检查点和oracle数据库的恢复（一）SCN

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lctyn 2011-01-24 10:13:09

文章标签 oracle 职场休闲 scn 文章分类 数据库

SCN号研究

什么是scn（system change number）号

scn号是ORACLE数据库一个内部时钟，是数据库的历史时间记录，用来衡量ORACLE事务（transaction）发生的顺序，是一个时间概念。 scn号不断增加，可以理解为直至无穷大。

什么时候scn号会增加

scn号会实时增加的，而且只会增加不会减少，反映数据库的更新历史。

怎样查询目前系统最新的scn号

SQL>select dbms_flashback.get_system_change_number from dual; （9i以前）

SQL>select current_scn# from v$database;（10g以后）

scn号的存在什么地方

1. 控制文件中当：

Database checkpoint SCN（系统常规检查点checkpoint对应的scn号，Oracle就把系统检查点的scn存储到控制文件中）

Resetlogs scn(系统在执行resetlogs操作时的scn号，当数据库系统做resetlogs操作完成之后会把这时scn号记录到控制文件)

SQL>select RESETLOGS_CHANGE# from v$database；

Datafile Checkpoint scn（一个检查点动作完成后，Oracle就把每个数据文件的scn单独存放在控制文件中,每个数据文件一个）

SQL> select name, CHECKPOINT_CHANGE# from v$datafile where name ＝'FILENAME';

End scn号（ORACLE将End scn号存放在控制文件中，这个scn号用于检查数据库启动过程是否需要做instance recovery,每个数据文件一个）

SQL> select name, LAST_CHANGE# from v$datafile where name ＝'FILENAME';

First scn、Next scn(控制文件中包含每个日志文件的low scn和next scn)

在线重做日志

2. 数据文件中（头部）：

Start scn（ORACLE将Start scn号存放在数据文件头中，这个scn用于检查数据库启动过程是否需要做media recovery,每个数据文件一个）

SQL> Select name,checkpoint_change# from v$datafile_header where name ＝'FILENAME';

3. 数据文件

4. 日志文件（日志文件头中包含了low scn,next scn,表示该日志文件包含有从low scn到next scn的redo record）

接下来我们实例演示一下

Dump控制文件

Alter session set events 'immediate trace name controlf level 8';

打开一个新的session

Dump数据文件头部

控制dump文件节选

第一段

***************************************************************************
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)
04/11/2009 11:07:20
DB Name "ORA"
Database flags = 0x00404001 0x00001000
Controlfile Creation Timestamp 04/11/2009 11:07:20
Incmplt recovery scn: 0x0000.00000000

Resetlogs scn: 0x0000.00089d08 Resetlogs Timestamp 04/11/2009 11:07:22

此处为Resetlogs scn

Prior resetlogs scn: 0x0000.00000001 Prior resetlogs Timestamp 12/19/2006 04:16:53
Redo Version: compatible=0xa200300
#Data files = 4, #Online files = 4
Database checkpoint: Thread=1 scn: 0x0000.000a326c

Database checkpoint SCN,系统常规检查点checkpoint对应的scn号，Oracle就把系统检查点的scn存储到控制文件中

Threads: #Enabled=1, #Open=1, Head=1, Tail=1
enabled threads: 01000000 00000000 00000000 00000000

。。。。。。。。。。。。。。。。。。。。。。。。。。。。。

Max log members = 3, Max data members = 1
Arch list: Head=2, Tail=2, Force scn: 0x0000.00096e74scn: 0x0000.000a326b
Activation ID: 1302138531

Controlfile Checkpointed at scn: 0x0000.000a32ca 04/17/2009 15:59:53

增量检查点更新的scn

。。。。。。。。。。。。。。。。。。。。。。。。。。。。

***************************************************************************
CHECKPOINT PROGRESS RECORDS
***************************************************************************
(size = 8180, compat size = 8180, section max = 11, section in-use = 0,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 2, numrecs = 11)
THREAD #1 - status:0x2 flags:0x0dirty:11

dirty buffer的数量

low cache rba:(0x8.b85.0) on disk rba:(0x8.bfb.0)
on disk scn: 0x0000.000a340b 04/17/2009 16:10:26

增量检查点会更新low cache rba
resetlogs scn: 0x0000.00089d08 04/11/2009 11:07:22

heartbeat: 684440041 mount id: 1302634282

心跳检验没三秒检查一次，用于验证实例的存活性。
THREAD #2 - status:0x0 flags:0x0 dirty:0
low cache rba:(0x0.0.0) on disk rba:(0x0.0.0)
on disk scn: 0x0000.00000000 01/01/1988 00:00:00
resetlogs scn: 0x0000.00000000 01/01/1988 00:00:00
。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。

第二段

***************************************************************************
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:0xf thread links forward:0 back:0
#logs:3 first:1 last:3 current:2 last used seq#:0x8
enabled at scn: 0x0000.00089d08 04/11/2009 11:07:22
disabled at scn: 0x0000.00000000 01/01/1988 00:00:00
opened at 04/17/2009 15:59:49 by instance ora

Checkpointed at scn: 0x0000.000a326c 04/17/2009 15:59:49

redo进程检查点，这个检查点和Database的检查点信息一致。

thread:1 rba:(0x8.2.10)
enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000

第三段

***************************************************************************
LOG FILE RECORDS
***************************************************************************
(size = 72, compat size = 72, section max = 16, section in-use = 3,
last-recid= 6, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 10, numrecs = 16)
LOG FILE #1:
(name #3) +DISKGROUP1/ora/onlinelog/group_1.261.683896043
Thread 1 redo log links: forward: 2 backward: 0
siz: 0x19000 seq: 0x00000006 hws: 0x6 bsz: 512 nab: 0x92aa flg: 0x1 dup: 1
Archive links: fwrd: 0 back: 0 Prev scn: 0x0000.0008e394
Low scn: 0x0000.00096e74 04/14/2009 12:57:09
Next scn: 0x0000.0009de44 04/17/2009 15:17:12
LOG FILE #2:
(name #2) +DISKGROUP1/ora/onlinelog/group_2.262.683896043
Thread 1 redo log links: forward: 3 backward: 1
siz: 0x19000 seq: 0x00000008 hws: 0x2 bsz: 512 nab: 0xffffffff flg: 0x8 dup: 1
Archive links: fwrd: 0 back: 0 Prev scn: 0x0000.0009de44
Low scn: 0x0000.000a326b 04/17/2009 15:59:48
Next scn: 0xffff.ffffffff 01/01/1988 00:00:00

日志文件中的first scn和next scn，当前文件的next scn为无穷大

LOG FILE #3:
(name #1) +DISKGROUP1/ora/onlinelog/group_3.263.683896045
Thread 1 redo log links: forward: 0 backward: 2
siz: 0x19000 seq: 0x00000007 hws: 0x5 bsz: 512 nab: 0xa8c flg: 0x1 dup: 1
Archive links: fwrd: 0 back: 0 Prev scn: 0x0000.00096e74
Low scn: 0x0000.0009de44 04/17/2009 15:17:12
Next scn: 0x0000.000a326b 04/17/2009 15:59:48

第四段

***************************************************************************
DATA FILE RECORDS
***************************************************************************
(size = 428, compat size = 428, section max = 100, section in-use = 4,
last-recid= 7, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 11, numrecs = 100)
DATA FILE #1:
(name #7) +DISKGROUP1/ora/datafile/system.256.683895993
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:63 scn: 0x0000.000a326c 04/17/2009 15:59:49
Stop scn: 0xffff.ffffffff 04/17/2009 15:57:34

checkpoint cnt号：checkpoint的次数

scn:为Datafile Checkpoint scn

stop scn当数据库正常打开时

这个值被设为无穷大。
Creation Checkpointed at scn: 0x0000.00000008 12/19/2006 04:17:12
thread:0 rba:(0x0.0.0)

数据文件dump

DATA FILE #1:
(name #7) +DISKGROUP1/ora/datafile/system.256.683895993
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:63 scn: 0x0000.000a326c 04/17/2009 15:59:49
Stop scn: 0xffff.ffffffff 04/17/2009 15:57:34
Creation Checkpointed at scn: 0x0000.00000008 12/19/2006 04:17:12
thread:0 rba:(0x0.0.0)
enabled threads: 00000000 00000000 00000000 00000000 00000000 00000000

。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。
00000000 00000000 00000000 00000000 00000000 00000000 00000000
Hot Backup end marker scn: 0x0000.00000000
aux_file is NOT DEFINED
V10 STYLE. FILEHEADER （header前面的信息来自控制文件，后面的来自数据文件头部）
Compatibility Vsn = 169870080=0xa200300
Db ID=1302124456=0x4d9cd7a8, Db Name='ORA'
Activation ID=0=0x0
Control Seq=457=0x1c9, File size=61440=0xf000
File Number=1, Blksiz=8192, File Type=3 DATA
Tablespace #0 - SYSTEM rel_fn:1
Creation at scn: 0x0000.00000008 12/19/2006 04:17:12
Backup taken at scn: 0x0000.00000000 01/01/1988 00:00:00 thread:0
reset logs count:0x28c36cea scn: 0x0000.00089d08 reset logs terminal rcv data:0x0 scn: 0x0000.00000000
prev reset logs count:0x245542b5 scn: 0x0000.00000001 prev reset logs terminal rcv data:0x0 scn: 0x0000.00000000
recovered at 04/17/2009 15:59:47
status:0x2004 root dba:0x00400179 chkpt cnt: 63 ctl cnt:62

datafile cnt号和control file cnt号

begin-hot-backup file size: 0

Checkpointed at scn: 0x0000.000a326c 04/17/2009 15:59:49

begin scn号
thread:1 rba:(0x8.2.10)

日志文件dump文件

DUMP OF REDO FROM FILE '+DISKGROUP1/ora/onlinelog/group_2.262.683896043'
Opcodes *.*
RBAs: 0x000000.00000000.0000 thru 0xffffffff.ffffffff.ffff
SCNs: scn: 0x0000.00000000 thru scn: 0xffff.ffffffff
Times: creation thru eternity
FILE HEADER:
Compatibility Vsn = 169870080=0xa200300
Db ID=1302124456=0x4d9cd7a8, Db Name='ORA'
Activation ID=1302138531=0x4d9d0ea3
Control Seq=457=0x1c9, File size=102400=0x19000
File Number=2, Blksiz=512, File Type=2 LOG
descrip:"Thread 0001, Seq# 0000000008, SCN 0x0000000a326b-0xffffffffffff"
thread: 1 nab: 0xffffffff seq: 0x00000008 hws: 0x2 eot: 1 dis: 0
resetlogs count: 0x28c36cea scn: 0x0000.00089d08 (564488)
resetlogs terminal rcv count: 0x0 scn: 0x0000.00000000
prev resetlogs count: 0x245542b5 scn: 0x0000.00000001 (1)
prev resetlogs terminal rcv count: 0x0 scn: 0x0000.00000000
Low scn: 0x0000.000a326b (668267) 04/17/2009 15:59:48
Next scn: 0xffff.ffffffff 01/01/1988 00:00:00

与控制文件中对应的low scn和next scn
Enabled scn: 0x0000.00089d08 (564488) 04/11/2009 11:07:22
Thread closed scn: 0x0000.000a326b (668267) 04/17/2009 15:59:48
Disk cksum: 0x5f9b Calc cksum: 0x5f9b
Terminal recovery stop scn: 0x0000.00000000
Terminal recovery 01/01/1988 00:00:00

Alter session set events 'immediate trace name file_hdrs level 10';

获得处于current状态的日子文件

SQL> select l.STATUS,lf.member from v$log l,v$logfile lf where l.GROUP#=lf.GROUP#;

STATUS ---------------- MEMBER -------------------------------------------------------------------------------- INACTIVE +DISKGROUP1/ora/onlinelog/group_3.263.683896045 CURRENT +DISKGROUP1/ora/onlinelog/group_2.262.683896043 INACTIVE +DISKGROUP1/ora/onlinelog/group_1.261.683896043

打开一个新的session

Dump日志文件

注意:

在每次dump文件后都要重新打开一个session才能重新dump下一个文件，在dump文件后用

SELECT    d.VALUE
       || '/'
       || LOWER (RTRIM (i.INSTANCE, CHR (0)))
       || '_ora_'
       || p.spid
       || '.trc' trace_file_name
FROM (SELECT p.spid
          FROM v$mystat m, v$session s, v$process p
         WHERE m.statistic# = 1 AND s.SID = m.SID AND p.addr = s.paddr) p,
       (SELECT t.INSTANCE
          FROM v$thread t, v$parameter v
         WHERE v.NAME = 'thread'
           AND (v.VALUE = 0 OR t.thread# = TO_NUMBER (v.VALUE))) i,
       (SELECT VALUE
          FROM v$parameter
         WHERE NAME = 'user_dump_dest') d
/

获得dump文件的路径。

Alter system dump logfile '+DISKGROUP1/ora/onlinelog/group_2.262.683896043';

select group#,thread#,sequence#,first_change# from v$log;

归档重做日志

select NAME,THREAD#,SEQUENCE#,FIRST_CHANGE#,NEXT_CHANGE# from v$archived_log;

SQL> select checkpoint_change# from v$database;