数据库控制文件中的SCN详解
在的启动和关闭数据库的时候控制文件起着重要的作用，从官方文档中我们可以知道控制文件中保存着下列的信息： 数据库的名称及数据库创建时间等 所有的数据文件、重做日志文件的名称和位置信息。 表空间的信息。 Offline 数据文件的信息。 重做日志及归档日志
在的启动和关闭数据库的时候控制文件起着重要的作用，从官方文档中我们可以知道控制 文件中保存着下列的信息：
数据库的名称及数据库创建时间等
所有的数据文件、重做日志文件的名称和位置信息。
表空间的信息。
Offline数据文件的信息。
重做日志及归档日志的信息。
备份集及备份文件的信息
检查点及SCN信息等。
正常关闭数据库时，控制文件中的SCN值
在控制文件的数据文件信息部分，对于每一个数据文件都有一个Checkpoint SCN 和 Stop SCN，在数据库启动时要对其进行校验。
我们知道数据库在一致性关闭的时候控制文件中的SCN是怎样的呢？
1.一致性关闭数据库。Dump控制文件
SYS@ENMOEDU > shutdown immediate
SYS@ENMOEDU > startup mount
Database mounted.
SYS@ENMOEDU > oradebug setmypid;
Statement processed.
SYS@ENMOEDU > oradebug dump controlf 3;
Statement processed.
2.数据库的相关信息，上次数据库成功完成检查点的信息
***************************************************************************
DATABASE ENTRY
***************************************************************************
(size = 316, compat size = 316, div max = 1, div in-use = 1,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 1, numrecs = 1)
01/13/2014 16:13:27
DB Name "ENMOEDU"
Database flags = 0x00404001 0x00001000
Controlfile Creation Timestamp 01/13/2014 16:13:28
Incmplt recovery scn: 0x0000.00000000
Resetlogs scn: 0x0000.000fc451 Resetlogs Timestamp 02/05/2014 15:44:46
Prior resetlogs scn: 0x0000.000fa657 Prior resetlogs Timestamp 02/05/2014 14:15:27
Redo Version: compatible=0xb200000
#Data files = 6, #Online files = 6
Database checkpoint: Thread=1 scn: 0x0000.0011b73b
Threads: #Enabled=1, #Open=0, Head=0, Tail=0
3.检查点进程记录。Low cache rba是恢复的起点，此时是fffffff说明数据库不需要恢复。On disk rba是恢复的终点。
***************************************************************************
CHECKPOINT PROGRESS RECORDS
***************************************************************************
(size = 8180, compat size = 8180, div max = 11, div in-use = 0,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 2, numrecs = 11)
THREAD #1 - status:0x1 flags:0x0 dirty:0
low cache rba:(0xffffffff.ffffffff.ffff) on disk rba:(0x6.6f7.0)
on disk scn: 0x0000.0011b58f 02/19/2014 21:58:48
resetlogs scn: 0x0000.000fc451 02/05/2014 15:44:46
heartbeat: 839919586 mount id: 99161229
Flashback log tail log# 2 thread# 1 seq 2 block 5337 byte 0
4.Redo线程记录中记录了检查点的信息，与数据库的检查点一样
***************************************************************************
REDO THREAD RECORDS
***************************************************************************
(size = 256, compat size = 256, div max = 8, div in-use = 1,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 9, numrecs = 8)
THREAD #1 - status:0xbe thread links forward:0 back:0
#logs:3 first:1 last:3 current:3 last used seq#:0x6
enabled at scn: 0x0000.000fc451 02/05/2014 15:44:46
disabled at scn: 0x0000.00000000 01/01/1988 00:00:00
opened at 02/19/2014 21:57:43 by instance ENMOEDU
Checkpointed at scn: 0x0000.0011b73b 02/19/2014 21:58:52
5.数据文件中记录的检查点信息与DATABASE ENTRY记录的一样
***************************************************************************
DATA FILE RECORDS
***************************************************************************
DATA FILE #6:
name #13: /u01/app/oracle/oradata/ENMOEDU/sample01.dbf
creation size=12800 block size=8192 status=0xe head=13 tail=13 dup=1
tablespace 7, index=7 krfil=6 prev_file=0
unrecoverable scn: 0x0000.001143f4 02/10/2014 15:34:23
Checkpoint cnt:7 scn: 0x0000.0011b73b 02/19/2014 21:58:52
Stop scn: 0x0000.0011b73b 02/19/2014 21:58:52
Creation Checkpointed at scn: 0x0000.00111736 02/10/2014 15:28:26
thread:1 rba:(0x4.1f5.10)
enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000
由于数据库是一致性关闭，数据文件中的Checkpoint scn 与Stop scn，database scn和redo中记录的scn值都一样，所以在下次启动数据库的时候可以直接启动，不用进行恢复。
数据库异常关闭的时候的SCN的值。
1.如果数据库异常关闭，则不会执行任何的检查点。通过shutdown abort可以模拟一次异常，当使用shutdown abort方式关闭数据库时，此时的SCN值是怎么样的。
SYS@ENMOEDU > shutdown abort
ORACLE instance shut down.
SYS@ENMOEDU > startup mount
Database mounted.
SYS@ENMOEDU > oradebug setmypid;
Statement processed.
SYS@ENMOEDU > oradebug dump controlf 3;
Statement processed.
2.在DATABASE ENTRY中我们可以看到数据库thread checkpoint信息。
***************************************************************************
DATABASE ENTRY
***************************************************************************
(size = 316, compat size = 316, div max = 1, div in-use = 1,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 1, numrecs = 1)
01/13/2014 16:13:27
DB Name "ENMOEDU"
Database flags = 0x00404001 0x00001200
Controlfile Creation Timestamp 01/13/2014 16:13:28
Incmplt recovery scn: 0x0000.00000000
Resetlogs scn: 0x0000.000fc451 Resetlogs Timestamp 02/05/2014 15:44:46
Prior resetlogs scn: 0x0000.000fa657 Prior resetlogs Timestamp 02/05/2014 14:15:27
Redo Version: compatible=0xb200000
#Data files = 6, #Online files = 6
Database checkpoint: Thread=1 scn: 0x0000.0011b73e
Threads: #Enabled=1, #Open=1, Head=1, Tail=1
3. 检查点信息其中包含 low chache rba 和on disk ora信息。low cache rba（recovery block address）是恢复的起点，on disk rba（确定已经写到磁盘上的重做值）是恢复的终点，
***************************************************************************
CHECKPOINT PROGRESS RECORDS
***************************************************************************
(size = 8180, compat size = 8180, div max = 11, div in-use = 0,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 2, numrecs = 11)
THREAD #1 - status:0x2 flags:0x0 dirty:510
low cache rba:(0x6.18e2.0) on disk rba:(0x6.ca99.0)
on disk scn: 0x0000.0011e785 02/19/2014 22:55:08
resetlogs scn: 0x0000.000fc451 02/05/2014 15:44:46
heartbeat: 839992040 mount id: 99202971
Flashback log tail log# 3 thread# 1 seq 3 block 2244 byte 0
4. Redo线程的记录。
***************************************************************************
REDO THREAD RECORDS
***************************************************************************
(size = 256, compat size = 256, div max = 8, div in-use = 1,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 9, numrecs = 8)
THREAD #1 - status:0xbf thread links forward:0 back:0
#logs:3 first:1 last:3 current:3 last used seq#:0x6
enabled at scn: 0x0000.000fc451 02/05/2014 15:44:46
disabled at scn: 0x0000.00000000 01/01/1988 00:00:00
opened at 02/19/2014 22:49:27 by instance ENMOEDU
Checkpointed at scn: 0x0000.0011b73e 02/19/2014 22:49:27
thread:1 rba:(0x6.9ce.10)
enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000
5. 数据文件的检查点信息；
DATA FILE #6:
name #13: /u01/app/oracle/oradata/ENMOEDU/sample01.dbf
creation size=12800 block size=8192 status=0xe head=13 tail=13 dup=1
tablespace 7, index=7 krfil=6 prev_file=0
unrecoverable scn: 0x0000.001143f4 02/10/2014 15:34:23
Checkpoint cnt:8 scn: 0x0000.0011b73e 02/19/2014 22:49:27
Stop scn: 0xffff.ffffffff 02/19/2014 21:58:52
Creation Checkpointed at scn: 0x0000.00111736 02/10/2014 15:28:26
thread:1 rba:(0x4.1f5.10)
enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000
可以看出由于是异常关闭数据库，数据库没有完成最后的检查点，数据库文件的stop scn仍然设定为最大的ffffffff。由以上信息可以看出，各部分的checkpoint scn 都是一样的，但是数据文件的stop scn不等于checkpoint scn，意味着数据库的上一次的关闭没有执行完全检查点，是异常关闭数据库，此时启动数据库需要进行完全恢复。
在进行恢复的时候，从检查点信息中读取low cache rba的值确定日志文件的地址。再读取日志，从最后的检查点开始，应用所有的重做记录，这个过程叫做前滚。数据库可以被打开提供访问和使用，但是此时数据库中包含已提交的和没有提交的事务，使用undo段回滚。
再对数据库进行实例恢复的时候，我们从alert日志中可以看到的信息。
alter database open
Beginning crash recovery of 1 threads
parallel recovery started with 2 processes
Started redo scan
Completed redo scan
read 22747 KB redo, 510 data blocks need recovery
Started redo application at
Thread 1: logseq 6, block 6370
Recovery of Online Redo Log: Thread 1 Group 3 Seq 6 Reading mem 0
Mem# 0: /u01/app/oracle/oradata/ENMOEDU/redo03.log
Completed redo application of 3.07MB
Wed Feb 19 23:13:55 2014
Completed crash recovery at
Thread 1: logseq 6, block 51865, scn 1193381
510 data blocks read, 493 data blocks written, 22747 redo k-bytes read
Wed Feb 19 23:13:55 2014
在实例恢复的时候。 Alter中的logseq 6, block 6370就是日志中的检查点中记录的low cache rba:(0x6.18e2.0) 。恢复完成时的logseq 6, block 51865就是on disk rba:(0x6.ca99.0)。把十六进制的数换成十进制。
总结：当一致性关闭数据库的时候，关闭数据库之前数据库会执行一个检查点动作，每个数据文件的你Stop SCN会被设置为Checkpoint SCN。数据库不需要恢复。当数据库是异常关闭的时候，Stop SCN 来不及设置为Checkpoint SCN会是无穷大，在重启数据库的时候需要恢复
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