rman crosscheck作用

Crosschecks are useful because they can do the following:
■ Update outdated information about backups that disappeared from disk or tape or 
became corrupted
更新備份的過期資訊
■ Update the repository if you delete archived redo logs or other files with operating 
system commands
如果之前使用的是作業系統命令，則更新倉庫資訊

Use the crosscheck feature to check the status of a backup on disk or tape. If the 
backup is on disk, then CROSSCHECK checks whether the header of the file is valid. If a 
backup is on tape, then the command checks that the backups exist in the media 
management software catalog.
使用crosscheck特性檢查磁碟和磁帶的備份狀態。
Backup pieces and image copies can have the status AVAILABLE, EXPIRED, or 
UNAVAILABLE. You can view the status of backups by running the RMAN LIST 
command or by querying V$BACKUP_FILES or recovery catalog views such as RC_
DATAFILE_COPY or RC_ARCHIVED_LOG. A crosscheck updates the RMAN repository 
so that all of these techniques provide accurate information. RMAN updates each 
backup in the RMAN repository to status EXPIRED if the backup is no longer 
available. If a new crosscheck determines that an expired backup is available again, 
then RMAN updates its status to AVAILABLE.
可以透過list命令檢視檔案獲取狀態，也可以檢視v$backup_files和rc_datafile_cope或rc_archived_log檢視。
Note: The CROSSCHECK command does not delete operating 
system files or remove repository records. You must use the 
DELETE command for these operations.
You can issue the DELETE EXPIRED command to delete all expired backups. RMAN 
removes the record for the expired file from the repository. If for some reason the file 
still exists on the media, then RMAN issues warnings and lists the mismatched objects 
that cannot be deleted.