ORA-600 [2662] Block SCN is ahead of Current SCN 处理方法 说明
一. ORA-600[2662] 说明
关于ORA-600[2662]的的错误，之前的blog 有说明：
ORA-600[2662] "Block SCN is ahead of Current SCN" [ID 28929.1]
http://blog.csdn.net/tianlesoftware/article/details/6106130
关于ORA-600 的各个参数说明，参考我的blog：
http://blog.csdn.net/tianlesoftware/article/details/6645809
简单的说，就是block 里的SCN 大于current SCN 时，就会报ORA-600[2662]错误。
导致这个问题的原因有如下可能：
(1) doing an open resetlogs with_ALLOW_RESETLOGS_CORRUPTION enabled
(2) a hardware problem, like a faultycontroller, resulting in a failed write to the control file or the redo logs
(3) restoring parts of the database frombackup and not doing the appropriate recovery
(4) restoring a control file and not doinga RECOVER DATABASE USING BACKUP CONTROLFILE
(5) having _DISABLE_LOGGING set duringcrash recovery
(6) problems with the DLM in a parallelserver environment
(7) a bug
二. 解决方法
(1)if the SCNs in the error are very close, attempting a startup several timeswill bump up the dscn every time we open the database even if open fails. Thedatabase will open when dscn=scn.
(2)Youcan bump the SCN either on open or while the database is open using<Event:ADJUST_SCN> (see Note:30681.1).
Beaware that you should rebuild the database if you use this option. Once this has occurred you would normally want to rebuild thedatabase via exp/rebuild/imp as there is no guarantee that some other blocksare not ahead of time.
关于这个rebuild 的另一段描述：
Forcing the database to startup using_ALLOW_RESETLOGS_CORRUPTION:
This parameter is undocumented andunsupported. The_allow_resetlogs_corruption should only be done as a lastresort. Usually when a database is opened with either the RESETLOGS or NORESETLOGSoption, the status and checkpoint structures in all the file headers of alldata files are checked to make sure that they are consistent. Once this is checked, the redo logs are zeroedout in case of RESETLOGS. When the_ALLOW_RESETLOGS_CORRUPTION parameter is set, the file header checks arebypassed. This means that we do not make sure that the files are consistent andopen the database. It will potentially cause some lost data and lost dataintegrity.
Thedatabase should be rebuilt since data and the data dictionary could be corruptin ways that are not immediately detectable. This could lead to future outagesor put the database in a state where it cannot be salvaged at all. There is noguarantee that this will work.
当我们设置_ALLOW_RESETLOGS_CORRUPTION参数后，讲不在检查file header，这样就不能保证files的一致性，就可能造成数据的丢失。 所以这种情况下，就是对数据库进行rebuild。 即：export DB, 重新创建实例，import 数据。
2.1 方法一
ORA-600的参数格式如下：
Arg[a] Current SCN WRAP
Arg[b] Current SCN BASE
Arg[c] dependent SCNWRAP
Arg[d] dependent SCN BASE
Arg[e] Where present this is the DBA wherethe dependent SCN came from.
当dependent SCN 与 current SCN 差距不大的时候，可以多shutdown 和 startup 数据库，这样拉大current SCN，当current SCN 大于dependent SCN, 就可以正常启动DB了。
2.2 方法二： 使用adjust_scn event
如果dependent SCN 和 current SCN 差距很大的时候，通过多次重启DB 来拉大SCN 明显不现实。 这时候，就可以通过adjust_scn event 来拉大current scn。 但是使用这种方法，需要重新rebuild 一下数据库，即exp 和imp。
关于adjust_scn event 使用说明，参考我的blog。 这篇文档在MOS 上没有找到，所以转帖过来了。
http://blog.csdn.net/tianlesoftware/article/details/6645633
2.2.1 数据库没有打开时使用说明
Take a backup.
You can use event 10015 to trigger an ADJUST_SCNon database open:
startup mount;
alter session set events '10015 trace nameadjust_scn level 1';
(NB: You can only use IMMEDIATE here onanOPEN database. If the database is only mounted use the 10015 trigger to adjustSCN,otherwise you get ORA 600 [2251], [65535], [4294967295] )
alter database open;
If you get an ORA 600:2256 shutdown, use ahigher level and reopen.
Do*NOT* set this event in init.ora or the instance will crash as soon as SMON orPMON try to do any clean up.Always use it with the "alter session"command.
--不要将该参数设置到init.ora 文件里，否则当SMON 或者PMON 进程进行cleanup时，DB 就会crash 掉。
2.2.2 数据库OPEN 状态使用
You can increase the SCN thus:
alter session set events 'IMMEDIATE tracename ADJUST_SCN level 1';
LEVEL:
Level1 is usually sufficient - it raises the SCN to 1 billion(1024*1024*1024)，Level 2 raises it to 2 billionetc...
--注意level1的值，1代表的是讲SCN 增加1亿，2代表2亿
If you try to raise the SCN to a level LESSTHAN or EQUAL to its current setting you will get <OERI:2256> - Seebelow.
Ie: The event steps the SCN to knownlevels. You cannot use the same level twice.
Calculating a Level from600 errors:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To get a LEVEL for ADJUST_SCN:
a) Determine the TARGET scn:
ora-600 [2662] See <OERI:2662> Use TARGET >= blocksSCN
ora-600 [2256] See<OERI:2256> Use TARGET >=CurrentSCN
b)Multiply the TARGET wrap number by 4.This will give you the level to use in the adjust_scn to get the correct wrapnumber.
c) Next, add the following value to the levelto get the desired base value as well :
Add to Level Base
~~~~~~~~~~~~ ~~~~~~~~~~~~
0 0
1 1073741824(1024*1024*1024)，
2 2147483648(2*1024*1024*1024)，
3 3221225472(3*1024*1024*1024)，
注意：
在Oracle 9i 下面，直接使用alter session 命令就可以增加SCN。 但是在Oracle 10g下面，还需要修改隐含参数：_allow_error_simulation，将该参数设置为true，才能真正增进scn。
隐含参数的查看，可以使用如下视图：
http://blog.csdn.net/tianlesoftware/article/details/6641281
三. adjust_scn 示例
db 版本信息：
SYS@dave2(db2)> select * from v$versionwhere rownum=1;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise EditionRelease 10.2.0.1.0 - Prod
3.1 db open 状态
SYS@dave2(db2)> select current_scn fromv$database;
CURRENT_SCN
-----------
4304475
SYS@dave2(db2)> select open_mode fromv$database;
OPEN_MODE
----------
READ WRITE
SYS@dave2(db2)> alter session set events'immediate trace name adjust_scn level 1';
Session altered.
SYS@dave2(db2)> select current_scn fromv$database;
CURRENT_SCN
-----------
4304523
注意这里的SCN 并没有大量的增加，我们设置一下_allow_error_simulation。
SYS@dave2(db2)> alter system set"_allow_error_simulation"=true scope=spfile;
System altered.
SYS@dave2(db2)> startup force
ORACLE instance started.
Total System Global Area 239075328 bytes
Fixed Size 1218724 bytes
Variable Size 79693660 bytes
Database Buffers 155189248 bytes
Redo Buffers 2973696 bytes
Database mounted.
Database opened.
SYS@dave2(db2)>
再次使用adjust_scn event：
SYS@dave2(db2)> alter session set events'immediate trace name adjust_scn level 1';
Session altered.
SYS@dave2(db2)> select current_scn fromv$database;
CURRENT_SCN
-----------
1073742111
--这次SCN 成功增加了。
3.2 db not open 状态
这里_allow_error_simulation 参数已经修改过了，我们就不进行重复修改，直接将将db 启动到mount，在使用adjust_scn 增加SCN值。
SYS@dave2(db2)> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SYS@dave2(db2)> startup mount
ORACLE instance started.
Total System Global Area 239075328 bytes
Fixed Size 1218724 bytes
Variable Size 79693660 bytes
Database Buffers 155189248 bytes
Redo Buffers 2973696 bytes
Database mounted.
SYS@dave2(db2)> select current_scnfrom v$database;
CURRENT_SCN
-----------
0
--如果DB 没有open，这个命令是不好使的
SYS@dave2(db2)> altersession set events '10015 trace name adjust_scn level 2';
Session altered.
SYS@dave2(db2)> select current_scn fromv$database;
CURRENT_SCN
-----------
0
SYS@dave2(db2)> alterdatabase open;
Database altered.
SYS@dave2(db2)> select current_scn from v$database;
CURRENT_SCN
-----------
2147483746
--SCN 已经增加
--最后查看一下_allow_error_simulation参数的值：
SYS@dave2(db2)> select name,value fromall_parameters where name='_allow_error_simulation';
NAME VALUE
-------------------------------------------------------------------------------
_allow_error_simulation TRUE
最后不要忘记对db进行rebuild。
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