今天遇到这样一个需求，两节点的RAC+两节点的RAC的DG环境，主库在做一些操作的时候，不能让备库应用日志，所以备库的MRP进程关闭了，但是可以正常接收日志，主库做完操作后确认操作失败，备库需要failover切换成主库，之前的主库重建为备库，为了不让备库应用主库传过来的错误日志，在存储上删掉了主库传过来的归档日志。
案例环境：OEL 5.5，ORACLE 10.2.0.4.0
在执行recover finish时报错。

SQL> alter database recover managed standby database finish force;
alter database recover managed standby database finish force
*
ERROR at line 1:
ORA-00283: recovery session canceled due to errors
ORA-16171: RECOVER...FINISH not allowed due to gap for thr 2, seq 12-26

此时无法failover切换。

SQL> alter database commit to switchover to primary;
alter database commit to switchover to primary
*
ERROR at line 1:
ORA-16139: media recovery required

在做不完全恢复后，还是不能切换。

SQL> recover standby database until cancel;
ORA-00279: change 235858 generated at 03/19/2014 08:33:10 needed for thread 2
ORA-00289: suggestion : +DATA
ORA-00280: change 235858 for thread 2 is in sequence #12


Specify log: {=suggested | filename | AUTO | CANCEL}
CANCEL
Media recovery cancelled.
SQL> alter database commit to switchover to primary;
alter database commit to switchover to primary
*
ERROR at line 1:
ORA-16139: media recovery required

这时要想打开备库，就需要使用ACTIVATE STANDBY DATABASE功能了。

SQL> alter database ACTIVATE STANDBY DATABASE;

Database altered.

此时数据库就是一个独立的数据库了。

SQL> select database_role,open_mode from v$database;

DATABASE_ROLE    OPEN_MODE
---------------- ----------
PRIMARY          MOUNTED

SQL> alter database open;

Database altered.

SQL> select database_role,open_mode from v$database;

DATABASE_ROLE    OPEN_MODE
---------------- ----------
PRIMARY          READ WRITE

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