不可预知原因导致 primary 数据库故障并且短期内不能恢复就需要 failover。如果是这种切换那你就要小心点了，有可能只是虚惊一场

环境：

OS：rhel63_64bit

DB: Oracle10gR2

主备切换 切换的顺序： 先从主库到备用，再从备库到主库
Switchover Switchover：
无损转换，通常是用户手动触发或者有计划的让其自动触发，比如硬件升级啦，软件升级啦之类的。
通常它给你带来的工作量非常小并且都是可预计的。其执行分两个阶段，第一步，primary 数据库转换为standby 角色，第二步，standby 数据库(之一)转换为 primary 角色，primary 和 standby 只是简单的角色互换，这也印证了我们前面关于角色转换是 primary/standby 互动的猜测。
Failover Failover：
不可预知原因导致 primary 数据库故障并且短期内不能恢复就需要 failover。如果是这种切换那你就要小心点了，有可能只是虚惊一场，甚至连你可能损失的脑细胞的数量都能预估，但如果运气不好又没有完备的备份恢复策略而且 primary 数据并非处于最大数据保护或最高可用性模式地话

SWITCHOVER_STATUSVARCHAR2(18)Indicates whether switchover is allowed:
■NOT ALLOWED - Either this is a standby database and the primary
database has not been switched first or this is a primary database
and there are no standby databases.
■SESSIONS ACTIVE - Indicates that there are active SQL sessions
attached to the primary or standby database that need to be
disconnected before the switchover operation is permitted. Query
theV$SESSIONviewtoidentifythespecificprocessesthatneedto
be terminated.
■SWITCHOVER PENDING - This is a standby database and the
primary database switchover request has been received but not
processed.
■SWITCHOVER LATENT - The switchover was in pending mode,
but did not complete and went back to the primary database.
■TOPRIMARY-Thisisastandbydatabaseandisallowedtoswitch
over to a primary database.
■TOSTANDBY-Thisisaprimarydatabaseandisallowedtoswitch
over to a standby database.
■RECOVERY NEEDED - This is a standby database that has not
received the switchover request.
主库操作
SQL> select process,pid,status,client_process from v$managed_standby;

PROCESS PID STATUS CLIENT_P
--------- ---------- ------------ --------
ARCH 7913 CONNECTED ARCH
ARCH 7915 CONNECTED ARCH
ARCH 7917 CLOSING ARCH
ARCH 7919 CONNECTED ARCH
ARCH 7921 CONNECTED ARCH
ARCH 7923 CONNECTED ARCH
ARCH 7925 CONNECTED ARCH
ARCH 7927 CONNECTED ARCH
ARCH 7929 CONNECTED ARCH
ARCH 7931 CONNECTED ARCH
LGWR 7893 WRITING LGWR

11 rows selected.

SQL> select DATABASE_ROLE,switchover_status from v$database;

DATABASE_ROLE SWITCHOVER_STATUS
---------------- --------------------
PRIMARY SESSIONS ACTIVE

SQL>
sys@ORA> SELECT COUNT(*) FROM V$SESSION WHERE USERNAME IS NOT NULL;

COUNT(*)
----------
1
SQL> startup force
ORACLE instance started.

Total System Global Area 213909504 bytes
Fixed Size 2095152 bytes
Variable Size 88082384 bytes
Database Buffers 117440512 bytes
Redo Buffers 6291456 bytes
Database mounted.
Database opened.
SQL> select DATABASE_ROLE,switchover_status from v$database;

DATABASE_ROLE SWITCHOVER_STATUS
---------------- --------------------
PRIMARY SESSIONS ACTIVE

SQL> SELECT COUNT(*) FROM V$SESSION WHERE USERNAME IS NOT NULL;

COUNT(*)
----------
1
没有其他的会话，直接切换
SQL> select sequence#,applied from dba_logstdby_log;

no rows selected

SQL> ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY WITH SESSION SHUTDOWN;

Database altered.
SQL> alter database commit to switchover to standby;

Database altered.

SQL> select instance_name,status from v$instance;

INSTANCE_NAME STATUS
---------------- ------------
ora STARTED
SQL> shutdown immediate
ORA-01507: database not mounted

ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area 213909504 bytes
Fixed Size 2095152 bytes
Variable Size 92276688 bytes
Database Buffers 113246208 bytes
Redo Buffers 6291456 bytes
Database mounted.
sys@ORA> select DATABASE_ROLE,switchover_status from v$database;

DATABASE_ROLE SWITCHOVER_STATUS
---------------- --------------------
PHYSICAL STANDBY TO PRIMARY

SQL> select process,pid,status,client_process from v$managed_standby;