Oracle 的 DBMS_SCN 修正以及 SCN 的 auto-rollover 新特性

在 Oracle 11.2.0.2 之后，随着一系列 SCN 耗尽问题的出现，很多补丁涌现出来，一个新的 Package 增加进来。

这个 Package 就是 DBMS_SCN。

如果你的数据库中存在这个 Package，也就意味着你已经安装具备了关于 DB Link 的修正补丁。

以下是这个包的主要函数过程以及说明，这个内容来自 Oracle 11.2.0.4 版本平台：

Rem Rem $Header: rdbms/admin/dbmsscnc.sql /st_rdbms_11.2.0/1 2013/04/18 23:05:40 vgokhale Exp $ Rem Rem dbmsscn.sql Rem Rem Copyright (c) 2012, 2013, Oracle and/or its affiliates. Rem All rights reserved. Rem Rem NAME Rem dbmsscnc.sql - dbms_scn package definition Rem Rem DESCRIPTION Rem Rem Rem NOTES Rem Rem Rem MODIFIED (MM/DD/YY) Rem mtiwary 05/26/12 - Declarations and definitions related to DBMS_SCN Rem package. Rem mtiwary 05/26/12 - Created Rem Rem Rem BEGIN SQL_FILE_METADATA Rem SQL_SOURCE_FILE: rdbms/admin/dbmsscn.sql Rem SQL_SHIPPED_FILE: Rem SQL_PHASE: Rem SQL_STARTUP_MODE: NORMAL Rem SQL_IGNORABLE_ERRORS: NONE Rem SQL_CALLING_FILE: Rem END SQL_FILE_METADATA SET ECHO ON SET FEEDBACK 1 SET NUMWIDTH 10 SET LINESIZE 80 SET TRIMSPOOL ON SET TAB OFF SET PAGESIZE 100 CREATE OR REPLACE LIBRARY DBMS_SCN_LIB TRUSTED AS STATIC; / CREATE OR REPLACE PACKAGE DBMS_SCN AUTHID CURRENT_USER IS DBMS_SCN_API_MAJOR_VERSION CONSTANT NUMBER := 1; DBMS_SCN_API_MINOR_VERSION CONSTANT NUMBER := 0; PROCEDURE GetCurrentSCNParams( rsl OUT number, headroom_in_scn OUT number, headroom_in_sec OUT number, cur_scn_compat OUT number, max_scn_compat OUT number); -- Currently no exceptions are thrown. -- rsl - Reasonable SCN Limit as of 'now' -- headroom_in_scn - Difference between current SCN and RSL -- headroom_in_sec - number of seconds it would take to reach RSL -- assuming a constant SCN consumption rate associated -- with current SCN compatibility level -- cur_scn_compat - current value of SCN compatibility -- max_scn_compat - max value of SCN compatibility this database -- understands FUNCTION GetSCNParamsByCompat( compat IN number, rsl OUT number, headroom_in_scn OUT number, headroom_in_sec OUT number ) RETURN boolean; -- compat -- SCN compatibility value -- rsl -- Reasonable SCN Limit -- headroom_in_scn -- Difference between current SCN and RSL -- headroom_in_sec -- number of seconds it would take to reach RSL -- assuming a constant SCN consumption rate associated -- with specified database SCN compatibility -- -- Returns False if 'compat' parameter value is invalid, and OUT parameters -- are not updated. PROCEDURE GetSCNAutoRolloverParams( effective_auto_rollover_ts OUT DATE, target_compat OUT number, is_enabled OUT boolean); -- effective_auto_rollover_ts - timestamp at which rollover becomes -- effective -- target_compat - SCN compatibility value this database -- will move to, as a result of -- auto-rollover -- is_enabled - TRUE if auto-rollover feature is -- currently enabled PROCEDURE EnableAutoRollover; PROCEDURE DisableAutoRollover; END DBMS_SCN; /

这里就可以看到 auto-rollover 的自动 SCN 兼容性终止时间，也就是说，在不同的兼容性设置中，SCN 的算法不同，但是内置了天然的算法过期时间。

在此之后，可以通过命令修改数据库的 SCN 兼容性算法：

SQL> ALTER DATABASE SET SCN COMPATIBILITY 2;

Database altered.

从高级别向低级别修改，需要数据库在 Mount 状态：

SQL> ALTER DATABASE SET SCN COMPATIBILITY 2; ALTER DATABASE SET SCN COMPATIBILITY 2 * ERROR at line 1: ORA-01126: database must be mounted in this instance and not open in any instance SQL> shutdown immediate; Database closed. Database dismounted. ORACLE instance shut down. SQL> startup mount; ORACLE instance started. Total System Global Area 4609830912 bytes Fixed Size 2260888 bytes Variable Size 989855848 bytes Database Buffers 3607101440 bytes Redo Buffers 10612736 bytes Database mounted. SQL> ALTER DATABASE SET SCN COMPATIBILITY 2; Database altered. SQL> alter database open; Database altered.

这是一个非常重要的变化。