Oracle备份恢复中，redo的恢复相对来说还是比较简单的，只要保证每组的成员不止一个，出现问题的几率就相当小，即使出了问题我们也可按照不同的方法将他们恢复，所以如果碰到日志文件损坏，完全不必紧张！按照下面的方法来做，基本上都能搞定！
试验一：用命令清空日志组方法
1、 查看原来表中数据
SQL> conn test/test
Connected.
SQL> select * from test;
TEL
----------
1
2
3
2、插入新数据
SQL> insert into test values(4);
1 row created.
SQL> commit;
Commit complete.
SQL>
3、 正常关闭数据库
4、 利用os command删除所有redo文件
5、 启动数据库
SQL> startup
ORACLE instance started.
Total System Global Area 353862792 bytes
Fixed Size 730248 bytes
Variable Size 285212672 bytes
Database Buffers 67108864 bytes
Redo Buffers 811008 bytes
Database mounted.
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: '/T3/ORACLE/oradata/ORA9/redo01.log'
6、 查看当前日志状态
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS
---------- ---------- ---------- ---------- ---------- --- ----------------
FIRST_CHANGE# FIRST_TIME
------------- ----------
1 1 2 104857600 1 YES INACTIVE
487837 01-9月 -05
2 1 4 104857600 1 NO CURRENT
487955 01-9月 -05
3 1 3 104857600 1 YES INACTIVE
487839 01-9月 -05
看来redo01.log不是当前日志，对于这类非当前日志可以直接clear,系统会重新自动生成一个redo文件
7、SQL> alter database clear logfile group 1;
Database altered.
7、 继续启动db
SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-00313: open failed for members of log group 2 of thread 1
ORA-00312: online log 2 thread 1: '/T3/ORACLE/oradata/ORA9/redo02.log'
8、 看来redo也得恢复，但是redo02是当前redo,直接clear是不行的
SQL> alter database clear logfile group 2;
alter database clear logfile group 2
*
ERROR at line 1:
ORA-00350: log 2 of thread 1 needs to be archived
ORA-00312: online log 2 thread 1: '/T3/ORACLE/oradata/ORA9/redo02.log'
9、 按照oracle的某些做法也是不行的
SQL> alter database clear unarchived logfile group 2 unrecoverable datafile;
alter database clear unarchived logfile group 2 unrecoverable datafile
*
ERROR at line 1:
ORA-00313: open failed for members of log group 2 of thread 1
ORA-00312: online log 2 thread 1: '/T3/ORACLE/oradata/ORA9/redo02.log'
ORA-27037: unable to obtain file status
SVR4 Error: 2: No such file or directory
Additional information: 3
10、 尝试从其他冷被分cp过来一个，再作clear,还是不行
SQL> host cp /T3/ORACLE/oradata2/ORA9/redo02.log /T3/ORACLE/oradata/ORA9/
SQL> alter database clear unarchived logfile group 2 unrecoverable datafile;
alter database clear unarchived logfile group 2 unrecoverable datafile
*
ERROR at line 1:
ORA-00322: log 2 of thread 1 is not current copy
ORA-00312: online log 2 thread 1: '/T3/ORACLE/oradata/ORA9/redo02.log'
SQL>
11、 但是对于非当前日志就都可以，下面看看redo03
SQL> alter database clear logfile group 3;
Database altered.
结论：
如果数据库是正常shutdown,非当前日志都可以直接clear来重新生成，而且不丢失数据，因为正常关闭db,数据已经写入dbf文件了。唯独当前日志不可以，当前日志必须用其他方法恢复，不管是不是正常关闭，
方法二：用cancel模式恢复数据库
前面的出错提示，步骤都一样，唯独恢复的方法不一样
SQL> startup
ORACLE instance started.
Total System Global Area 353862792 bytes
Fixed Size 730248 bytes
Variable Size 285212672 bytes
Database Buffers 67108864 bytes
Redo Buffers 811008 bytes
Database mounted.
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: '/T3/ORACLE/oradata/ORA9/redo01.log'
看看丢失了哪些redo
SQL> host ls /T3/ORACLE/oradarta/ORA9/redo*
/T3/ORACLE/oradarta/ORA9/redo*: No such file or directory
看来redo都丢了
直接recover
SQL> recover database until cancel;
Media recovery complete.
这个时候redo还没有生成
SQL> host ls /T3/ORACLE/oradata/ORA9/redo*
/T3/ORACLE/oradata/ORA9/redo*: No such file or directory
启动数据库
SQL> alter database open ;
alter database open
*
ERROR at line 1:
ORA-01589: must use RESETLOGS or NORESETLOGS option for database open
SQL> alter database open resetlogs;
Database altered.
（注意，这里必须用resetlogs,否则会错误的
SQL> alter database open noresetlogs;
alter database open noresetlogs
*
ERROR at line 1:
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: '/T3/ORACLE/oradata/ORA9/redo01.log'
SQL>
Resetlogs其实就是根据控制文件让系统自动重新生成redo,如果noresetlog的话，就不会重新生成redo,缺少了文件，db自然无法启动）
SQL> host ls /T3/ORACLE/oradata/ORA9/redo*
/T3/ORACLE/oradata/ORA9/redo01.log /T3/ORACLE/oradata/ORA9/redo02.log /T3/ORACLE/oradata/ORA9/redo03.log
SQL>
检验
SQL> select * from test.test;
TEL
----------
1
2
3
4
SQL>
数据一点儿都没有丢失
结论：
如果数据库是正常关闭的，用recover database until cancel可以轻松恢复或者说重新建立所有的redo，不再区分是否是当前日志，而且由于正常关闭，不会丢失任何数据，唯一可能丢失的情况就是如果日志还没有归档
这种恢复方法由于要resetlogs,所以在恢复完成后，日志清零，以前的备份不再起作用，所以建议立即备份
SQL> archive log list;
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /T3/ORACLE/arch
Oldest online log sequence 0
Next log sequence to archive 1
Current log sequence 1
SQL>
实验三：通过重新生成控制文件来恢复redo
前面的都一样，只是处理方法不一样
SQL> startup
ORACLE instance started.
Total System Global Area 353862792 bytes
Fixed Size 730248 bytes
Variable Size 285212672 bytes
Database Buffers 67108864 bytes
Redo Buffers 811008 bytes
Database mounted.
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: '/T3/ORACLE/oradata/ORA9/redo01.log'
SQL> alter database backup controlfile to trace;
Database altered.
SQL> shutdown immediate
ORA-01109: database not open
Database dismounted.
ORACLE instance shut down.
SQL>
2、 修改一下刚才生成的那个文件
CREATE CONTROLFILE REUSE DATABASE "ORA9" RESETLOGS ARCHIVELOG
-- SET STANDBY TO MAXIMIZE PERFORMANCE
MAXLOGFILES 50
MAXLOGMEMBERS 5
MAXDATAFILES 100
MAXINSTANCES 1
MAXLOGHISTORY 226
LOGFILE
GROUP 1 '/T3/ORACLE/oradata/ORA9/redo01.log' SIZE 100M,
GROUP 2 '/T3/ORACLE/oradata/ORA9/redo02.log' SIZE 100M,
GROUP 3 '/T3/ORACLE/oradata/ORA9/redo03.log' SIZE 100M
-- STANDBY LOGFILE
DATAFILE
'/T3/ORACLE/oradata/ORA9/system01.dbf',
'/T3/ORACLE/oradata/ORA9/undotbs01.dbf',
'/T3/ORACLE/oradata/ORA9/cwmlite01.dbf',
'/T3/ORACLE/oradata/ORA9/drsys01.dbf',
'/T3/ORACLE/oradata/ORA9/example01.dbf',
'/T3/ORACLE/oradata/ORA9/indx01.dbf',
'/T3/ORACLE/oradata/ORA9/odm01.dbf',
'/T3/ORACLE/oradata/ORA9/tools01.dbf',
'/T3/ORACLE/oradata/ORA9/users01.dbf',
'/T3/ORACLE/oradata/ORA9/xdb01.dbf',
'/T3/ORACLE/oradata/ORA9/test01.dbf'
CHARACTER SET ZHS16GBK
;
另存为一个脚本，运行他
SQL> @clone.sql
Control file created.
SQL> alter database open resetlogs;
Database altered.
SQL>
搞定……………
结论：这种方法的关键是重新创建控制文件，后面的步骤和前面的道理一样的
前面的三种方法都是假设db是正常关闭的，数据已经写入数据库文件中，所以不会由数据存在redo种，所以clear的话也不会有数据丢失