECORD_KEEP_TIME is 7 days. If you are not using a recovery catalog, then set the CONTROL_FILE_RECORD_KEEP_TIME value to slightly longer than the oldest file that
you need to keep. For example, if you back up the whole database once a week, then you need to keep every backup for at least seven days. Set CONTROL_FILE_RECORD_KEEP_TIME to a value such as 10 or 14.
Regardless of whether you use a recovery catalog, never use RMAN when CONTROL_FILE_RECORD_KEEP_TIME is set to 0. If you do, then you may lose backup records.

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.

由上可知,CONTROL_FILE_RECORD_KEEP_TIME的设置会覆盖RMAN保留策略,所以,尽量将CONTROL_FILE_RECORD_KEEP_TIME的值设置的比保留策略大,以免control_file备份被清除,没达到备份的目的.

参考:http://www.itpub.net/thread-1624579-1-1.html