听不止一个人说起，学技术就要不断地折腾、搞破坏，比如说备份恢复，你就可以尝试删文件，不管是日志文件、临时文件、数据文件还是system文件。删了之后，重启数据库肯定报错，有的甚至当时数据库就挂掉，这样你就可以学着恢复，一破一立之间，很多常规的备份恢复手段也就算是领教了。我今天就尝试着把虚拟机上的一个重做日志文件组删除。
1.环境准备
我们在Oracle11g中进行测试，数据库处于非归档状态。
SQL>
SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - Production
PL/SQL Release 11.2.0.3.0 - Production
CORE 11.2.0.3.0 Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production
SQL>
SQL> archive log list;
Database log mode No Archive Mode
Automatic archival Disabled
Archive destination /u01/app/oracle/product/11.2.0/dbhome_1/dbs/arch
Oldest online log sequence 9
Current log sequence 11
SQL>
2.删除一个重做日志文件组，重启数据库报错
首先，通过查询v$log视图来获取数据库重做日志文件组的状态。
SQL> select GROUP#,MEMBERS,ARCHIVED,STATUS from v$log;
GROUP# MEMBERS ARC STATUS
---------- ---------- --- ----------------
1 1 NO INACTIVE
2 1 NO CURRENT
3 1 NO INACTIVE
SQL>
然后，通过ls命令查看数据文件，删除第一个重做日志文件组（该文件组只有一个日志成员）。
[oracle@ hoegh HOEGH]$ ls
control01.ctl redo01.log sysaux01.dbf undotbs01.dbf
control02.ctl redo02.log system01.dbf users01.dbf
example01.dbf redo03.log temp01.dbf
[oracle@hoegh HOEGH]$
[oracle@hoegh HOEGH]$
[oracle@hoegh HOEGH]$ rm redo01.log
[oracle@hoegh HOEGH]$ ls
control01.ctl control02.ctl example01.dbf redo02.log redo03.log sysaux01.dbf system01.dbf temp01.dbf undotbs01.dbf users01.dbf
重启数据库，数据库报错。
SQL>
SQL> shu immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 941600768 bytes
Fixed Size 1348860 bytes
Variable Size 515902212 bytes
Database Buffers 419430400 bytes
Redo Buffers 4919296 bytes
Database mounted.
ORA-03113: end-of-file on communication channel
Process ID: 5196
Session ID: 125 Serial number: 5
SQL>
SQL> select status from v$instance;
ERROR:
ORA-03114: not connected to ORACLE
SQL>
3.查看报警日志文件，定位问题
ORA-03113报错是一个非常经典的报错，报错原因多种多样，从报错信息中并看不出是什么原因导致的报错，我们可以到报警日志文件中查看有价值的线索。
[oracle@enmoedu1 trace]$ tail -40 alert_HOEGH.log
Wed Jul 08 21:59:30 2015
MMON started with pid=15, OS id=5443
Wed Jul 08 21:59:30 2015
MMNL started with pid=16, OS id=5445
starting up 1 dispatcher(s) for network address '(ADDRESS=(PARTIAL=YES)(PROTOCOL=TCP))'...
starting up 1 shared server(s) ...
ORACLE_BASE from environment = /u01/app/oracle
Wed Jul 08 21:59:39 2015
alter database mount
Wed Jul 08 21:59:43 2015
Successful mount of redo thread 1, with mount id 2105928075
Database mounted in Exclusive Mode
Lost write protection disabled
Completed: alter database mount
Wed Jul 08 22:11:45 2015
Time drift detected. Please check VKTM trace file for more details.
Wed Jul 08 22:11:59 2015
alter database open
Wed Jul 08 22:11:59 2015
Errors in file /u01/app/oracle/diag/rdbms/HOEGH/HOEGH/trace/HOEGH_lgwr_5435.trc:
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: '/u01/app/oracle/oradata/HOEGH/redo01.log'
ORA-27037: unable to obtain file status
Linux Error: 2: No such file or directory
Additional information: 3
Errors in file /u01/app/oracle/diag/rdbms/HOEGH/HOEGH/trace/HOEGH_lgwr_5435.trc:
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: '/u01/app/oracle/oradata/HOEGH/redo01.log'
ORA-27037: unable to obtain file status
Linux Error: 2: No such file or directory
Additional information: 3
Errors in file /u01/app/oracle/diag/rdbms/HOEGH/HOEGH/trace/HOEGH_ora_5451.trc:
ORA-00313: open failed for members of log group 1 of thread
ORA-00312: online log 1 thread 1: '/u01/app/oracle/oradata/HOEGH/redo01.log'
USER (ospid: 5451): terminating the instance due to error 313
Wed Jul 08 22:12:00 2015
System state dump requested by (instance=1, osid=5451), summary=[abnormal instance termination].
System State dumped to trace file /u01/app/oracle/diag/rdbms/HOEGH/HOEGH/trace/HOEGH_diag_5425.trc
Dumping diagnostic data in directory=[cdmp_20150708221200], requested by (instance=1, osid=5451), summary=[abnormal instance termination].
Instance terminated by USER, pid = 5451
其中，黄色标注部分为关键信息，我们知道“/u01/app/oracle/oradata/HOEGH/redo01.log”这个文件找不到了。
4.启动数据库到mount状态，重建重做日志文件组
从报警日志可以看出，第一组重做日志文件组丢了，我们可以通过sql语句“alter database clear logfile group 1;”重建日志文件组；确认日志文件创建成功后，将数据库切换到open状态。
SQL> startup nomount
ORACLE instance started.
Total System Global Area 941600768 bytes
Fixed Size 1348860 bytes
Variable Size 515902212 bytes
Database Buffers 419430400 bytes
Redo Buffers 4919296 bytes
SQL> alter database mount;
Database altered.
SQL> select group#,sequence#,archived,status from v$log;
GROUP# SEQUENCE# ARC STATUS
---------- ---------- --- ----------------
1 7 NO INACTIVE
3 6 NO INACTIVE
2 8 NO CURRENT
SQL>
SQL>
SQL>
SQL>
SQL> alter database clear logfile group 1;
Database altered.
SQL> select group#,sequence#,archived,status from v$log;
GROUP# SEQUENCE# ARC STATUS
---------- ---------- --- ----------------
1 0 NO UNUSED
3 6 NO INACTIVE
2 8 NO CURRENT
启动数据库到open状态
SQL>
SQL> alter database open;
Database altered.
SQL>
SQL> select group#,sequence#,archived,status from v$log;
GROUP# SEQUENCE# ARC STATUS
---------- ---------- --- ----------------
1 0 NO UNUSED
2 8 NO CURRENT
3 6 NO INACTIVE
此时我们再次查看文件列表，结果如下。
[oracle@hoegh HOEGH]$ ls
control01.ctl control02.ctl example01.dbf redo01.log redo02.log redo03.log sysaux01.dbf system01.dbf temp01.dbf undotbs01.dbf users01.dbf
[oracle@hoegh HOEGH]$
5.手动切换重做日志文件组
为了确保新建的日志文件组可用，我们可以手动切换日志文件组，改变新建日志文件组的状态（由UNUSED改为其他）。
SQL>
SQL> alter system switch logfile;
System altered.
SQL> select group#,sequence#,archived,status from v$log;
GROUP# SEQUENCE# ARC STATUS
---------- ---------- --- ----------------
1 9 NO CURRENT
2 8 NO ACTIVE
3 6 NO INACTIVE
SQL> alter system switch logfile;
System altered.
SQL> select group#,sequence#,archived,status from v$log;
GROUP# SEQUENCE# ARC STATUS
---------- ---------- --- ----------------
1 9 NO ACTIVE
2 8 NO ACTIVE
3 10 NO CURRENT
SQL> alter system switch logfile;
System altered.
SQL> select group#,sequence#,archived,status from v$log;
GROUP# SEQUENCE# ARC STATUS
---------- ---------- --- ----------------
1 9 NO INACTIVE
2 11 NO CURRENT
3 10 NO INACTIVE
SQL>
其中，
current:表示该日志组为当前日志组,oracle正在使用该日志组;
active:当current redo组发生日志切换时,状态会改变为active,在这个状态下,如果数据库为归档模式,archive进程会归档active日志组;如果发生数据库crash,该日志组也是实例恢复必需的日志组;
inactive:当active日志组归档完毕并且oracle判断不需要进行实例恢复时,会将其状态修改为inactive,等待下一轮的使用;所以当日志组为inactive的时候,如果数据库为归档模式.那么日志肯定是归档完成了。
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