在DataGuard环境中如果我们在做failover的时候，可能会碰到ORA-00261错误，下面是该错误的产生原因和解决方法。
如果是由于网络问题而导致需要切换，那么通常standby端的RFS进程并不会意识到primary已经不可访问，所以RFS进程也不会释放当前的standby redo log文件。
如果是primary端的数据库实例由于故障中断，那么一般情况下standby端的RFS进程会立刻意识到primary已经不可访问，也就会立刻释放当前的standby redo log文件。
只要RFS进程没有释放standby redo log文件，那么执行ALTER DATABASE RECOVER MANAGED STANDBY DATABASE FINISH命令就会在alertlog文件中发现如下的报错信息
Warning: log 4 of thread 1 is being archived or modified
Recovery interrupted.
Media Recovery failed with error 261
如果在报上述错误的时候，执行SWITCH，那么将会出现下面的错误：
ORA-16139: media recovery required

所以必须检查alertlog文件，直到发现如下信息才表示RFS进程已经释放了standby redo log文件，这时候才可以作FINISH：
RFS: Possible network disconnect with primary database

促使RFS进程释放standby redo log 文件有两种方法：
1． 等待RFS进程的network timeout，通常需要等待8分钟左右
2． 关闭standby数据库，再重新开启，这样会强制RFS进程释放standby redo log
我们可以通过v$managed_standby视图来监控RFS进程何时释放

实行Failover：
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE FINISH;
alertlog中将显示如下信息，表示finish成功：
Terminal Incomplete Recovery: UNTIL CHANGE 3738452
Terminal Incomplete Recovery: End-Of-Redo log allocation
Terminal Incomplete Recovery: log 4 reserved for thread 1 seq# 8772
TERMINAL RECOVERY changing datafile format version from 8.0.0.0.0 to 9.0.0.0.0
Switching logfile format version from 8.0.0.0.0 to 9.0.0.0.0
Terminal Incomplete Recovery: clearing standby redo logs.
Terminal Incomplete Recovery: thread 1 seq# 8772 redo required
Terminal Incomplete Recovery: End-Of-Redo log /global/oradata/ctsdb/stdby_redo04.log
Identified end-of-REDO for thread 1 sequence 8772
Terminal Incomplete Recovery: end checkpoint SCN 3738453
Media Recovery Complete
Switching logfile format version from 9.0.0.0.0 to 8.0.0.0.0
Terminal Incomplete Recovery: successful completion
Begin: Wait for standby logfiles to be archived
Wed Sep  1 13:42:28 2004
ARC1: Evaluating archive   log 4 thread 1 sequence 8772
Wed Sep  1 13:42:28 2004
ARC0: Evaluating archive   log 4 thread 1 sequence 8772
Wed Sep  1 13:42:28 2004
ARC1: Beginning to archive log 4 thread 1 sequence 8772
Wed Sep  1 13:42:28 2004
ARC0: Unable to archive log 4 thread 1 sequence 8772
Wed Sep  1 13:42:28 2004
Creating archive destination LOG_ARCHIVE_DEST_1: '/global/oradata/ctsdb/archive/arch1_8772.log'
Wed Sep  1 13:42:28 2004
      Log actively being archived by another process
Wed Sep  1 13:42:28 2004
ARC1: Completed archiving  log 4 thread 1 sequence 8772
Wed Sep  1 13:42:43 2004
End: All standby logfiles have been archived
Resetting standby activation ID 4038461969 (0xf0b60a11)
Completed: ALTER DATABASE RECOVER MANAGED STANDBY DATABASE FINISH

FINSH成功之后再执行SWITCH：
ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY;

SWITCH成功之后，重新启动数据库：
SHUTDOWN IMMEDIATE;
STARTUP;