Oracle的redo log非常重要，redo log损坏将导致数据库开法开启或数据丢失，针对redo log在各种场景下如何打开或恢复数据库，特别模拟测试说明：
各场景包括如下(共6个场景):
场景一.非归档下inactive状态的redo 恢复
场景二.非归档下active状态的redo 恢复
场景三.非归档下current状态的redo恢复
场景四.归档模式下inactive状态的redo 恢复
场景五.归档模式下的active状态的redo 恢复
场景六.归档模式下的current状态的redo恢复
oracle版本为oracle 11.1.0.7
各场景恢复操作如下：
一.非归档下inactive状态的redo 恢复
session 1:
查看归档模式：---非归档
SQL> archive log list
Database log mode No Archive Mode
Automatic archival Disabled
Archive destination /kttest1-1_data1/emsdev/arch
Oldest online log sequence 3
Current log sequence 8
查看log情况：
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM
---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- ---------
1 1 10 104857600 1 NO INACTIVE 1.2790E+13 30-MAY-14
2 1 13 104857600 1 NO CURRENT 1.2790E+13 30-MAY-14
3 1 12 104857600 1 NO ACTIVE 1.2790E+13 30-MAY-14
SQL> col member for a70
SQL> select * from v$logfile;
GROUP# STATUS TYPE MEMBER
---------- ------- ------- ----------------------------------------------------------------------
IS_
---
3 ONLINE /kttest1-1_data1/emsdev/redo03.dbf
NO
1 ONLINE /kttest1-1_data1/emsdev/redo01.dbf
NO
2 ONLINE /kttest1-1_data1/emsdev/redo02.dbf
NO
SQL> alter system checkpoint;
System altered.
SQL> alter system checkpoint;
System altered.
session 2:
root@kttest1-1 # dd if=/dev/null of=/kttest1-1_data1/emsdev/redo02.dbf bs=512 count=10
0+0 records in
0+0 records out
session 1:
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS
---------- ---------- ---------- ---------- ---------- --- ----------------
FIRST_CHANGE# FIRST_TIM
------------- ---------
1 1 23 104857600 1 NO INACTIVE
1.2792E+13 03-JUN-14
2 1 22 104857600 1 NO INACTIVE
1.2792E+13 02-JUN-14
3 1 24 104857600 1 NO CURRENT
1.2792E+13 03-JUN-14
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 3207790592 bytes
Fixed Size 2119072 bytes
Variable Size 381586016 bytes
Database Buffers 2818572288 bytes
Redo Buffers 5513216 bytes
Database mounted.
ORA-00320: cannot read file header from log 2 of thread 1
ORA-00312: online log 2 thread 1: '/kttest1-1_data1/emsdev/redo02.dbf'
SQL> alter database clear logfile group 2;
Database altered.
SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-00320: cannot read file header from log 2 of thread 1
ORA-00312: online log 2 thread 1: '/kttest1-1_data1/emsdev/redo02.dbf'
SQL> alter database drop logfile group 2;
Database altered.
SQL> alter database open;
Database altered.
SQL> alter database add logfile group 2 ('/kttest1-1_data1/emsdev/redo02.dbf') size 100m;
alter database add logfile group 2 ('/kttest1-1_data1/emsdev/redo02.dbf') size 300m
*
ERROR at line 1:
ORA-00301: error in adding log file '/kttest1-1_data1/emsdev/redo02.dbf' - file
cannot be created
ORA-27038: created file already exists
Additional information: 1
SQL> alter database add logfile group 2 ('/kttest1-1_data1/emsdev/redo02.dbf') size 100m reuse;
Database altered.
二.非归档下active状态的redo 恢复
如果非current redo 损坏，但是包含active事务，那么情况完全不同。
----Session 1
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS
---------- ---------- ---------- ---------- ---------- --- ----------------
FIRST_CHANGE# FIRST_TIM
------------- ---------
1 1 26 104857600 1 NO CURRENT
1.2792E+13 04-JUN-14
2 1 25 314572800 1 NO ACTIVE
1.2792E+13 04-JUN-14
3 1 24 104857600 1 NO INACTIVE
1.2792E+13 03-JUN-14
SQL> col member for a70
SQL> select * from v$logfile;