Oracle启动后对控制文件的依赖

    今天测试的时候纠正了一个错误的印象，一直认为如果当前控制文件被误删除，会导致实例关闭。

    而实际上，Oracle并不会结束当前的实例：

 SQL> show parameter control_files

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
control_files string /u01/oracle/oradata/db01/contr
ol01.ctl, /u01/oracle/oradata/
db01/control02.ctl, /u01/oracl
e/oradata/db01/control03.ctl
SQL> select * from dual;

D
-
X

SQL> host
oracle:/home/oracle> cd /u01/oracle/oradata/db01
oracle:/u01/oracle/oradata/db01> ls -l *.ctl
-rw-r----- 1 oracle dba 9748480 May 24 12:55 control01.ctl
-rw-r----- 1 oracle dba 9748480 May 24 12:55 control02.ctl
-rw-r----- 1 oracle dba 9748480 May 24 12:55 control03.ctl
oracle:/u01/oracle/oradata/db01> rm control02.ctl
oracle:/u01/oracle/oradata/db01> exit
exit

SQL> select * from dual;

D
-
X

SQL> select instance_name, status from v$instance;

INSTANCE_NAME STATUS
---------------- ------------
db01 OPEN

SQL> alter system checkpoint;

System altered.

SQL> select instance_name, status from v$instance;

INSTANCE_NAME STATUS
---------------- ------------
db01 OPEN

SQL> alter system switch logfile;

System altered.

SQL> select instance_name, status from v$instance;

INSTANCE_NAME STATUS
---------------- ------------
db01 OPEN

SQL> col name format a50
SQL> select * from v$controlfile;

STATUS NAME IS_ BLOCK_SIZE FILE_SIZE_BLKS
------- -------------------------------------------------- --- ---------- --------------
/u01/oracle/oradata/db01/control01.ctl NO 16384 594
/u01/oracle/oradata/db01/control02.ctl NO 16384 594
/u01/oracle/oradata/db01/control03.ctl NO 16384 594

SQL> select * from v$controlfile_record_section;
select * from v$controlfile_record_section
*
ERROR at line 1:
ORA-00210: cannot open the specified control file
ORA-00202: control file: '/u01/oracle/oradata/db01/control02.ctl'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3

SQL> select * from v$controlfile;

STATUS NAME IS_ BLOCK_SIZE FILE_SIZE_BLKS
------- -------------------------------------------------- --- ---------- --------------
/u01/oracle/oradata/db01/control01.ctl NO 16384 594
/u01/oracle/oradata/db01/control02.ctl NO 16384 594
/u01/oracle/oradata/db01/control03.ctl NO 16384 594

    可以看到，删除了一个控制文件之后，数据库照常运行，甚至在发现一个控制文件丢失之后，数据库实例仍然处于打开状态，检查一下alert文件：
 Thread 1 advanced to log sequence 21 (LGWR switch)
Current log# 3 seq# 21 mem# 0: /u01/oracle/oradata/db01/redo03.log
Sun May 24 13:00:58 2009
Errors in file /u01/oracle/diag/rdbms/db01/db01/trace/db01_m002_27935.trc:
ORA-00210: cannot open the specified control file
ORA-00202: control file: '/u01/oracle/oradata/db01/control02.ctl'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
Sun May 24 13:24:09 2009
Errors in file /u01/oracle/diag/rdbms/db01/db01/trace/db01_m000_27986.trc:
ORA-00210: cannot open the specified control file
ORA-00202: control file: '/u01/oracle/oradata/db01/control02.ctl'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
Sun May 24 14:01:49 2009
Errors in file /u01/oracle/diag/rdbms/db01/db01/trace/db01_m002_28082.trc:
ORA-00210: cannot open the specified control file
ORA-00202: control file: '/u01/oracle/oradata/db01/control02.ctl'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
Sun May 24 14:24:59 2009
Errors in file /u01/oracle/diag/rdbms/db01/db01/trace/db01_m000_28132.trc:
ORA-00210: cannot open the specified control file
ORA-00202: control file: '/u01/oracle/oradata/db01/control02.ctl'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
Sun May 24 14:59:29 2009
Errors in file /u01/oracle/diag/rdbms/db01/db01/trace/db01_m000_28204.trc:
ORA-00210: cannot open the specified control file
ORA-00202: control file: '/u01/oracle/oradata/db01/control02.ctl'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3

    虽然Oracle认识到一个控制文件出现了异常，但是Oracle并没有关闭当前实例。测试发现，即使删除了所有当前的控制文件，当前实例依然处于打开的状态，并且只有需要访问控制文件的操作会报错，其他操作仍然可以运行。

    Oracle既然不会自动关闭，那么万一出现了控制文件误删除的情况，一定不要手工关闭当前的实例，只要实例不关闭，就可以轻松的查询数据库中的数据文件、日志文件等信息来重建控制文件，而一旦关闭了实例，由于缺失控制文件，实例将无法启动，这时候要找到所有控制文件中包含的信息就比较困难了。