使用oradebug增进scn

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本文介绍如何通过oradebug命令手动修改Oracle数据库的系统更改号(SCN)，包括使用oradebug poke命令的具体步骤，并提供了修改SCN的示例。

http://blog.itpub.net/29154652/viewspace-774532/

使用oradebug增进scn，网上案例很多，这里自己测试了下，发布上来。

首先来看下命令：
POKE
To poke memory locations use
ORADEBUG POKE address length value
where address and value can be decimal or hexadecimal and length is in bytes
For Example
    ORADEBUG POKE 0x20005F0C 4 0x46495845
    ORADEBUG POKE 0x20005F10 4 0x44205349
    ORADEBUG POKE 0x20005F14 2 0x5A45
WARNING Do not use the POKE command on a production system

查看数据库当前的SCN：
oracle@GDB_01:~/run> sqlplus / as sysdba

SQL*Plus: Release 11.2.0.2.0 Production on Thu Oct 17 12:19:54 2013
Copyright (c) 1982, 2010, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> select current_scn from v$database;

CURRENT_SCN
-----------
    2625512

SQL> select to_char(2625512,'xxxxxxx') from dual;

TO_CHAR(2625512,'XXXXXXX')
--------------------------------------------------------------------------------
280fe8

使用oradebug查看kcsgscn_
SQL> oradebug setmypid
Statement processed.

SQL> oradebug dumpvar sga kcsgscn_
kcslf kcsgscn_ [060019360, 060019390) = 00280FFC 00000000 00000000 00000000 00089F7B 00000000 00000000 00000000 00000000 00000000 60019040 00000000

跟V$DATABASE.CURRENT_SCN相近。
此外，我们使用的Linux平台，属于Little Endian，所以其SCN base在前，SCN wrap在后，这点需要注意。

假设假设我们要修改成1G大小：

SQL> select 1*1024*1024*1024 from dual;

1*1024*1024*1024
----------------
      1073741824

SQL> select to_char(1073741824,'xxxxxxxxx') from dual;

TO_CHAR(1073741824,'XXXXXXXXX')
--------------------------------------------------------------------------------
40000000

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area 4175568896 bytes
Fixed Size                  2233088 bytes
Variable Size            1560284416 bytes
Database Buffers         2566914048 bytes
Redo Buffers               46137344 bytes
Database mounted.

SQL> oradebug setmypid
Statement processed.
SQL> oradebug dumpvar sga kcsgscn_
kcslf kcsgscn_ [060019360, 060019390) = 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 60019040 00000000
在mount状态下，不能看到scn的信息。

我们在这里修改scn为1G。
SQL> oradebug poke 0x060019360 4 0x40000000
BEFORE: [060019360, 060019364) = 00000000
AFTER: [060019360, 060019364) = 40000000
SQL> oradebug dumpvar sga kcsgscn_
kcslf kcsgscn_ [060019360, 060019390) = 40000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 60019040 00000000
SQL> alter database open;

Database altered.

SQL> select current_scn from v$database;

CURRENT_SCN
-----------
1073742107

SQL> select current_scn/1024/1024/1024 from v$database;

CURRENT_SCN/1024/1024/1024
--------------------------
                1.00000027

修改成功。

备注：此命令危险，生产环境勿乱操作，可能后果很严重。
延伸阅读：
http://www.dbsnake.net/how_to_dirty_adjust_scn.html
oradebug：http://blog.chinaunix.net/attachment/attach/22/94/87/7322948773d50faeda7c405c55399f07a0acf48ecc.pdf