Oracle 隐含参数 _disable_logging 详解

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提示：Oracle 的隐含参数只应该在测试环境或者在 Oracle Support 的支持下使用。
从 Oracle 的系统表中，我们知道 Oracle 存在一个隐含参数_disable_logging 可以用于禁用日志生成，这个参数显然只能用于测试目的（可以极大提高 Benchmark 测试的性能），禁止日志生成必然导致事务的不可恢复性, 而且会导致日志文件损坏。

SQL> select ksppinm,ksppdesc from x$ksppi where ksppinm like '%logging';

KSPPINM              KSPPDESC
-------------------- ------------------------------
_disable_logging     Disable logging

因为特殊的需要，对这个参数进行了一点简单测试:

1. 设置参数

[oracle@jumper bdump]$ sqlplus "/ as sysdba"

SQL*Plus: Release 9.2.0.4.0 - Production on Wed Oct 19 11:01:19 2005

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.


Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production

SQL> alter system set "_disable_logging"=true scope=both;

System altered.

2. 事务测试

SQL> create table t as select * from dba_users;

Table created.

SQL> select count(*) from t;

  COUNT(*)
----------
        12

SQL> shutdown abort;
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area   97588504 bytes
Fixed Size                   451864 bytes
Variable Size              33554432 bytes
Database Buffers           62914560 bytes
Redo Buffers                 667648 bytes
Database mounted.
Database opened.
SQL> select count(*) from t;
select count(*) from t
                     *
ERROR at line 1:
ORA-00942: table or view does not exist

由于未产生相应日志，数据库 crash 或 shutdown abort 之后，上一次成功完成的检查点之后变化的数据将无法恢复。

Wed Oct 19 20:38:38 2005
Beginning crash recovery of 1 threads
Wed Oct 19 20:38:38 2005
Started first pass scan
Wed Oct 19 20:38:39 2005
Completed first pass scan
 0 redo blocks read, 0 data blocks need recovery
Wed Oct 19 20:38:39 2005
Started recovery at
 Thread 1: logseq 2, block 201, scn 0.897632464
Recovery of Online Redo Log: Thread 1 Group 1 Seq 2 Reading mem 0
  Mem# 0 errs 0: /opt/oracle/oradata/conner/redo01.log
Wed Oct 19 20:38:39 2005
Completed redo application
Wed Oct 19 20:38:39 2005
Ended recovery at
 Thread 1: logseq 2, block 201, scn 0.897652465
 0 data blocks read, 0 data blocks written, 0 redo blocks read
Crash recovery completed successfully

更多 Oracle 相关信息见Oracle 专题页面 http://www.linuxidc.com/topicnews.aspx?tid=12

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