對於生成當中的資料庫，不同公司不同業務需求或者不同時間，對主庫與備庫的保護模式需求不一樣，
這樣，就需要切換主資料庫與備用資料庫之間的保護模式。以下是模式之間的切換過程。

--檢視主庫的保護模式：
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

--檢視備庫的保護模式：
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三種保護模組式之間的轉換：
--最大效能轉換至最大可用：
--關閉主庫，重啟到mount狀態：
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.

--在主庫修改引數資訊：
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.

--開啟主庫並驗證：
SQL> alter database open;
Database altered.

--驗證主庫：
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

--確認備庫：
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
備庫也已經把模式改變到最大可用模式。

--將主庫與備庫調至最大保護模式：
--由於最大保護模式與最大可用模式的重做歸檔程式、網路傳輸與磁碟寫選項是一樣的
--所以可以直接更改模式就可以了，不需要先修改引數資訊：

--把主庫與備庫調至最大保護模式：
--直接修改到maximize protection模式：
SQL> alter database set standby database to maximize protection;
Database altered.
--驗證主庫：
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

--確認備庫：
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
可以看到主庫與備庫都調至到最大保護模式。

--把資料庫重新調至回到最大效能模式：
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.

--驗證主庫：
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

--確認備庫：
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

---最後開啟主庫：
SQL> alter database open;
Database altered.

DG中3種保護模式之間的切換

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