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在Unix/Linux上删除所有的Controlfile控制文件后实例并不会在第一时间crash，相反居然还可以顺利完成FULL checkpoint 。
这是为什么呢？ ---注意这个问题仅适用于Unix/Linux， 在Windows平台上不允许删除 已经被其他进程打开的文件。
究其根本原因是在Linux/Unix上Read 或 Write一个文件，进程都会打开一个与此文件相关联的 打开文件描述符 Open File Descriptors (a file descriptor (FD) is an abstract indicator for accessing a file. The term is generally used in POSIX operating systems.In POSIX, a file descriptor is an integer, specifically
of the C type int.)
而在Linux/Unix上当一个文件被删除时，它只是简单地被”unlink”即删除一个硬链接hard link； 实际包含文件数据的inode 在所有与该文件关联的 打开文件描述符(Open File Descriptors)被关闭之前 都不会被实际删除，已经获得 打开文件描述符的进程 只要不用close函数关闭掉这些描述符， 都可以继续正常地对文件进行读写。 当所有该文件管理的打开文件符都被关闭时 文件才真正意义上被删除。
注意：--SYSTEM不能在线RECOVER，需要重启数据库。
UNDO可以在线RECOVER，不过UNDO 文件OFFLINE了不能做操作了--不能DML，查询可以--不涉及回滚段中数据的可以。
其它数据文件可以在线恢复。
使用的命令主要是：
ps -ef |grep dbw |grep -v grep
ps -ef |grep ckpt |grep -v grep
ll /proc/10986/fd
cp /proc/10986/fd/258 /u01/oradata/bys3/system01.dbf
alter database datafile 6 offline;
recover datafile 6;
alter database datafile 6 online;
1.删除SYSTEM文件的恢复实验：
会话1：
SYS@ bys3>select * from v$dbfile;
FILE# NAME
---------- ----------------------------------------
1 /u01/oradata/bys3/system01.dbf
2 /u01/oradata/bys3/sysaux01.dbf
3 /u01/oradata/bys3/undotbs01.dbf
4 /u01/oradata/bys3/user01.dbf
5 /u01/oradata/bys3/test1.dbf
6 /u01/oradata/bys3/test2.dbf
7 /u01/oradata/bys3/test22.dbf
8 /u01/oradata/bys3/test3.dbf
SYS@ bys3>select open_mode from v$database;
OPEN_MODE
--------------------
READ WRITE
#############
新打开一个会话2：
查看DBWR的进程号，并删除/u01/oradata/bys3/system01.dbf
[oracle@bys3 bys3]$ ps -ef |grep dbw |grep -v grep 查出进程PID---10986，
注意这里查的是DBWR进程，如是控制文件，则查CKPT进程。
oracle 10986 1 0 Mar01 ? 00:04:10 ora_dbw0_bys3
[oracle@bys3 bys3]$ pwd
/u01/oradata/bys3
[oracle@bys3 bys3]$ ls
control01.ctl redo02.log system01.dbf test22.dbf undotbs01.dbf
control02.ctl redo03.log temp01.dbf test2.dbf user01.dbf
redo01.log sysaux01.dbf test1.dbf test3.dbf
[oracle@bys3 bys3]$ rm -rf system01.dbf
#########################
回到会话1：
SYS@ bys3>conn bys/bys
ERROR:
ORA-00604: error occurred at recursive SQL level 2
ORA-01116: error in opening database file 1
ORA-01110: data file 1: '/u01/oradata/bys3/system01.dbf'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
ORA-00604: error occurred at recursive SQL level 2
ORA-01116: error in opening database file 1
ORA-01110: data file 1: '/u01/oradata/bys3/system01.dbf'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
Warning: You are no longer connected to ORACLE.
@ >conn / as sysdba
Connected.
SYS@ bys3>select open_mode from v$database;
OPEN_MODE
--------------------
READ WRITE
##########################################
会话2：
[oracle@bys3 ~]$ ll /proc/10986/fd
total 0
lr-x------ 1 oracle oinstall 64 Mar 7 11:23 0 -> /dev/null
l-wx------ 1 oracle oinstall 64 Mar 7 11:23 1 -> /dev/null
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 10 -> /u01/app/oracle/product/11.2.0/dbhome_1/dbs/lkBYS3
lr-x------ 1 oracle oinstall 64 Mar 7 11:23 11 -> /u01/app/oracle/product/11.2.0/dbhome_1/rdbms/mesg/oraus.msb
l-wx------ 1 oracle oinstall 64 Mar 7 11:23 2 -> /dev/null
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 256 -> /u01/oradata/bys3/control01.ctl
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 257 -> /u01/oradata/bys3/control02.ctl
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 258 -> /u01/oradata/bys3/system01.dbf (deleted) ---这里会一直闪
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 259 -> /u01/oradata/bys3/sysaux01.dbf
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 260 -> /u01/oradata/bys3/undotbs01.dbf
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 261 -> /u01/oradata/bys3/user01.dbf
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 262 -> /u01/oradata/bys3/test1.dbf
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 263 -> /u01/oradata/bys3/test2.dbf
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 264 -> /u01/oradata/bys3/test22.dbf
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 265 -> /u01/oradata/bys3/test3.dbf
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 266 -> /u01/oradata/bys3/temp01.dbf
lr-x------ 1 oracle oinstall 64 Mar 7 11:23 3 -> /dev/null
lr-x------ 1 oracle oinstall 64 Mar 7 11:23 4 -> /dev/null
lr-x------ 1 oracle oinstall 64 Mar 7 11:23 5 -> /dev/null
lr-x------ 1 oracle oinstall 64 Mar 7 11:23 6 -> /u01/app/oracle/product/11.2.0/dbhome_1/rdbms/mesg/oraus.msb
lr-x------ 1 oracle oinstall 64 Mar 7 11:23 7 -> /proc/10986/fd
lr-x------ 1 oracle oinstall 64 Mar 7 11:23 8 -> /dev/zero
lrwx------ 1 oracle oinstall 64 Mar 7 11:23 9 -> /u01/app/oracle/product/11.2.0/dbhome_1/dbs/hc_bys3.dat
[oracle@bys3 ~]$ cp /proc/10986/fd/258 /u01/oradata/bys3/system01.dbf
[oracle@bys3 ~]$ ll /u01/oradata/bys3/system01.dbf
-rw-r----- 1 oracle oinstall 524296192 Mar 7 11:25 /u01/oradata/bys3/system01.dbf
此时，ll /proc/10986/fd还是会显示文件状态是：(deleted)，如果是非SYSTEM文件，在线RECOVER后这里会变正常。
####################
SYS@ bys3>conn bys/bys
Connected.
BYS@ bys3>select TABLESPACE_NAME,STATUS,ONLINE_STATUS from dba_data_files;
TABLESPACE_NAME STATUS ONLINE_
------------------------------ --------- -------
SYSTEM AVAILABLE SYSTEM
SYSAUX AVAILABLE ONLINE
UNDOTBS1 AVAILABLE ONLINE
USERS AVAILABLE ONLINE
TEST1 AVAILABLE ONLINE
TEST1 AVAILABLE ONLINE
TEST2 AVAILABLE ONLINE
TEST3 AVAILABLE ONLINE
ALERT日志中的警告信息：
Fri Mar 07 11:17:39 2014
Errors in file /u01/diag/rdbms/bys3/bys3/trace/bys3_m000_14498.trc:
ORA-00604: error occurred at recursive SQL level 2
ORA-01116: error in opening database file 1
ORA-01110: data file 1: '/u01/oradata/bys3/system01.dbf'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
Checker run found 1 new persistent data failures
Fri Mar 07 11:18:41 2014
Errors in file /u01/diag/rdbms/bys3/bys3/trace/bys3_m000_14516.trc:
ORA-01116: error in opening database file 1
ORA-01110: data file 1: '/u01/oradata/bys3/system01.dbf'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
Fri Mar 07 11:18:42 2014
Errors in file /u01/diag/rdbms/bys3/bys3/trace/bys3_m001_14520.trc:
ORA-00604: error occurred at recursive SQL level 2
ORA-01116: error in opening database file 1
ORA-01110: data file 1: '/u01/oradata/bys3/system01.dbf'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
############
此时，应该关闭数据库，打开到MOUNT，恢复SYSTEM文件，再打开数据库，就可以正常使用了。
SYS@ bys3>shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SYS@ bys3>startup
ORACLE instance started.
Total System Global Area 225996800 bytes
Fixed Size 1363692 bytes
Variable Size 150995220 bytes
Database Buffers 67108864 bytes
Redo Buffers 6529024 bytes
Database mounted.
ORA-01113: file 1 needs media recovery
ORA-01110: data file 1: '/u01/oradata/bys3/system01.dbf'
SYS@ bys3>
recover datafile 1;
Media recovery complete.
SYS@ bys3>alter database open;
Database altered.
#############
2.如果丢失的是非SYSTEM/UNDO，则可以在线恢复：方法如下：
BYS@ bys3>alter database datafile 6 offline;
Database altered.
BYS@ bys3>recover datafile 6;
Media recovery complete.
BYS@ bys3>alter database datafile 6 online;
Database altered.
################
3.UNDO的OFFLINE及恢复和上面一样：
SYS@ bys3>alter database datafile 3 offline;
alter database datafile 3 offline
*
ERROR at line 1:
ORA-00603: ORACLE server session terminated by fatal error
ORA-00376: file 3 cannot be read at this time
ORA-01110: data file 3: '/u01/oradata/bys3/undotbs01.dbf'
ORA-00376: file 3 cannot be read at this time
ORA-01110: data file 3: '/u01/oradata/bys3/undotbs01.dbf'
ORA-00376: file 3 cannot be read at this time
ORA-01110: data file 3: '/u01/oradata/bys3/undotbs01.dbf'
Process ID: 15106
Session ID: 1 Serial number: 5
BYS@ bys3>update dept set deptno=22;
update dept set deptno=22
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-00376: file 3 cannot be read at this time
ORA-01110: data file 3: '/u01/oradata/bys3/undotbs01.dbf'
#########
4.控制文件误删除的恢复的上面数据文件的恢复方法基本一样：
[oracle@bys3 ~]$ rm -rf /u01/oradata/bys3/control*
[oracle@bys3 ~]$ ps -ef|grep ckpt|grep -v grep ----注意这里查的
是CKPT进程。
oracle 15076 1 0 11:53 ? 00:00:01 ora_ckpt_bys3
[oracle@bys3 ~]$ ll /proc/15076/fd
total 0
lr-x------ 1 oracle oinstall 64 Mar 7 12:12 0 -> /dev/null
l-wx------ 1 oracle oinstall 64 Mar 7 12:12 1 -> /dev/null
lrwx------ 1 oracle oinstall 64 Mar 7 12:12 10 -> /u01/app/oracle/product/11.2.0/dbhome_1/dbs/lkBYS3
lr-x------ 1 oracle oinstall 64 Mar 7 12:12 11 -> /u01/app/oracle/product/11.2.0/dbhome_1/rdbms/mesg/oraus.msb
l-wx------ 1 oracle oinstall 64 Mar 7 12:12 2 -> /dev/null
lrwx------ 1 oracle oinstall 64 Mar 7 12:12 256 -> /u01/oradata/bys3/control01.ctl (deleted)
lrwx------ 1 oracle oinstall 64 Mar 7 12:12 257 -> /u01/oradata/bys3/control02.ctl (deleted)
lr-x------ 1 oracle oinstall 64 Mar 7 12:12 3 -> /dev/null
lr-x------ 1 oracle oinstall 64 Mar 7 12:12 4 -> /dev/null
lr-x------ 1 oracle oinstall 64 Mar 7 12:12 5 -> /dev/null
lr-x------ 1 oracle oinstall 64 Mar 7 12:12 6 -> /u01/app/oracle/product/11.2.0/dbhome_1/rdbms/mesg/oraus.msb
lr-x------ 1 oracle oinstall 64 Mar 7 12:12 7 -> /proc/15076/fd
lr-x------ 1 oracle oinstall 64 Mar 7 12:12 8 -> /dev/zero
lrwx------ 1 oracle oinstall 64 Mar 7 12:12 9 -> /u01/app/oracle/product/11.2.0/dbhome_1/dbs/hc_bys3.dat
[oracle@bys3 ~]$ cp /proc/15076/fd/256 /u01/oradata/bys3/control01.ctl
[oracle@bys3 ~]$ cp /proc/15076/fd/257 /u01/oradata/bys3/control02.ctl
[oracle@bys3 ~]$ ls /u01/oradata/bys3/control*
/u01/oradata/bys3/control01.ctl /u01/oradata/bys3/control02.ctl
日志：
Fri Mar 07 12:12:04 2014
Errors in file /u01/diag/rdbms/bys3/bys3/trace/bys3_m000_15540.trc:
ORA-00210: cannot open the specified control file
ORA-00202: control file: '/u01/oradata/bys3/control01.ctl'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
关闭数据库：
BYS@ bys3>conn / as sysdba
Connected.
SYS@ bys3>shutdown immediate;
Database closed.
ORA-03113: end-of-file on communication channel
Process ID: 15566
Session ID: 51 Serial number: 223
关库时日志：
ALTER DATABASE DISMOUNT
Shutting down archive processes
Archiving is disabled
********************* ATTENTION: ********************
The controlfile header block returned by the OS
has a sequence number that is too old.
The controlfile might be corrupted.
PLEASE DO NOT ATTEMPT TO START UP THE INSTANCE
without following the steps below.
RE-STARTING THE INSTANCE CAN CAUSE SERIOUS DAMAGE
TO THE DATABASE, if the controlfile is truly corrupted.
In order to re-start the instance safely,
please do the following:
(1) Save all copies of the controlfile for later
analysis and contact your OS vendor and Oracle support.
(2) Mount the instance and issue:
ALTER DATABASE BACKUP CONTROLFILE TO TRACE;
(3) Unmount the instance.
(4) Use the script in the trace file to
RE-CREATE THE CONTROLFILE and open the database.
打开数据库：
[oracle@bys3 ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.4.0 Production on Fri Mar 7 12:16:10 2014
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to an idle instance.
SYS@ bys3>startup
ORACLE instance started.
Total System Global Area 225996800 bytes
Fixed Size 1363692 bytes
Variable Size 150995220 bytes
Database Buffers 67108864 bytes
Redo Buffers 6529024 bytes
Database mounted.
Database opened.
打开库时的日志
ALTER DATABASE MOUNT
Fri Mar 07 12:16:21 2014
Sweep [inc][246535]: completed
……………………………………
Sweep [inc2][246423]: completed
Sweep [inc2][246415]: completed
Sweep [inc2][246407]: completed
Successful mount of redo thread 1, with mount id 3368434514
Database mounted in Exclusive Mode
Lost write protection disabled
Completed: ALTER DATABASE MOUNT
