环境：Oracle 11.2.0.4 DG
故障现象：
客户在备库告警日志中发现GAP sequence提示信息：
Mon Nov 21 09:53:29 2016
Media Recovery Waiting for thread 1 sequence 12034
Fetching gap sequence in thread 1, gap sequence 12034-12078
Mon Nov 21 09:55:20 2016
FAL[client]: Failed to request gap sequence
GAP - thread 1 sequence 12034-12078
DBID 3493955325 branch 881855745
FAL[client]: All defined FAL servers have been attempted.
------------------------------------------------------------
Check that the CONTROL_FILE_RECORD_KEEP_TIME initialization
parameter is defined to a value that's sufficiently large
enough to maintain adequate log switch information to resolve
archivelog gaps.
------------------------------------------------------------
修复过程：
- 1.查询备库SCN
- 2.确定主库是否添加数据文件
- 3.备库停止日志应用
- 4.主库增量备份并传输到备库上
- 5.备库上进行恢复
- 6.主库上创建standby controlfile文件并传输到备库
- 7.备库恢复控制文件
- 8.清空备库日志组
- 9.备库重设flashback
- 10.备库重新接收并应用日志
- 11.备库重新开启read only模式
- 12.验证修复是否成功
- Reference
1.查询备库SCN
查询备库当前SCN，如果人为造成控制文件、数据文件、数据文件头的SCN不一致，那么需要根据日志中gap的起始sequence#找出对应的SCN。可以查看文末reference中惜分飞的博客评论部分。SQL> col CURRENT_SCN for 999999999999999999
SQL> SELECT CURRENT_SCN FROM V$DATABASE;
CURRENT_SCN
-------------------
11906842766974
2.确定主库是否添加数据文件（这里没有）
select FILE#,name from v$datafile where CREATION_CHANGE#> =11906842766974;
确定主库在这个scn之后是否有添加数据文件，如果添加文件，需要手工在备库添加。本次没有遇到。
3.备库停止日志应用
SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;
4.主库增量备份并传输到备库上
主库进行增量备份然后传输到备库上
RMAN> backup as compressed backupset INCREMENTAL from scn 11906842766974 database format '/backup/dumpfile/%u.bak';
$ scp *.bak 192.168.56.158:/oradata/rman/
5.备库上进行恢复
RMAN> CATALOG START WITH '/oradata/rman/';
--注意如果此时库是read only则需要置换为mount后再进行recover操作；
RMAN> RECOVER DATABASE NOREDO;
6.主库上创建standby controlfile文件并传输到备库
主库上创建standby controlfile文件并传输到备库RMAN> BACKUP CURRENT CONTROLFILE FOR STANDBY FORMAT '/home/oracle/std_ctl.bak';
[oracle@localhost ~]$ scp std_ctl.bak 192.168.56.158:/home/oracle/
7.备库恢复控制文件
备库关库，启动到nomount状态后恢复控制文件，最后启动到mount状态RMAN> shutdown;
RMAN> STARTUP NOMOUNT;
RMAN> RESTORE STANDBY CONTROLFILE FROM '/home/oracle/std_ctl.bak';
RMAN> alter database mount;
8.清空备库日志组（这里不用）
本次DG中使用了standby log模式，不需要此步骤。SQL> ALTER DATABASE CLEAR LOGFILE GROUP 1;
如果配置了physical standby redo log则不需该步骤；
如果没有采用standby log模式，有几组需要清空几组。
9.备库重设flashback（根据实际情况选做，这里本身就没开启）
备库重设flashback（根据实际情况选做，这里DG环境备库本身就没开启，所以不用操作）SQL> ALTER DATABASE FLASHBACK OFF;
SQL> ALTER DATABASE FLASHBACK ON;
10.备库重新接收并应用日志
备库重新接收并应用日志：SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE using current logfile DISCONNECT FROM SESSION;
恢复过程备库最后的日志（最后需要出现Media Recovery Waiting for 字样）：
Mon Nov 21 17:17:05 2016
Managed Standby Recovery starting Real Time Apply
Parallel Media Recovery started with 32 slaves
Waiting for all non-current ORLs to be archived...
All non-current ORLs have been archived.
Completed: ALTER DATABASE RECOVER MANAGED STANDBY DATABASE using current logfile DISCONNECT FROM SESSION
Media Recovery Log /oradata/arch/1_12131_881855745.dbf
Mon Nov 21 17:18:59 2016
Media Recovery Log /oradata/arch/1_12132_881855745.dbf
Mon Nov 21 17:20:44 2016
Media Recovery Log /oradata/arch/1_12133_881855745.dbf
Mon Nov 21 17:21:02 2016
Media Recovery Log /oradata/arch/1_12134_881855745.dbf
Mon Nov 21 17:22:22 2016
Media Recovery Waiting for thread 1 sequence 12135 (in transit)
11.备库重新开启read only模式
根据实际情况，备库重新开启read only模式，本次需求是需要备库read only状态应用日志（11g ADG特性）SQL> alter database RECOVER MANAGED STANDBY DATABASE CANCEL;
SQL> alter database open;
SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE using current logfile DISCONNECT FROM SESSION;
12.验证修复是否成功
12.1 对比最大sequence#
不一定准确（比如中间产生过gap，但是后期的归档日志正常传输，那么实际上虽然结果相同，但是还是有gap）
在主库中执行alter system switch logfile;
分别主备库中执行：select max(sequence#) from v$archived_log;
12.2 通过跟踪alert文件
主库告警：
tail -200f /oracle/diag/rdbms/shoucall/shoucall/trace/alert_shoucall.log
备库告警：
tail -200f /u01/app/oracle/diag/rdbms/shoucall_dg/shoucall/trace/alert_shoucall.log