主備庫切換原因：
1. switch：用戶主動(dòng)切換；
2. failover：主庫出現(xiàn)故障，強(qiáng)行切換；
switch切換過程：先主庫再備庫
主庫
1. 查看switchover_status：select database_role, switchover_status from v$database;
如果switchover_status為：to standby 則：
ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY
否則用：ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY WITH SESSION SHUTDOWN;
2. 重啟數(shù)據(jù)庫：
shutdown immediate;
startup nomount;
3. 以備庫mount
alter database mount standby database;
備庫
1. 應(yīng)用redo：alter database recover managed standby database disconnect from session;
2. 查看switchover_status：select database_role, switchover_status from v$database;
如果switchover_status為：to primary 則：ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY
否則用：ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY WITH SESSION SHUTDOWN;
3. 重啟數(shù)據(jù)庫
shutdown immediate;
startup;
failover 主備庫切換
1.. 停止日志應(yīng)用
alter database recover managed standby database cancel;
2. 關(guān)閉standby日志傳輸
alter database recover managed standby database finish force;
3. 切換到primary
alter database commit to switchover to primary with session shutdown;
做這一步的時(shí)候，若存在gap，則會(huì)報(bào)ORA-16139:Switchover: Media recovery required - standby not in limbo 錯(cuò)誤。
做測(cè)試的時(shí)候，若先起主庫再起備庫，且未等待備庫相關(guān)日志傳輸完畢，就會(huì)出現(xiàn)這個(gè)問題。此時(shí)需要強(qiáng)制切換
alter database activate physical standby database;
4. 重啟數(shù)據(jù)庫到open狀態(tài)
當(dāng)前題目：ORACLEDataGuard主備庫角色切換
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