对于生成当中的数据库，不同公司不同业务需求或者不同时间，对主库与备库的保护模式需求不一样，
这样，就需要切换主数据库与备用数据库之间的保护模式。以下是模式之间的切换过程。

--查看主库的保护模式：
SQL> select open_mode,protection_mode,switchover_status,database_role from v$database;
OPEN_MODE PROTECTION_MODE SWITCHOVER_STATUS DATABASE_ROLE
-------------------- -------------------- -------------------- ----------------
READ WRITE MAXIMUM PERFORMANCE TO STANDBY PRIMARY

--查看备库的保护模式：
SQL> select open_mode,protection_mode,switchover_status,database_role from v$database;
OPEN_MODE PROTECTION_MODE SWITCHOVER_STATUS DATABASE_ROLE
-------------------- -------------------- -------------------- ----------------
READ ONLY MAXIMUM PERFORMANCE NOT ALLOWED PHYSICAL STANDBY

----Data Guard三种保护模块式之间的转换：
--最大性能转换至最大可用：
--关闭主库，重启到mount状态：
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.

Total System Global Area 835104768 bytes
Fixed Size 2257840 bytes
Variable Size 683674704 bytes
Database Buffers 146800640 bytes
Redo Buffers 2371584 bytes
Database mounted.

--在主库修改参数信息：
SQL> alter system set
2 LOG_ARCHIVE_DEST_2='SERVICE=PROD LGWR SYNC AFFIRM VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=PROD';
System altered.

--SQL> alter database set standby database to maximize availability;
Database altered.

--打开主库并验证：
SQL> alter database open;
Database altered.

--验证主库：
SQL> select open_mode,protection_mode,switchover_status,database_role from v$database;
OPEN_MODE PROTECTION_MODE SWITCHOVER_STATUS DATABASE_ROLE
-------------------- -------------------- -------------------- ----------------
READ WRITE MAXIMUM AVAILABILITY TO STANDBY PRIMARY

--确认备库：
SQL> select open_mode,protection_mode,switchover_status,database_role from v$database;
OPEN_MODE PROTECTION_MODE SWITCHOVER_STATUS DATABASE_ROLE
-------------------- -------------------- -------------------- ----------------
READ ONLY WITH APPLY MAXIMUM AVAILABILITY NOT ALLOWED PHYSICAL STANDBY
备库也已经把模式改变到最大可用模式。

--将主库与备库调至最大保护模式：
--由于最大保护模式与最大可用模式的重做归档进程、网络传输与磁盘写选项是一样的
--所以可以直接更改模式就可以了，不需要先修改参数信息：

--把主库与备库调至最大保护模式：
--直接修改到maximize protection模式：
SQL> alter database set standby database to maximize protection;
Database altered.
--验证主库：
SQL> select open_mode,protection_mode,switchover_status,database_role from v$database;
OPEN_MODE PROTECTION_MODE SWITCHOVER_STATUS DATABASE_ROLE
-------------------- -------------------- -------------------- ----------------
READ WRITE MAXIMUM PROTECTION TO STANDBY PRIMARY

--确认备库：
SQL> select open_mode,protection_mode,switchover_status,database_role from v$database;
OPEN_MODE PROTECTION_MODE SWITCHOVER_STATUS DATABASE_ROLE
-------------------- -------------------- -------------------- ----------------
READ ONLY WITH APPLY MAXIMUM PROTECTION NOT ALLOWED PHYSICAL STANDBY
可以看到主库与备库都调至到最大保护模式。

--把数据库重新调至回到最大性能模式：
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.

Total System Global Area 835104768 bytes
Fixed Size 2257840 bytes
Variable Size 687869008 bytes
Database Buffers 142606336 bytes
Redo Buffers 2371584 bytes
Database mounted.
SQL>
SQL> alter system set
2 LOG_ARCHIVE_DEST_2='SERVICE=PROD ASYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=PROD';
System altered.

SQL> alter database set standby database to maximize performance;
Database altered.

--验证主库：
SQL> select open_mode,protection_mode,switchover_status,database_role from v$database;
OPEN_MODE PROTECTION_MODE SWITCHOVER_STATUS DATABASE_ROLE
-------------------- -------------------- -------------------- ----------------
MOUNTED MAXIMUM PERFORMANCE NOT ALLOWED PRIMARY

--确认备库：
SQL> select open_mode,protection_mode,switchover_status,database_role from v$database;
OPEN_MODE PROTECTION_MODE SWITCHOVER_STATUS DATABASE_ROLE
-------------------- -------------------- -------------------- ----------------
MOUNTED MAXIMUM PERFORMANCE NOT ALLOWED PRIMARY

---最后打开主库：
SQL> alter database open;
Database altered.

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DG中3种保护模式之间的切换

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