What is an obsolete backup set?
A. A backup set that is missing one or more backup set pieces
B. A backup that has exceeded the retention criteria and is no longer needed
C. A backup set that does not include archived redo logs
D. A backup set that can not be recovered due to corruption
E. A backup set superceded by a datafile copy
An obseloete backup set is one that has exceeded the retention criteria.As a result,
it is subject to automatic removal in the flash recovery area.
原文：http://www.cnblogs.com/Babylon/p/7794308.html