The Log_buffer Default Size Cannot Be Reduced In 10g R2 [ID 351857.1]-优快云博客

在Oracle 10gR2中，无论如何设置log_buffer参数，其大小通常保持在约14MB。这是因为在10gR2版本里，固定SGA区域与重做日志缓冲区被合并，并且如果分配后的粒度中有剩余空间，则会被加入到重做日志缓冲区中。

The Log_buffer Default Size Cannot Be Reduced In 10g R2 [ID 351857.1]

		Modified 01-JUN-2010 Type PROBLEM Status MODERATED

In this Document
  Symptoms
  Cause
  Solution
  References

Platforms: 1-914CU;

This document is being delivered to you via Oracle Support's Rapid Visibility (RaV) process and therefore has not been subject to an independent technical review.

Applies to:

Oracle Server - Enterprise Edition - Version: 10.2.0.1 and later [Release: 10.2 and later ]
Information in this document applies to any platform.

Symptoms

In customer environment, no matter what value is specified for the log_buffer, the buffer size remains at around 14mbs.

In 10G R2 Reference manual, it states that this parameter can no longer be modified and its set by default to 512K or 128K * the cpu count in the server. The document is in error.

Cause

In 10G R2, Oracle combines fixed SGA area and redo buffer [log buffer] together. If there is a free space after Oracle puts the combined buffers into a granule, that space is added to the redo buffer. Thus you see redo buffer has more space than expected. This is an expected behavior.

"In 10.2 the log buffer is rounded up to use the rest of the granule...

Solution

The Log_buffer will be set by default, by Oracle internal algorithm and can no longer be made smaller. However, the buffer size can still be increased from the default setting..
ie:.
Total System Global Area 612368384 bytes
Fixed Size 1290916 bytes
Variable Size 297799004 bytes
Database Buffers 310378496 bytes
Redo Buffers 2899968 bytes <--- default value

SQL> alter system set log_buffer=20000000 scope=spfile;
System altered.

Total System Global Area 612368384 bytes
Fixed Size 1290796 bytes
Variable Size 276827604 bytes
Database Buffers 310378496 bytes
Redo Buffers 23871488 bytes <--- note the increased value

SQL> alter system set log_buffer=1048576 scope=spfile;
System altered.

Total System Global Area 612368384 bytes
Fixed Size 1290916 bytes
Variable Size 276827484 bytes
Database Buffers 331350016 bytes
Redo Buffers 2899968 bytes <--- this should only be 1mb, but it does not get lower than the calculated default