如何利用閃回數據庫特性恢復failover後的dataguard環境？
11g dataguard standby 切成主庫，測試完成後恢復為原standby 環境
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概述：
11204 單機對單機實施dg，因局方要求需要（讀寫模式）打開standby ；而這時原生產環境不能有任何影響動，依然對外服務；
采用的思路是：standby 直接failover 為primary db；這時原有dg關系被破壞，互不影響；
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思路概要：
1.確認主庫歸檔日志存放空間是否足夠？（需考慮歸檔保留刪除策略？）；關閉主庫到備庫的日志傳輸
2.備庫確認是否開啟flashback database，以及閃回日志存放空間是否充足？
3.備庫failover to primary （切換前確認是否日志延遲傳輸？手工注冊）
4.業務測試
5.恢復failover 備庫
6.確認dg 環境恢復是否正常（日志自動傳輸和應用？）？
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具體實施步驟：
------主庫defer 日志傳輸
alter system set log_archive_dest_2=defer;
---enable 日志傳輸：
alter system set log_archive_dest_2=enable；
-----備庫(mount)配置 flashback database：
STANDBY DATABASE: Stop redo apply, configure flashback retention,
start flashback database, open the database and start redo apply (Is active DG).
---檢查備庫是否啟用flashback database:
select flashback_on from v$database;
注意這裡需要確認下備庫打開模式： mount？readonly with apply？
在11g 環境下備庫可能啟用了 ADG 特性 備庫日志處於實時應用，數據庫模式為 readonly with apply
這時需要重啟數據庫到mount狀態修改flashback database 模式；
如果備庫處於mount 狀態，可以先取消日志apply ，直接打開閃回數據庫特性；
---取消備庫日志應用：
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;
---需要配置一下兩個參數來打開flashback database 特性：
ALTER SYSTEM SET db_recover_file_dest='/lixora/lixora/lixora/';
ALTER SYSTEM SET db_recover_file_dest_size=100G;
ALTER SYSTEM SET DB_FLASHBACK_RETENTION_TARGET=240; ---4hours
ALTER DATABASE FLASHBACK ON;
--手工創建還原點（該步驟沒有測試過）：
Creating Restore point in Physical Standby：
CREATE RESTORE POINT before_damage GUARANTEE FLASHBACK DATABASE
-------備庫failover to primary db 應急切換步驟：
（注：模擬主庫由於故障無法正常switchover，需要執行failover，強制備庫->pridb並接管業務）
1.備庫：
由於是failover，所以理解主庫這時候已經無法正常使用，只需備庫切換至pridb
【前提主庫還是可用的：可選】查詢沒有應用的日志：
SELECT THREAD#, LOW_SEQUENCE#, HIGH_SEQUENCE# FROM V$ARCHIVE_GAP;
select distinct thread#,max(sequence#) over(partition by thread#) a from v$archived_log;
該語句取得當前數據庫各線程已歸檔文件最大序號，如果primary 與standby 最大序號不相同，
必須將多出的序號對應的歸檔文件復制到待轉換的standby服務器。
Cp過來並register
ALTER DATABASE REGISTER PHYSICAL LOGFILE 'filespec1'
停止應用恢復模式
alter database recover managed standby database finish;
or
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE FINISH FORCE;
轉換standbydb為primary db
alter database commit to switchover to primary;
重啟數據庫，恢復正常業務
alter database open；
數據庫角色查看：
select open_mode,database_role from v$database;
OPEN_MODE DATABASE_ROLE
---------- ----------------
OPEN PRIMARY
------恢復failover 的備庫：
C. Using SQL*PLUS
Step 1 Determine the Standby Became Primary SCN.
Step 2 Flashback the Failed Primary Database.
Step 3 Convert to physical standby database.
Step 4 Restart Redo Transport.
Step 5 Start Redo Apply.
Step 1 Determine the SCN at which the old standby database became the primary database.
SQL> SELECT TO_CHAR(STANDBY_BECAME_PRIMARY_SCN) FROM V$DATABASE;
Step 2 Flashback the Failed Primary Database to SCN standby_became_primary_scn.
SQL> SHUTDOWN IMMEDIATE;
SQL> startup mount
SQL> FLASHBACK DATABASE TO SCN
;
Step 3 Convert the database to a physical standby database and Restart database in mount stage.
SQL> ALTER DATABASE CONVERT TO PHYSICAL STANDBY;
SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP MOUNT;
Step 4 Restart Redo Transport to the New Physical Standby Database.
1. If you have not set the remote archive destination on current primary then set remote archive destination:
SQL>ALTER SYSTEM SET LOG_ARCHIVE_DEST_2 = 'SERVICE=lixora VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=lixora' SCOPE=BOTH;
2. Enable the destination
SQL>ALTER SYSTEM SET LOG_ARCHIVE_DEST_STATE_2=ENABLE;
3. Perform a log switch to ensure that standby database begins receiving redo data from the new primary database
SQL> ALTER SYSTEM SWITCH LOGFILE;
SQL> SELECT DEST_ID, STATUS, ERROR FROM V$ARCHIVE_DEST WHERE DEST_ID=2;
--確認日志是否都apply了？
select applied from v$archived_log;
select message from v$dataguard_status;
Step 5 Start Redo Apply.
SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT FROM SESSION;
Please see also fallowing docu:
TIPS：
Oracle? Data Guard Concepts and Administration
11g Release 2 (11.2)
13.2 Converting a Failed Primary Into a Standby Database Using Flashback Database
