Oracle体系结构及备份（八）——sga-log_buffer

一 什么是Log Buffer

 

        Log Buffer，重做日志缓冲区，暂存数据库中所有数据块的改变。内存管理方法是FIFO。重做日志被用于提供数据恢复功能，暂存重做日志的目的是为了提高语句的执行速度。大小由参数LOG_BUFFER决定，但这个内存不能动态调整大小。

 

        log buffer space

        This event occurs when server processes are waiting for free space in the log buffer, because all the redo is generated faster than LGWR can write it out.

 

        Actions

 

        Modify the redo log buffer size. If the size of the log buffer is already reasonable, then ensure that the disks on which the online redo logs reside do not suffer from I/O contention. The log buffer space wait event could be indicative of either disk I/O contention on the disks where the redo logs reside, or of a too-small log buffer. Check the I/O profile of the disks containing the redo logs to investigate whether the I/O system is the bottleneck. If the I/O system is not a problem, then the redo log buffer could be too small. Increase the size of the redo log buffer until this event is no longer significant.

 

 

二 操作示例

--不能被sga_target自动管理，需要手动设置
--1M——2M，不能设置太大，也不需设置太大
SQL> show parameter log_buffer;

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
log_buffer			     integer	 2923520
SQL> SELECT 2923520/1024/1024 FROM dual;

2923520/1024/1024
-----------------
       2.78808594

三 总结

 

        1.Log Buffer，重做日志缓冲区，暂存数据库中所有数据块的改变。

        2.可以通过参数log_buffer查询日志缓冲区的大小。

 

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