rman backup database force 功能
FORCE
Causes RMAN to ignore backup optimization. In other words, even if CONFIGURE BACKUP OPTIMIZATION is set to ON, RMAN backs up all specified files.
Note: You can also specify this option in the backupSpecOperand clause.
加上FORCE选项，则RMAN忽略调CONFIGURE里的优化功能选项．

比如：你配置了RMAN优化功能为ON，之前对只读表空间作了备份，这时，再备份全库，不加FORCE.
则RMAN不会备份只读表空间，因为之前已经备份过，加上FORCE，则RMAN忽略掉优化功能，
作全库备份．