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理解redo(9)10g R2后oracle对redo写的增强

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Release: 2016-06-07 17:58:20
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10G R2后,Oracle提供了一个新参数commit_write来控制客户端commit后,ORACLE写日志的行为。这个参数值有四种组合。 immediate , wait 注释:Server process在提交时使用semctl发送信号给LGWR,通知其Flush Log Buffer,并等待LGWR发回的消息(semtimedop). LGWR

10G R2后,Oracle提供了一个新参数commit_write来控制客户端commit后,ORACLE写日志的行为。这个参数值有四种组合。

·  immediate , wait

注释:Server process在提交时使用semctl发送信号给LGWR,通知其Flush Log Buffer,并等待LGWR发回的消息(semtimedop).

LGWR在接收到消息后, Flush log buffer到磁盘,通过semctl发送信号给Server Process,告知其commit成功.

·  immediate , nowait

·  batch , wait

注释:Server Process在提交时使用semctl发送信号给LGWR,要求其提交后通知此Server Process进程,并等待此消息返回(运行semtmdop)

LGWR在经过一定的内部时间(Timeout)后, Flush Log Buffer到磁盘,并使用semctl通知此Server Process进程

·  batch , nowait

传统的,我们的默认写方式都是immediate , wait。代表server process立即通知LGWR刷新日志,LGWR刷新完log buffer后通知server process,COMMIT成功。

如果用了nowait选项,your session doesn’t send a message to lgwr at all。不管是batch,还是immediate,在指定nowait时候,事务都是在in the near future进行提交,

而不是立即提交。因为,由下图可知:Immediate nowait和Batch nowait的redo synch writes次数都是0,即没有立即进行日志写。
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