自己手头有一套dataguard环境，因为也有些日子没有用了，结果突然心血来潮准备启动起来学习一下，突然发现在敲了命令 recover managed standby database disconnect from session之后,命令运行正常,但是后台却报了ora错误。
Sat Jun 27 23:16:39 2015
Recovery Slave PR00 previously exited with exception 1157
Errors in file /u02/dg11g/diag/rdbms/dg11g/DG11G/trace/DG11G_mrp0_6514.trc:
ORA-01157: cannot identify/lock data file 7 - see DBWR trace file
ORA-01110: data file 7: '/u02/dg11g/oradata/DG11G/test_new01.dbf'
MRP0: Background Media Recovery process shutdown (DG11G)
Sat Jun 27 23:16:39 2015
Completed: ALTER DATABASE RECOVER  managed standby database disconnect from session
RFS[162]: Opened log for thread 1 sequence 171 dbid 1028247664 branch 880742847
RFS[161]: Opened log for thread 1 sequence 173 dbid 1028247664 branch 880742847
RFS[160]: Opened log for thread 1 sequence 172 dbid 1028247664 branch 880742847
通过上面的日志我们可以看到，MRP进程是在做数据恢复的时候报了ora错误ora-01157
但是RFS还是没有问题，RFS主要是从主库来传输归档文件的，可以看到能够正常从主库中传输归档日志，sequence#号为171,173,172的归档日志都传输到了备库。

本来这个问题没有引起多大的关注，想可能是哪些归档文件没有用到导致的，但是发现MRP压根用不了。所以尽管归档传输完成了，但是数据变更还是应用不到备库。
查看v$archive_gap没有任何记录，说明没有归档日志apply的时候出现问题。

我们来看看这个ora问题的一些明细信息,提示是在7号数据文件的地方报了ora-01157错误。
Errors in file /u02/dg11g/diag/rdbms/dg11g/DG11G/trace/DG11G_mrp0_6514.trc:
ORA-01157: cannot identify/lock data file 7 - see DBWR trace file
ORA-01110: data file 7: '/u02/dg11g/oradata/DG11G/test_new01.dbf'
从官方对于这个问题的描述来看，似乎是数据文件出了问题。
$ oerr ora 01157
01157, 00000, "cannot identify/lock data file %s - see DBWR trace file"
// *Cause:  The background process was either unable to find one of the data 
//         files or failed to lock it because the file was already in use.
//         The database will prohibit access to this file but other files will
//         be unaffected. However the first instance to open the database will
//         need to access all online data files. Accompanying error from the
//         operating system describes why the file could not be identified.
// *Action: Have operating system make file available to database. Then either
//         open the database or do ALTER SYSTEM CHECK DATAFILES.
因为这个环境被折腾了不知道多少遍，反复切换，反复测试，我都不记得是哪些特殊的操作导致了这个问题了。所以这个问题还得从头来分析。
首先查看了一下/u02/dg11g/oradata/DG11G/test_new01.dbf 这个文件，发现在文件系统中竟然不存在。
但是在数据字典信息中却存在，使用的sql语句为，可以返回对应的记录来。
select name,file# from v$datafile where file#=7;

从这个情况来看，可能是在备库端误删除了这个数据文件造成的。对于删除的数据文件我们怎么来评估呢，首先得查看主库，查看主库中的文件情况,但是在主库中这个数据文件和表空间压根不存在。
这样一来这个问题就有些棘手了。
如果能够修复MRP的问题，看似这个问题就引刃而解，如果修复不了，可能这个dataguard就不可用了，可能得考虑重建一个物理备库了。
对此我们采取保守态度，带着一丝尝试看看备库能不能启动到open read only状态。
但是这三个操作的结果让我有些迷茫了。
open不了，说可能需要恢复，恢复的文件竟然是system01.dbf，尝试recover until cancel也未果。
idle> alter database open read only;
alter database open read only
*
ERROR at line 1:
ORA-10458: standby database requires recovery
ORA-01196: file 1 is inconsistent due to a failed media recovery session
ORA-01110: data file 1: '/u02/dg11g/oradata/DG11G/system01.dbf'

idle> recover database until cancel;
ORA-00283: recovery session canceled due to errors
ORA-01610: recovery using the BACKUP CONTROLFILE option must be done

idle> alter database open read only;
alter database open read only
*
ERROR at line 1:
ORA-10458: standby database requires recovery
ORA-01196: file 1 is inconsistent due to a failed media recovery session
ORA-01110: data file 1: '/u02/dg11g/oradata/DG11G/system01.dbf'

对于这个问题，如果有一个sql语句能够一针见血的解决问题就好了，自己在反复尝试之后发现还是有的，问题的解决思路就是先解决ORA-01157问题，然后dataguard中的MRP问题就能引刃而解。
对于ora-01157这个问题中的数据文件在主库中不存在，但是在备库的数据字典中存在，我们可以直接在备库中把数据字典中的问题先解决了。
idle>alter database datafile '/u02/dg11g/oradata/DG11G/test_new01.dbf' offline drop;
Database altered.
然后dataguard的日志中就出现而来转机，在后台会去校验这个文件的问题，只是抛出了一个警告。Warning: Datafile 7 (/u02/ora11g/oradata/TEST11G/test_new01.dbf) is offline during full database recovery and will not be recovered
然后MRP就正常启动了。后台开始使用归档文件做数据恢复了。

alter database datafile '/u02/dg11g/oradata/DG11G/test_new01.dbf' offline drop
Completed: alter database datafile '/u02/dg11g/oradata/DG11G/test_new01.dbf' offline drop
Sat Jun 27 23:24:08 2015
ALTER DATABASE RECOVER  managed standby database disconnect from session
Attempt to start background Managed Standby Recovery process (DG11G)
Sat Jun 27 23:24:08 2015
MRP0 started with pid=25, OS id=8431
MRP0: Background Managed Standby Recovery process started (DG11G)
 started logmerger process
Sat Jun 27 23:24:13 2015
Managed Standby Recovery not using Real Time Apply
Parallel Media Recovery started with 2 slaves
Warning: Datafile 7 (/u02/ora11g/oradata/TEST11G/test_new01.dbf) is offline during full database recovery and will not be recovered
Waiting for all non-current ORLs to be archived...
All non-current ORLs have been archived.
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_121_880742847.dbf
Completed: ALTER DATABASE RECOVER  managed standby database disconnect from session
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_122_880742847.dbf
Sat Jun 27 23:24:31 2015
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_123_880742847.dbf
Recovery deleting file #7:'/u02/dg11g/oradata/DG11G/test_new01.dbf' from controlfile.
Deleted file /u02/dg11g/oradata/DG11G/test_new01.dbf
Recovery dropped tablespace 'TEST_NEW'
Recovery created file /u02/dg11g/oradata/DG11G/test_new01.dbf
Successfully added datafile 7 to media recovery
Datafile #7: '/u02/dg11g/oradata/DG11G/test_new01.dbf'
Recovery deleting file #7:'/u02/dg11g/oradata/DG11G/test_new01.dbf' from controlfile.
Deleted file /u02/dg11g/oradata/DG11G/test_new01.dbf
Recovery dropped tablespace 'TEST_NEW'
Recovery deleting file #7:'/u02/dg11g/oradata/DG11G/test_new01.dbf' from controlfile.
Deleted file /u02/dg11g/oradata/DG11G/test_new01.dbf
Recovery dropped tablespace 'TEST_NEW'
Recovery deleting file #7:'/u02/dg11g/oradata/DG11G/test_new01.dbf' from controlfile.
Deleted file /u02/dg11g/oradata/DG11G/test_new01.dbf
Recovery dropped tablespace 'TEST_NEW'
Media Recovery Log /u02/dg11g/switchover/DG11G/archivelog/1_124_880742847.dbf
Media Recovery Log /u02/dg11g/switchover/DG11G/archivelog/1_125_880742847.dbf
Recovery deleting file #7:'/u02/dg11g/oradata/DG11G/test_new01.dbf' from controlfile.
Deleted file /u02/dg11g/oradata/DG11G/test_new01.dbf
Recovery dropped tablespace 'TEST_NEW'
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_126_880742847.dbf
Sat Jun 27 23:24:49 2015
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_127_880742847.dbf
Sat Jun 27 23:25:01 2015
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_128_880742847.dbf
Sat Jun 27 23:25:17 2015
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_129_880742847.dbf
Sat Jun 27 23:25:29 2015

比较有意思的是查看日志可以看到，数据文件被反复创建删除了很多次。最后还是以drop终止。
然后就开始使用一大堆的归档文件做数据恢复了。

Sat Jun 27 23:28:30 2015
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_172_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_173_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_174_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_175_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_176_880742847.dbf
Sat Jun 27 23:28:40 2015
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_177_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_178_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_179_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_180_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_181_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_182_880742847.dbf
Sat Jun 27 23:28:52 2015
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_183_880742847.dbf
Media Recovery Log /u02/dg11g/flash_recovery_area/DG11G/archivelog/1_184_880742847.dbf

在主库中查看，redo的序列号185,备库中的序列号是184。
sys@TEST11G> select sequence#,status from v$log;
 SEQUENCE# STATUS
---------- ----------------
       184 INACTIVE
       185 CURRENT
       183 INACTIVE