Oracle scn介绍

SCN是当Oracle数据库更新后，由DBMS自动维护去累积递增的一个数字。当一笔交易commit 时，LGWR会将log buffer写入redo log file，

上次做了下基于scn恢复delete掉的数据后，觉得应该好好理解下scn的知识，今天在网上找了下相关的介绍，参考了某些文章，，在此我通过实验总结一下。

SCN是当Oracle数据库更新后，由DBMS自动维护去累积递增的一个数字。当一笔交易commit 时，LGWR会将log buffer写入redo log file，同时也会将该笔交易的SCN同步写入到redo log file内（wait-until-completed）。因此当你commit transaction时，在交易成功的讯息返回之前，LGWR必须先完整的完成上述行为之后，否则你是看不到提交成功的回应讯息。
那系统是如何产生一个最新的
select dbms_flashback.get_system_change_number, SCN_TO_TIMESTAMP(dbms_flashback
  2  .get_system_change_number) from dual;

GET_SYSTEM_CHANGE_NUMBER
------------------------
SCN_TO_TIMESTAMP(DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER)
---------------------------------------------------------------------------
           117947354
24-MAR-11 11.32.22.000000000 AM

也可以用函数

SQL> select timestamp_to_scn(SYSTIMESTAMP) as scn from dual;
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$datafile_header中checkpoint_change#得到。
数据库在正常启动后下，system scn,datafile scn,start scn会相等，而last scn会被置于无穷大，这里为null。
正常关闭后（immediate,noraml,translate），上面四个scn会应执行full checkpoint 而相等。
当系统在非正常关闭后，如shutdown abort，这个时候last scn依然为无穷大，那么当重新启动实例时，系统首先会比较start scn与system scn，如果一致，那么再比较start scn 与last scan是否一样大，因为是非正常关闭，这里会不一样大，那么就需要例程恢复。
如果打开数据库时发现system scn>datafile scn，那么以为着使用旧的备份数据文件，也就是需要介质恢复
如果是system scn1、正常启动时
SQL> SELECT checkpoint_change# FROM v$database;

CHECKPOINT_CHANGE#
------------------
     117866282

SQL> SELECT file#, checkpoint_change# FROM v$datafile_header;

     FILE# CHECKPOINT_CHANGE#
---------- ------------------
     1        117866282
     2        117866282
     3        117866282
     4        117866282
     5        117866282
     6        117866282
     7        117866282
     8        117866282
     9        117866282
    10        117866282
    11        117866282

     FILE# CHECKPOINT_CHANGE#
---------- ------------------
    12        117866282
    13        117866282
    14        117866282

14 rows selected.

SQL> SELECT file#, checkpoint_change#, last_change# FROM v$datafile;

     FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
     1        117866282
     2        117866282
     3        117866282
     4        117866282
     5        117866282
     6        117866282
     7        117866282
     8        117866282
     9        117866282
    10        117866282
    11        117866282

     FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
    12        117866282
    13        117866282
    14        117866282

14 rows selected.
2、正常关闭后，然后在startup mount;
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.

Total System Global Area  835104768 bytes
Fixed Size            2217952 bytes
Variable Size          624953376 bytes
Database Buffers      201326592 bytes
Redo Buffers            6606848 bytes
Database mounted.
SQL> SELECT file#, checkpoint_change# FROM v$datafile_header;

     FILE# CHECKPOINT_CHANGE#
---------- ------------------
     1        117925094
     2        117925094
     3        117925094
     4        117925094
     5        117925094
     6        117925094
     7        117925094
     8        117925094
     9        117925094
    10        117925094
    11        117925094

     FILE# CHECKPOINT_CHANGE#
---------- ------------------
    12        117925094
    13        117925094
    14        117925094

14 rows selected.

SQL> SELECT checkpoint_change# FROM v$database;

CHECKPOINT_CHANGE#
------------------
     117925094

SQL> SELECT file#, checkpoint_change#, last_change# FROM v$datafile;

     FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
     1        117925094     117925094
     2        117925094     117925094
     3        117925094     117925094
     4        117925094     117925094
     5        117925094     117925094
     6        117925094     117925094
     7        117925094     117925094
     8        117925094     117925094
     9        117925094     117925094
    10        117925094     117925094
    11        117925094     117925094

     FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
    12        117925094     117925094
    13        117925094     117925094
    14        117925094     117925094

14 rows selected.
--发现start scn=last scn,证明系统是正常关闭
SQL> alter database open;

Database altered.
3、在正常打开状态下进行事务操作
SQL> CREATE TABLE w(a number);

Table created.

SQL> INSERT INTO w VALUES(1);

1 row created.

SQL> commit;

Commit complete.

SQL> INSERT INTO w VALUES(2);

1 row created.
4、非正常关闭
SQL> shutdown abort;
ORACLE instance shut down.
5、打开到mount状态下，观看scn
SQL> startup mount;
ORACLE instance started.

Total System Global Area  835104768 bytes
Fixed Size            2217952 bytes
Variable Size          624953376 bytes
Database Buffers      201326592 bytes
Redo Buffers            6606848 bytes
Database mounted.
SQL> SELECT file#,checkpoint_change#, last_change# FROM v$datafile;

     FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
     1        117925097
     2        117925097
     3        117925097
     4        117925097
     5        117925097
     6        117925097
     7        117925097
     8        117925097
     9        117925097
    10        117925097
    11        117925097

     FILE# CHECKPOINT_CHANGE# LAST_CHANGE#
---------- ------------------ ------------
    12        117925097
    13        117925097
    14        117925097

14 rows selected.

SQL> SELECT checkpoint_change# FROM v$database;

CHECKPOINT_CHANGE#
------------------
     117925097

SQL> SELECT file#,checkpoint_chnge# FROM v$datafile_header;
SELECT file#,checkpoint_chnge# FROM v$datafile_header
             *
ERROR at line 1:
ORA-00904: "CHECKPOINT_CHNGE#": invalid identifier

SQL> SELECT file#, checkpoint_change# FROM v$datafile_header;

     FILE# CHECKPOINT_CHANGE#
---------- ------------------
     1        117925097
     2        117925097
     3        117925097
     4        117925097
     5        117925097
     6        117925097
     7        117925097
     8        117925097
     9        117925097
    10        117925097
    11        117925097

     FILE# CHECKPOINT_CHANGE#
---------- ------------------
    12        117925097
    13        117925097
    14        117925097

14 rows selected.
--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> ALTER DATABASE open;

Database altered.

SQL> SELECT * FROM w;

     A
----------
     1
--发现没有提交的事务丢失。
查看日志如下：
Thu Mar 24 10:47:24 2011
ALTER DATABASE   MOUNT
Successful mount of redo thread 1, with mount id 1274403260
Database mounted in Exclusive Mode
Lost write protection disabled
Completed: ALTER DATABASE   MOUNT
Thu Mar 24 10:50:12 2011
ALTER DATABASE open
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
 parallel recovery started with 2 processes
Started redo scan
Completed redo scan
 read 162 KB redo, 112 data blocks need recovery
Started redo application at
 Thread 1: logseq 2635, block 491121
Recovery of Online Redo Log: Thread 1 Group 1 Seq 2635 Reading mem 0   --恢复用的在线重做日志
  Mem# 0: /u01/oradata/orcl/redo01a.log
  Mem# 1: /u01/oradata/orcl/redo01b.log
Completed redo application of 0.13MB
Completed crash recovery at                             --恢复完成
 Thread 1: logseq 2635, block 491446, scn 117945330
 112 data blocks read, 112 data blocks written, 162 redo k-bytes read
Thu Mar 24 10:50:13 2011
LGWR: STARTING ARCH PROCESSES
Thu Mar 24 10:50:13 2011
ARC0 started with pid=22, OS id=31059
ARC0: Archival started
LGWR: STARTING ARCH PROCESSES COMPLETE
ARC0: STARTING ARCH PROCESSES
Thu Mar 24 10:50:14 2011
ARC1 started with pid=23, OS id=31061
Thread 1 advanced to log sequence 2636 (thread open)
Thu Mar 24 10:50:15 2011
ARC2 started with pid=24, OS id=31063
Thread 1 opened at log sequence 2636
  Current log# 2 seq# 2636 mem# 0: /u01/oradata/orcl/redo02a.log