详解Oracle中的隐含参数_disable_logging
注意：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。
3.观察alert文件
从日志中我们可以看到在instance recovery中，没有数据被恢复，只有成功完成的上次检查点之前数据可以被获取，之后数据都将丢失。
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
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