当前状态是dg1为主库，备库为dg2
先检查主备库状态：
dg1
SQL> select name,database_role,OPEN_MODE,SWITCHOVER_STATUS from v$database;
DATABASE_ROLE OPEN_MODE SWITCHOVER_STATUS
---------------- ---------- --------------------
PRIMARY READ WRITE SESSIONS ACTIVE
dg2
SQL> select database_role,OPEN_MODE,SWITCHOVER_STATUS from v$database;
DATABASE_ROLE OPEN_MODE SWITCHOVER_STATUS
---------------- ---------- --------------------
PHYSICAL STANDBY MOUNTED NOT ALLOWED
现在dg1（当前主库）上操作以下步骤：
SQL> alter database commit to switchover to physical standby with session shutdown;
Database altered.
SQL> shutdown immediate;
ORA-01507: database not mounted
Oracle instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 390070272 bytes
Fixed Size 2021024 bytes
Variable Size 150997344 bytes
Database Buffers 234881024 bytes
Redo Buffers 2170880 bytes
Database mounted.
SQL> select switchover_status from v$database;
SWITCHOVER_STATUS
--------------------
TO PRIMARY
现在dg2（当前备库）上操作以下步骤：
SQL> select name,database_role,OPEN_MODE,SWITCHOVER_STATUS from v$database;
NAME DATABASE_ROLE OPEN_MODE SWITCHOVER_STATUS
--------- ---------------- ---------- --------------------
ORCLDB PHYSICAL STANDBY MOUNTED TO PRIMARY
SQL> select switchover_status from v$database;
SWITCHOVER_STATUS
--------------------
TO PRIMARY
SQL> alter database commit to switchover to primary;
Database altered.
SQL> alter database open;
Database altered.
SQL> select name,database_role,OPEN_MODE,SWITCHOVER_STATUS from v$database;
NAME DATABASE_ROLE OPEN_MODE SWITCHOVER_STATUS
--------- ---------------- ---------- --------------------
ORCLDB PRIMARY READ WRITE SESSIONS ACTIVE
以上的操作已经完成了主备的切换，现在dg2为主库，dg1为备库，接下来我们在现在的主库dg2上做几次日志变化，看dg1能否同步过来。
dg1上看已经归档的日志：
SQL> select registrar,creator thread#,sequence#,first_change#,next_change# from v$archived_log;
REGISTR THREAD# SEQUENCE# FIRST_CHANGE# NEXT_CHANGE#
------- ------- ---------- ------------- ------------
FGRD FGRD 29 586174 586196
dg2上经过几次日志切换，
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM
---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- ---------
1 1 36 52428800 1 NO CURRENT 586469 05-JAN-14
2 1 34 52428800 1 YES INACTIVE 586464 05-JAN-14
3 1 35 52428800 1 YES INACTIVE 586467 05-JAN-14
可以看出现在日志已经到了36，那么我们在dg1上应用日志，应该会应用到日志35；
以下操作验证：
SQL> alter database recover managed standby database disconnect from session;
Database altered.
SQL> select registrar,creator thread#,sequence#,first_change#,next_change# from v$archived_log;
REGISTR THREAD# SEQUENCE# FIRST_CHANGE# NEXT_CHANGE#
------- ------- ---------- ------------- ------------
RFS ARCH 34 586464 586467
RFS ARCH 35 586467 586469
以上的主备切换过程就已经顺利进行完成。
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