这篇文章重点讨论Redo丢失的几种情况，及每种情况的处理方法。
一.说明：
1.以下所说的当前日志指日志状态为CURRENT,ACTIVE,非当前日志指日志状态为INACTIVE
2.不用考虑归档和非归档模式，2种模式下的Redo丢失情况一样。
二.丢失Redo的4种情况：
第一种情况：非当前日志，正常关闭。
第二种情况：非当前日志，非正常关闭。
第三种情况：当前日志，正常关闭。
第四种情况：当前日志，非正常关闭。
三.处理方法：
第一、二种情况的处理方法一样，直接把日志文件clear即可。
SQL> alter database clear logfile group 3;
SQL> alter database clear unarchived logfile group 3;//如果INACTIVE状态的在线Redo还未归档，增加关键字unarchived完成clear操作。（ACTIVE,INACTIVE都有可能未完成归档，归档是否完成可以查看v$log.archived字段）。
例子：
SQL> startup mount
ORACLE 例程已经启动。
Total System Global Area 263639040 bytes
Fixed Size 1384012 bytes
Variable Size 167772596 bytes
Database Buffers 88080384 bytes
Redo Buffers 6402048 bytes
数据库装载完毕。
SQL> select group#,thread#,status,archived from v$log;
GROUP# THREAD# STATUS ARCHIV
---------- ---------- -------------------------------- ------
1 1 CURRENT NO
3 1 ACTIVE NO
2 1 INACTIVE YES
SQL> alter database clear logfile group 3;
alter database clear logfile group 3
*
第 1 行出现错误:
ORA-01624: 日志 3 是紧急恢复实例 orcl (线程 1) 所必需的
ORA-00312: 联机日志 3 线程 1: 'E:\APP\ORADATA\ORCL\REDO03.LOG'
SQL> alter database clear logfile group 2;
数据库已更改。
第三种情况的处理办法：
SQL>startup mount;
SQL>recover database until cancel;
SQL>alter database open resetlogs;
例子1：
例子2（第三种情况的第二个处理方法）：
SQL> shutdown immediate
数据库已经关闭。
已经卸载数据库。
ORACLE 例程已经关闭。
SQL> startup mount
ORACLE 例程已经启动。
Total System Global Area 263639040 bytes
Fixed Size 1384012 bytes
Variable Size 167772596 bytes
Database Buffers 88080384 bytes
Redo Buffers 6402048 bytes
数据库装载完毕。
SQL> select group#,thread#,status,archived from v$log;
GROUP# THREAD# STATUS ARCHIV
---------- ---------- -------------------------------- ------
1 1 CURRENT NO
3 1 INACTIVE YES
2 1 INACTIVE YES
SQL> alter database clear logfile group 2;
数据库已更改。
SQL> alter database clear logfile group 3;
数据库已更改。
SQL> alter database clear unarchived logfile group 1;
数据库已更改。
这里CURRENT的Redo日志文件组能被clear unarchived。
SQL> alter database open;
数据库已更改。
如果Redo日志文件丢失，clear操作完成之后将在原有位置创建新的Redo日志文件。
第四种情况的处理方法：
1.通过备份来还原、恢复数据。
2.通过修改参数文件中的参数
_allow_resetlogs_corruption=TRUE
来强制启动数据库。<<<< 虽然能够启动数据库到open状态，但是启动后的数据库数据字典、数据有可能导致不一致的情况出现，故需要在open下把整个数据库export，然后删除库，重建，再将export的数据import到新的数据库中。
四.验证数据库是否正常关闭的方法
SQL> select open_mode from v$database;
OPEN_MODE
--------------------
READ WRITE
SQL> select status from v$instance;
STATUS
------------
OPEN
SQL> select file#,checkpoint_change#,fuzzy from v$datafile_header;
FILE# CHECKPOINT_CHANGE# FUZ
---------- ------------------ ---
1 1165820 YES
2 1165820 YES
3 1165820 YES
4 1165820 YES
FUZZY bit in datafile header means that there may have been writes into a datafile after the last checkpoint. E.g. there may be changes written to datafile with higher SCN than checkpoint_change# stored in datafile header (seen from v$datafile_header.checkpoint_change#).
FUZYY表示模糊性，意思是，该数据文件处于模糊状态，在最近一次CHECKPOINT后，该文件上的数据可能被修改过了，但没来得及更新到该文件上（或者该文件不知道），需要读取日志信息来判断。
SQL> select file#,checkpoint_change#,last_change# from v$datafile;
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 1165820
2 1165820
3 1165820
4 1165820
由于数据库是打开的状态，所以终止SCN是空，SCN的内容可参考文章：http://space.itpub.net/23135684/viewspace-627343
SQL> shutdown immediate
数据库已经关闭。
已经卸载数据库。
ORACLE 例程已经关闭。
SQL> startup mount
ORACLE 例程已经启动。
Total System Global Area 313860096 bytes
Fixed Size 1384352 bytes
Variable Size 155189344 bytes
Database Buffers 150994944 bytes
Redo Buffers 6291456 bytes
数据库装载完毕。
SQL> select file#,checkpoint_change#,fuzzy from v$datafile_header;
FILE# CHECKPOINT_CHANGE# FUZ
---------- ------------------ ---
1 1166324 NO
2 1166324 NO
3 1166324 NO
4 1166324 NO
在正常管理数据库的情况下，FUZZY字段都应该是NO，表示没有模糊不清的SCN存储在数据文件中。
SQL> select file#,checkpoint_change#,last_change# from v$datafile;
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 1166324 1166324
2 1166324 1166324
3 1166324 1166324
4 1166324 1166324
正常关闭数据库的终止SCN应该和启动SCN相同。FUZZY等于NO，且数据库的终止SCN等于启动SCN等于数据文件SCN，那么可以认为数据库是正常关闭，且在打开数据库之前不需要执行实例恢复或Crash恢复。
SQL> alter database open;
数据库已更改。
SQL> shutdown abort
ORACLE 例程已经关闭。
SQL> startup mount
ORACLE 例程已经启动。
Total System Global Area 313860096 bytes
Fixed Size 1384352 bytes
Variable Size 155189344 bytes
Database Buffers 150994944 bytes
Redo Buffers 6291456 bytes
数据库装载完毕。
SQL> select file#,checkpoint_change#,fuzzy from v$datafile_header;
FILE# CHECKPOINT_CHANGE# FUZ
---------- ------------------ ---
1 1166327 YES
2 1166327 YES
3 1166327 YES
4 1166327 YES
非正常关闭数据库实例，FUZZY字段的值是YES。
SQL> select file#,checkpoint_change#,last_change# from v$datafile;
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 1166327
2 1166327
3 1166327
4 1166327
非正常关闭数据库实例，终止SCN依然为空。那么，在数据库被打开之前必须使用归档Redo日志完成实例恢复或Crash恢复。
第四种的测试案例：
[oracle@prod dbs]$
[oracle@prod dbs]$
[oracle@prod dbs]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.4.0 Production on Mon Oct 22 14:01:31 2018
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES BLOCKSIZE MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM NEXT_CHANGE# NEXT_TIME
---------- ---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- --------- ------------ ---------
1 1 1 52428800 512 1 NO CURRENT 986240 22-OCT-18 2.8147E+14
2 1 0 52428800 512 1 YES UNUSED 0 0
3 1 0 52428800 512 1 YES UNUSED 0 0
SQL> select * from v$logfile ;
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ----------------------------------------- -------------------------
3 ONLINE /data/datafile/prod3/redo03.log NO
2 ONLINE /data/datafile/prod3/redo02.log NO
1 ONLINE /data/datafile/prod3/redo01.log NO
SQL>
SQL>
SQL> exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@prod dbs]$ rm /data/datafile/prod3/redo01.log
[oracle@prod dbs]$ rm /data/datafile/prod3/redo02.log
[oracle@prod dbs]$ rm /data/datafile/prod3/redo03.log
[oracle@prod dbs]$
[oracle@prod dbs]$
[oracle@prod dbs]$
[oracle@prod dbs]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.4.0 Production on Mon Oct 22 14:02:17 2018
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> shutdown abort
ORACLE instance shut down.
SQL>
SQL>
SQL> exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@prod dbs]$
[oracle@prod dbs]$
[oracle@prod dbs]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.4.0 Production on Mon Oct 22 14:02:29 2018
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to an idle instance.
SQL> startup
ORACLE instance started.
Total System Global Area 1185853440 bytes
Fixed Size 2252664 bytes
Variable Size 754974856 bytes
Database Buffers 419430400 bytes
Redo Buffers 9195520 bytes
Database mounted.
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: '/data/datafile/prod3/redo01.log'
ORA-27037: unable to obtain file status
Linux-x86_64 Error: 2: No such file or directory
Additional information: 3
SQL>
SQL> create pfile from spfile ;
File created.
SQL> exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options[oracle@prod dbs]$
[oracle@prod dbs]$
[oracle@prod dbs]$
[oracle@prod dbs]$ echo "*._allow_resetlogs_corruption=TRUE">> initprod3.ora
[oracle@prod dbs]$ cat initprod3.ora
prod3.__db_cache_size=419430400
prod3.__java_pool_size=16777216
prod3.__large_pool_size=83886080
prod3.__oracle_base='/u01/app/oracle'#ORACLE_BASE set from environment
prod3.__pga_aggregate_target=486539264
prod3.__sga_target=704643072
prod3.__shared_io_pool_size=0
prod3.__shared_pool_size=167772160
prod3.__streams_pool_size=0
*.audit_file_dest='/u01/app/oracle/admin/prod3/adump'
*.audit_trail='db'
*.compatible='11.2.0.4.0'
*.control_files='/data/datafile/prod3/control01.ctl','/data/datafile/prod3/control02.ctl'
*.db_block_size=8192
*.db_domain=''
*.db_name='prod3'
*.diagnostic_dest='/u01/app/oracle'
*.dispatchers='(PROTOCOL=TCP) (SERVICE=prod3XDB)'
*.local_listener='LISTENER_PROD3'
*.memory_target=1189085184
*.open_cursors=300
*.processes=150
*.remote_login_passwordfile='EXCLUSIVE'
*.undo_tablespace='UNDOTBS1'
*._allow_resetlogs_corruption=TRUE
[oracle@prod dbs]$
[oracle@prod dbs]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.4.0 Production on Mon Oct 22 14:04:20 2018
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> startup nomount pfile='/u01/app/oracle/product/11.2.0/db_1/dbs/initprod3.ora';
ORA-01081: cannot start already-running ORACLE - shut it down first
SQL> shutdown immediate
ORA-01109: database not open
Database dismounted.
ORACLE instance shut down.
SQL> startup nomount pfile='/u01/app/oracle/product/11.2.0/db_1/dbs/initprod3.ora';
ORACLE instance started.
Total System Global Area 1185853440 bytes
Fixed Size 2252664 bytes
Variable Size 754974856 bytes
Database Buffers 419430400 bytes
Redo Buffers 9195520 bytes
SQL> alter database mount ;
Database altered.
SQL> alter database open ;
alter database open
*
ERROR at line 1:
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: '/data/datafile/prod3/redo01.log'
ORA-27037: unable to obtain file status
Linux-x86_64 Error: 2: No such file or directory
Additional information: 3
SQL> recover database until cancel ;
ORA-00279: change 986243 generated at 10/22/2018 13:59:43 needed for thread 1
ORA-00289: suggestion : /u01/app/oracle/product/11.2.0/db_1/dbs/arch1_1_990194382.dbf
ORA-00280: change 986243 for thread 1 is in sequence #1
Specify log: {<RET>=suggested | filename | AUTO | CANCEL}
auto
ORA-00308: cannot open archived log '/u01/app/oracle/product/11.2.0/db_1/dbs/arch1_1_990194382.dbf'
ORA-27037: unable to obtain file status
Linux-x86_64 Error: 2: No such file or directory
Additional information: 3
ORA-00308: cannot open archived log '/u01/app/oracle/product/11.2.0/db_1/dbs/arch1_1_990194382.dbf'
ORA-27037: unable to obtain file status
Linux-x86_64 Error: 2: No such file or directory
Additional information: 3
ORA-01547: warning: RECOVER succeeded but OPEN RESETLOGS would get error below
ORA-01194: file 1 needs more recovery to be consistent
ORA-01110: data file 1: '/data/datafile/prod3/system01.dbf'
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.
SQL> select file#,checkpoint_change#,fuzzy from v$datafile_header;
FILE# CHECKPOINT_CHANGE# FUZ
---------- ------------------ ---
1 986247 YES
2 986247 YES
3 986247 YES
4 986247 YES
SQL> select file#,checkpoint_change#,last_change# from v$datafile;
FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 986247
2 986247
3 986247
4 986247
SQL
五.结论：
非正常关闭的当前日志丢失，可能导致数据库启动后的混乱，并可能造成少量数据的丢失。其他情况不会导致数据的丢失。