scn(系统改变号)信息与恢复

control中有三种SCN分别为，system SCN、datafile SCN、last SCN，数据文件头中有一种SCN start SCN
system scn从视图v$database中获得，对应checkpoint_change#字段，datafile scn、last scn分别对应视图v$datafile中的checkpoint_change#，last_change#，而 start scn则从v$datafile_header中checkpoint_change#得到。
数据库在正常启动后下，system scn,datafile scn,start scn会相等，而last scn会被置于无穷大，这里为null。
正常关闭后（immediate,noraml,translate），上面四个scn会应执行full checkpoint 而相等。
当系统在非正常关闭后，如shutdown abort，这个时候last scn依然为无穷大，那么当重新启动实例时，系统首先会比较start scn与system scn，如果一致，那么再比较start scn 与last scan是否一样大，因为是非正常关闭，这里会不一样大，那么就需要例程恢复。
如果打开数据库时发现system scn>datafile scn，那么以为着使用旧的备份数据文件，也就是需要介质恢复
如果是system scn<datafile scn，及控制文件scn是旧的，代表使用了老的控制文件，需要recover using backup controlfile进行恢复。

1、正常启动时
SQL> select checkpoint_change# from v$database; --控制文件中的scnCHECKPOINT_CHANGE#
------------------
5534071
SQL> select file#,checkpoint_change# from v$datafile_header; --start scnFILE# CHECKPOINT_CHANGE#
---------- ------------------
1 5534071
2 5534071
3 5534071
4 5534071
5 5534071
SQL> select file#,checkpoint_change#,last_change# from v$datafile; --datafile scn & last scnFILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 5534071
2 5534071
3 5534071
4 5534071
5 5534071
2、正常关闭后，然后在startup mount;
SQL> shutdown immediate;
数据库已经关闭。
已经卸载数据库。
ORACLE 例程已经关闭。
SQL> startup mount;
ORACLE 例程已经启动。Total System Global Area 319888364 bytes
Fixed Size 453612 bytes
Variable Size 192937984 bytes
Database Buffers 125829120 bytes
Redo Buffers 667648 bytes
数据库装载完毕。
SQL> select file#,checkpoint_change# from v$datafile_header;FILE# CHECKPOINT_CHANGE#
---------- ------------------
1 5534485
2 5534485
3 5534485
4 5534485
5 5534485SQL> select checkpoint_change# from v$database;CHECKPOINT_CHANGE#
------------------
5534485SQL> select file#,checkpoint_change#,last_change# from v$datafile;FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 5534485 5534485
2 5534485 5534485
3 5534485 5534485
4 5534485 5534485
5 5534485 5534485
--发现start scn=last scn,证明系统是正常关闭SQL> alter database open;数据库已更改。3、在正常打开状态下进行事务操作
SQL> create table t(a number);表已创建。SQL> insert into t values (1);已创建 1 行。SQL> commit;提交完成。SQL> insert into t values(2);已创建 1 行。4、非正常关闭
SQL> shutdown abort;
ORACLE 例程已经关闭。
SQL>5、打开到mount状态下，观看scn
SQL> startup mount;
ORACLE 例程已经启动。Total System Global Area 319888364 bytes
Fixed Size 453612 bytes
Variable Size 192937984 bytes
Database Buffers 125829120 bytes
Redo Buffers 667648 bytes
数据库装载完毕。
SQL> select file#,checkpoint_change#,last_change# from v$datafile;FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
1 5534486
2 5534486
3 5534486
4 5534486
5 5534486SQL> select checkpoint_change# from v$database;CHECKPOINT_CHANGE#
------------------
5534486SQL> select file#,checkpoint_change# from v$datafile_header;FILE# CHECKPOINT_CHANGE#
---------- ------------------
1 5534486
2 5534486
3 5534486
4 5534486
5 5534486
--这时发现start scn 与last scn不等，last scn为无穷大，需要例程恢复6、改变数据库状态为open，并查看该阶段运行日志
SQL> select * from wen.t;
select * from wen.t
*
ERROR 位于第 1 行:
ORA-01219: 数据库未打开: 仅允许在固定表/视图中查询
SQL> alter database open;数据库已更改。
SQL> select * from wen.t;A
----------
1
--发现没有提交的事务丢失。
查看日志如下：
Completed: ALTER DATABASE MOUNT
Wed May 17 21:35:46 2006
alter database open
Wed May 17 21:35:46 2006
Beginning crash recovery of 1 threads --会自动判断是否需要恢复，这里开始例程恢复
Wed May 17 21:35:46 2006
Started first pass scan
Wed May 17 21:35:47 2006
Completed first pass scan
206 redo blocks read, 90 data blocks need recovery
Wed May 17 21:35:47 2006
Started recovery at
Thread 1: logseq 167, block 271, scn 0.0
Recovery of Online Redo Log: Thread 1 Group 2 Seq 167 Reading mem 0 --恢复用的在线重做日志
Mem# 0 errs 0: D:ORACLEORADATADB1REDO02.LOG
Wed May 17 21:35:47 2006
Ended recovery at
Thread 1: logseq 167, block 477, scn 0.5554724
90 data blocks read, 90 data blocks written, 206 redo blocks read
Crash recovery completed successfully --恢复完成
Wed May 17 21:35:47 2006
LGWR: Primary database is in CLUSTER CONSISTENT mode
Thread 1 advanced to log sequence 168
Thread 1 opened at log sequence 168
Current log# 3 seq# 168 mem# 0: D:ORACLEORADATADB1REDO03.LOG
Successful open of redo thread 1.
Wed May 17 21:35:48 2006
SMON: enabling cache recovery
Wed May 17 21:35:48 2006
ARC0: Evaluating archive log 2 thread 1 sequence 167
ARC0: Beginning to archive log 2 thread 1 sequence 167
Creating archive destination LOG_ARCHIVE_DEST_1: 'D:DBBKARC00167.001'
ARC0: Completed archiving log 2 thread 1 sequence 167
Wed May 17 21:35:48 2006
Undo Segment 1 Onlined
Undo Segment 2 Onlined
Undo Segment 3 Onlined
Undo Segment 4 Onlined
Undo Segment 5 Onlined
Undo Segment 6 Onlined

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