重做日誌檔案中的SCN

日誌檔案頭中包含LOW SCN和NEXT SCN

LOW SCN和NEXT SCN表示改日誌檔案中包含在LOW SCN和NEXT SCN之間的重做資訊。對於當前重做日誌來說，NEXT SCN為無窮大，即ffffffff，因為最終的SCN是個未知數。

下面就看一下日誌檔案中的SCN：

SQL> select * from v$log;

    GROUP#    THREAD# SEQUENCE#      BYTES    MEMBERS ARC STATUS
---------- ---------- ---------- ---------- ---------- --- ----------------
FIRST_CHANGE# FIRST_TIME
------------- --------------
         1          1        141   52428800          1 YES INACTIVE
      6043116 13-7月 -11

2 1 142 52428800 1 NO CURRENT
6043132 13-7月 -11

3 1 139 52428800 1 YES INACTIVE
6022046 12-7月 -11

    GROUP#    THREAD# SEQUENCE#      BYTES    MEMBERS ARC STATUS
---------- ---------- ---------- ---------- ---------- --- ----------------
FIRST_CHANGE# FIRST_TIME
------------- --------------
         4          1        140   52428800          1 YES INACTIVE
      6042663 13-7月 -11

SQL> select dbms_flashback.get_system_change_number from dual;

GET_SYSTEM_CHANGE_NUMBER
------------------------
6057820

由上可以看出當前日誌組2的LOW SCN為6043132，先DUMP當前日誌檔案，然後在檢視TRC如下：

SQL> select member from v$logfile where group# = 2;

MEMBER

--------------------------------------------------------------------------------

D:\ORACLE\PRODUCT\10.2.0\ORADATA\WEIBIN\REDO02.LOG

SQL> alter system dump logfile 'D:\ORACLE\PRODUCT\10.2.0\ORADATA\WEIBIN\REDO02.LOG';

系統已更改。

檢視TRC:

DUMP OF REDO FROM FILE 'D:\ORACLE\PRODUCT\10.2.0\ORADATA\WEIBIN\REDO02.LOG'

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=4184897545=0xf9707809, Db Name='WEIBIN'

Activation ID=4203641025=0xfa8e78c1

Control Seq=7208=0x1c28, File size=102400=0x19000

File Number=2, Blksiz=512, File Type=2 LOG

descrip:"Thread 0001, Seq# 0000000142, SCN 0x0000005c35fc-0xffffffffffff"

thread: 1 nab: 0xffffffff seq: 0x0000008e hws: 0x1 eot: 1 dis: 0

resetlogs count: 0x2cad510c scn: 0x0000.001489a6 (1345958)

resetlogs terminal rcv count: 0x0 scn: 0x0000.00000000

prev resetlogs count: 0x2b8f370b scn: 0x0000.00089c75 (564341)

prev resetlogs terminal rcv count: 0x0 scn: 0x0000.00000000

Low scn: 0x0000.005c35fc (6043132) 07/13/2011 08:39:53

Next scn: 0xffff.ffffffff 01/01/1988 00:00:00

Enabled scn: 0x0000.001489a6 (1345958) 04/27/2011 09:59:40

SQL> alter system switch logfile;

系統已更改。

SQL> select * from v$log;

GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS

---------- ---------- ---------- ---------- ---------- --- ----------------

FIRST_CHANGE# FIRST_TIME

------------- --------------

1 1 141 52428800 1 YES INACTIVE

6043116 13-7月 -11

2 1 142 52428800 1 YES ACTIVE

6043132 13-7月 -11

3 1 143 52428800 1 NO CURRENT

6058893 13-7月 -11

GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS

---------- ---------- ---------- ---------- ---------- --- ----------------

FIRST_CHANGE# FIRST_TIME

------------- --------------

4 1 140 52428800 1 YES INACTIVE

6042663 13-7月 -11

SQL> alter system dump logfile 'D:\ORACLE\PRODUCT\10.2.0\ORADATA\WEIBIN\REDO02.LOG';

系統已更改。

檢視TRC:

DUMP OF REDO FROM FILE 'D:\ORACLE\PRODUCT\10.2.0\ORADATA\WEIBIN\REDO02.LOG'

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=4184897545=0xf9707809, Db Name='WEIBIN'

Activation ID=4203641025=0xfa8e78c1

Control Seq=7293=0x1c7d, File size=102400=0x19000

File Number=2, Blksiz=512, File Type=2 LOG

descrip:"Thread 0001, Seq# 0000000142, SCN 0x0000005c35fc-0x0000005c738d"

thread: 1 nab: 0x639b seq: 0x0000008e hws: 0x2 eot: 0 dis: 0

resetlogs count: 0x2cad510c scn: 0x0000.001489a6 (1345958)

resetlogs terminal rcv count: 0x0 scn: 0x0000.00000000

prev resetlogs count: 0x2b8f370b scn: 0x0000.00089c75 (564341)

prev resetlogs terminal rcv count: 0x0 scn: 0x0000.00000000

Low scn: 0x0000.005c35fc (6043132) 07/13/2011 08:39:53

Next scn: 0x0000.005c738d (6058893) 07/13/2011 15:19:39

從以上紅色部分我們可以注意到LOG FILE2的LOW SCN為6043132，NEXT SCN為6058893，我們可以知道LOGFILE2包含SCN介於6043132和6058893之間的重做資訊。我們之前查出的當前SCN(6057820）也包括在LOGFILE2中。

來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/25964700/viewspace-702117/，如需轉載，請註明出處，否則將追究法律責任。

重做日誌檔案中的SCN

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