1、模拟故障：

[oracle@dg1 dbs]$ cp -r initdg1.ora /oracle/initdg1.ora
[oracle@dg1 dbs]$ mv spfiledg1.ora spfiledg1.orabak
[oracle@dg1 dbs]$ vi initdg1.ora
修改“*.undo_tablespace='UNDOTBS1'” 为“*.undo_tablespace='UNDOTBS2'”

2、启动数据库：

SQL> startup
ORACLE instance started.

Total System Global Area  511705088 bytes
Fixed Size                  1220288 bytes
Variable Size             163578176 bytes
Database Buffers          343932928 bytes
Redo Buffers                2973696 bytes
Database mounted.
ORA-01092: ORACLE instance terminated. Disconnection forced

3、查看ALERT日志：

Errors in file /oracle/admin/dg1/udump/dg1_ora_3488.trc:
ORA-30012: undo tablespace 'UNDOTBS2' does not exist or of wrong type
Mon Sep 15 21:26:13 2008
Error 30012 happened during db open, shutting down database
USER: terminating instance due to error 30012
Instance terminated by USER, pid = 3488
ORA-1092 signalled during: ALTER DATABASE OPEN...

4、解决办法：
[oracle@dg1 bdump]$  oerr ORA 30012
30012, 00000, "undo tablespace '%s' does not exist or of wrong type"
// *Cause:   the specified undo tablespace does not exist or of the
//           wrong type.
// *Action:  Correct the tablespace name and reissue the statement.

5、分析原因：

SQL> select ts#,name from v$datafile;

         0
/oracle/oradata/dg1/system01.dbf

         1
/oracle/oradata/dg1/undotbs01.dbf

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS2


6、解决办法：

将PFILE文件中的表空间修改成UNDOTBS1，就解决问题。