结论：
backup命令中的 keep forever关键字会 屏蔽rman的retention policy。
backup命令中的keepforever关键字会屏蔽rman的retentionpolicy(三)
kup Set 222 26-NOV-14
Backup Piece 224 26-NOV-14 /home/oracle/backup/0ppojm71_1_1
Backup Set 232 26-NOV-14
Backup Piece 234 26-NOV-14 /home/oracle/backup/control/c-275920358-20141126-00
Backup Set 269 26-NOV-14
Backup Piece 272 26-NOV-14 /home/oracle/backup/control/c-275920358-20141126-01
Backup Set 280 15-DEC-14
Backup Piece 283 15-DEC-14 /home/oracle/keep_test_0spq8ni4_1_1 -------------------->此时，该备份就出现在 report obsolete redundancy 1; 的结果中了。
Backup Set 298 15-DEC-14
Backup Piece 308 15-DEC-14 /home/oracle/backup/control/c-275920358-20141215-00
Backup Set 377 15-DEC-14
Backup Piece 391 15-DEC-14 /home/oracle/backup/control/c-275920358-20141215-01
Backup Set 411 16-DEC-14
Backup Piece 414 16-DEC-14 /home/oracle/full_after_keep_10pq96j9_1_1
Backup Set 434 16-DEC-14
Backup Piece 444 16-DEC-14 /home/oracle/backup/control/c-275920358-20141216-00
RMAN>