SCN号在ORACLE中起着保证数据库一致性的作用，在ORACLE数据库的控制文件和数据文件头部，对于每一个数据文件都有2个SCN号，分别是：
| Checkpoint SCN | Stop SCN |
|---|
ORACLE通过比较两个SCN的值来确定控制文件和数据库文件是否保持一致，是否需要进行恢复。
实验如下：
1、正常关闭数据库
[oracle@db1 ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.4.0 Production on Fri Sep 15 21:23:54 2017
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SYS@oradb3> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
2、启动到mount状态后转储获取控制文件内容
SYS@oradb3> startup mount
ORACLE instance started.
Total System Global Area 1553305600 bytes
Fixed Size 2253544 bytes
Variable Size 469765400 bytes
Database Buffers 1073741824 bytes
Redo Buffers 7544832 bytes
Database mounted.
SYS@oradb3> alter session set events 'immediate trace name controlf level 12';
Session altered.
SYS@oradb3> select VALUE from v$diag_info where NAME='Default Trace File';
VALUE
--------------------------------------------------------------------------------
/u01/app/oracle/diag/rdbms/oradb3/oradb3/trace/oradb3_ora_109658.trc
3、打开转储的控制文件查看详细内容
[oracle@db1 ~]$ ll /u01/app/oracle/diag/rdbms/oradb3/oradb3/trace/oradb3_ora_109658.trc
-rw-r-----. 1 oracle oinstall 44185 Sep 15 21:25 /u01/app/oracle/diag/rdbms/oradb3/oradb3/trace/oradb3_ora_109658.trc
数据库的SCN信息
***************************************************************************
DATABASE ENTRY
***************************************************************************
(size = 316, compat size = 316, section max = 1, section in-use = 1,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 1, numrecs = 1)
09/15/2017 14:42:25
DB Name "ORADB3"
Database flags = 0x00404000 0x00001000
Controlfile Creation Timestamp 09/15/2017 14:42:25
Incmplt recovery scn: 0x0000.00000000
Resetlogs scn: 0x0000.000e2006 Resetlogs Timestamp 09/15/2017 14:42:27
Prior resetlogs scn: 0x0000.00000001 Prior resetlogs Timestamp 08/24/2013 11:37:30
Redo Version: compatible=0xb200400
#Data files = 5, #Online files = 5
Database checkpoint: Thread=1 scn: 0x0000.000f56cc ——>此处为Checkpoint SCN
Threads: #Enabled=1, #Open=0, Head=0, Tail=0| Database checkpoint: Thread=1 scn: 0x0000.000f56cc ——>此处为Checkpoint SCN |
REDO SCN信息
***************************************************************************
REDO THREAD RECORDS
***************************************************************************
(size = 256, compat size = 256, section max = 8, section in-use = 1,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 9, numrecs = 8)
THREAD #1 - status:0xe thread links forward:0 back:0
#logs:3 first:1 last:3 current:3 last used seq#:0x6
enabled at scn: 0x0000.000e2006 09/15/2017 14:42:27
disabled at scn: 0x0000.00000000 01/01/1988 00:00:00
opened at 09/15/2017 14:45:14 by instance oradb3
Checkpointed at scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Checkpoint SCN
thread:1 rba:(0x6.e7f3.10)
| Checkpointed at scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Checkpoint SCN |
数据文件SCN信息
***************************************************************************
DATA FILE RECORDS
***************************************************************************
(size = 520, compat size = 520, section max = 100, section in-use = 5,
last-recid= 30, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 11, numrecs = 100)
DATA FILE #1:
name #7: /u01/app/oracle/oradata/oradb3/system01.dbf
creation size=0 block size=8192 status=0xe head=7 tail=7 dup=1
tablespace 0, index=1 krfil=1 prev_file=0
unrecoverable scn: 0x0000.00000000 01/01/1988 00:00:00
Checkpoint cnt:95 scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Checkpoint SCN
Stop scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Stop SCN
DATA FILE #2:
name #6: /u01/app/oracle/oradata/oradb3/sysaux01.dbf
creation size=0 block size=8192 status=0xe head=6 tail=6 dup=1
tablespace 1, index=2 krfil=2 prev_file=0
unrecoverable scn: 0x0000.00000000 01/01/1988 00:00:00
Checkpoint cnt:95 scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Checkpoint SCN
Stop scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Stop SCN
DATA FILE #3:
name #5: /u01/app/oracle/oradata/oradb3/undotbs01.dbf
creation size=0 block size=8192 status=0xe head=5 tail=5 dup=1
tablespace 2, index=3 krfil=3 prev_file=0
unrecoverable scn: 0x0000.00000000 01/01/1988 00:00:00
Checkpoint cnt:16 scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Checkpoint SCN
Stop scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Stop SCN
DATA FILE #4:
name #4: /u01/app/oracle/oradata/oradb3/users01.dbf
creation size=0 block size=8192 status=0xe head=4 tail=4 dup=1
tablespace 4, index=4 krfil=4 prev_file=0
unrecoverable scn: 0x0000.00000000 01/01/1988 00:00:00
Checkpoint cnt:94 scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Checkpoint SCN
Stop scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Stop SCN
| Checkpoint cnt:94 scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Checkpoint SCN |
| Stop scn: 0x0000.000f56cc 09/15/2017 21:24:36 ——>此处为Stop SCN |
4、由于数据库正常关闭，执行了完全检查点，此时数据库、REDO、数据文件2个SCN全部相等，表明数据库处于一致状态，在下次启动时就能顺利通过验证，正常启动。
| Checkpoint SCN | Stop SCN |
|---|---|
| 0x0000.000f56cc | 0x0000.000f56cc |