天遇到一个数据库无法启动，看到alertlog中主要是ora 600和[2662]的报错：
SQL> startupORACLE instance started.Total System Global Area 1076850392 bytesFixed Size 736984 bytesVariable Size 536870912 bytesDatabase Buffers 536870912 bytesRedo Buffers 2371584 bytesDatabase mounted.ORA-01092: ORACLE instance terminated. Disconnection forced
SQL>
其中alertlog中报错：
Completed: ALTER DATABASE MOUNT
Thu Jan 22 13:05:08 2009
ALTER DATABASE OPEN
Thu Jan 22 13:05:09 2009
Beginning crash recovery of 1 threads
Thu Jan 22 13:05:09 2009
Started first pass scan
Thu Jan 22 13:05:09 2009
Completed first pass scan
0 redo blocks read, 0 data blocks need recovery
Thu Jan 22 13:05:09 2009
Started recovery at
Thread 1: logseq 2, block 3, scn 0.43536037
Recovery of Online Redo Log: Thread 1 Group 1 Seq 2 Reading mem 0
Mem# 0 errs 0: /oracle/oradata/ora9i/redo01.log
Thu Jan 22 13:05:09 2009
Ended recovery at
Thread 1: logseq 2, block 3, scn 0.43556038
0 data blocks read, 0 data blocks written, 0 redo blocks read
Crash recovery completed successfully
Thu Jan 22 13:05:10 2009
Thread 1 advanced to log sequence 3
Thread 1 opened at log sequence 3
Current log# 2 seq# 3 mem# 0: /oracle/oradata/ora9i/redo02.log
Successful open of redo thread 1.
Thu Jan 22 13:05:10 2009
SMON: enabling cache recovery
Thu Jan 22 13:05:10 2009
Errors in file /oracle/admin/ora9i/udump/ora9i_ora_12968.trc:
ORA-00600: internal error code, arguments: [2662], [0], [43556042], [261], [2396789971], [4194729], [], []
遇到ora-600和2662的问题，我们一般有2种方法解决：
一种是在open的状态下：用alter session set events 'IMMEDIATE trace name adjust_scn level n';
一种是在mount状态下：用alter session set events '10015 trace name adjust_scn level n';
其中n的运算如下：
根据alertlog中的报错：
ORA-00600: internal error code, arguments: [2662], [0], [43556042], [261], [2396789971], [4194729], [], []
这边，我们把2662后的参数[2662],[a],,[c],[d],[e]…
[a] Current SCN WRAP
Current SCN BASE
[c] dependent SCN WRAP
[d] dependent SCN BASE
[e] Where present this is the DBA where the dependent SCN came from.
其中scn可以用十六进制表示0Xffff.ffffffff。为了方便，oracle把前面的4个字节表示scn wrap，后面的8个字节表示scn base。scn最低值是0X0000.00000000，最高值是0Xffff.ffffffff。高位是scn wrap，低位是scn base。根据报错，我们需要把scn增进到dependent SCN WRAP为261。
而我们增进的level n，n是表示1g（即1024×1024×1024），也就是说，调整是以g为单位进行的。
而高位的scn wrap的一个1，即0X0001.00000000=0X000100000000（去掉便于分隔高低位的点）=100000000000000000000000000000000=2^32（即2乘以10的32次方）=4×2^30（4乘以2的30次方）=4×（1024×1024×1024）=4g。因此我们要增加到的scn，根据level n，n表示g，调整的level为4×261。即1044，再比这个数字大一些，我们可以设置成1045，1047都可以。
尝试用上述的方法去解决。由于是mount状态，因此只能用10015 trace name的adjust scn：
其中的隐含参数：
cat initora9i.ora
……
*.user_dump_dest='/oracle/admin/ora9i/udump'
*._allow_resetlogs_corruption=TRUE
"initora9i.ora" 47 lines, 1465 characters
SQL> startup nomount pfile='?/dbs/initora9i.ora'ORACLE instance started.Total System Global Area 1076850392 bytesFixed Size 736984 bytesVariable Size 536870912 bytesDatabase Buffers 536870912 bytesRedo Buffers 2371584 bytesSQL> alter database mount;Database altered.SQL> alter session set events '10015 trace name ADJUST_SCN level 1045';Session altered.SQL> alter database open;alter database open
*ERROR at line 1:ORA-01092: ORACLE instance terminated. Disconnection forced
alertlog中：
Thu Jan 22 13:27:56 2009
SMON: enabling cache recovery
Thu Jan 22 13:27:56 2009
Errors in file /oracle/admin/ora9i/udump/ora9i_ora_13322.trc:
ORA-00600: internal error code, arguments: [2662], [0], [43576046], [261], [2396789971], [4194729], [], []
Thu Jan 22 13:28:37 2009
Errors in file /oracle/admin/ora9i/udump/ora9i_ora_13322.trc:
ORA-00704: bootstrap process failure
ORA-00600: internal error code, arguments: [2662], [0], [43576046], [261], [2396789971], [4194729], [], []
Thu Jan 22 13:28:37 2009
Error 704 happened during db open, shutting down database
USER: terminating instance due to error 704
Instance terminated by USER, pid = 13322
ORA-1092 signalled during: alter database open...
看到报错信息中的scn还是没有到达目标scn。版本是9i的，应该不会限制啊，根据在某些10g版本中需要另外一个隐含参数_allow_error_simulation，才能增进scn，继续修改初始化参数，尝试启动：
cat initora9i.ora
……
*.user_dump_dest='/oracle/admin/ora9i/udump'
*._allow_resetlogs_corruption=TRUE
*._allow_error_simulation=TRUE
"initora9i.ora" 47 lines, 1465 characters
SQL> startup mount pfile='?/dbs/initora9i.ora'ORACLE instance started.Total System Global Area 1076850392 bytesFixed Size 736984 bytesVariable Size 536870912 bytesDatabase Buffers 536870912 bytesRedo Buffers 2371584 bytesDatabase mounted.SQL> alter session set events '10015 trace name ADJUST_SCN level 20000';Session altered.SQL> alter database open;
alertlog中报错依旧：
Ended recovery at
Thread 1: logseq 5, block 3, scn 0.43616049
0 data blocks read, 0 data blocks written, 1 redo blocks read
Crash recovery completed successfully
Thu Jan 22 13:41:49 2009
Thread 1 advanced to log sequence 6
Thread 1 opened at log sequence 6
Current log# 2 seq# 6 mem# 0: /oracle/oradata/ora9i/redo02.log
Successful open of redo thread 1.
Thu Jan 22 13:41:49 2009
SMON: enabling cache recovery
Thu Jan 22 13:41:49 2009
Errors in file /oracle/admin/ora9i/udump/ora9i_ora_13660.trc:
ORA-00600: internal error code, arguments: [2662], [0], [43616053], [261], [2396789971], [4194729], [], []
看来是不能用上述的方法了，小熊这个时候再次提出了一个隐含参数：_minimum_giga_scn，把该参数设置成1047再尝试启动：
cat initora9i.ora
……
*.user_dump_dest='/oracle/admin/ora9i/udump'
#*._allow_resetlogs_corruption=TRUE
*._allow_error_simulation=TRUE
*._minimum_giga_scn=1047
"initora9i.ora" 47 lines, 1465 characters
SQL> startup mount pfile='?/dbs/initora9i.ora'ORACLE instance started.Total System Global Area 1076850392 bytesFixed Size 736984 bytesVariable Size 536870912 bytesDatabase Buffers 536870912 bytesRedo Buffers 2371584 bytesDatabase mounted.SQL> alter database open;Database altered.SQL>
数据库终于起来了！
查询了一下orafaq，这个参数是表示Minimum SCN to start with in 2^30 units ，2乘以10的三十次方，也就是1024×1024×1024，也就是g了。这个参数是oracle723就开始有了，表示最小scn的起始值1g，我们这边的scn wrap有261，因此需要4×261，再比这个稍微大一些，就得出1047了。
总结：在一般情况下，遇到ora-600，2662的报错，可以通过10015的adjust scn起来，但是遇到Current SCN WRAP和dependent SCN WRAP相距比较远，通过上述方法起不来，我们可以通过隐含参数_minimum_giga_scn直接设置最小scn，启动数据库。
