A status of "expired" means that the backup piece or backup set is not found in the backup destination. With tape management systems, the automated tape library routines will expire files on the tape once a certain period of time has passed. RMAN uses this terminology to say that the tape management system has expired the file. It is no longer available. Therefore, the backup piece is expired as well.
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A status of "obsolete" means the backup piece is still available, but it is no longer needed.
expired表明，备份片或备份集找不到了。在使用磁带机的时代，自动运行的磁带库策略将会定期将磁带上的文件过期，RMAN使用了这种叫法。如果备份集或备份片过期了，表明备份集或备份片已经没有了。如果我们运行crosscheck，文件不存在的记录将会被记录为expired。
obsolete表明，备份片依然在，但是不在需要了。这个判断依据是，我们在configure命令中设定的相关部分。比如我们设定我们的保留策略是redundancy2，表明我们保留2个备份，如果我们在做一个备份的话，最老的那个就成为obsolete，如果FRA满了，obsolete状态的backup可以被复写。
**REF**
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https://searchoracle.techtarget.com/answer/What-is-the-difference-between-obsolete-and-expired-status
https://neeraj-dba.blogspot.com/2011/08/difference-between-obsolete-and-expired.html