log file sync和log file parallel write的关系

     log file sync：

     When a user session commits, the session's redo information needs to be flushed to the redo logfile. The user session will post the LGWR to write the log buffer to the redo log file. When the LGWR has finished writing, it will post the user session.

Wait Time: The wait time includes the writing of the log buffer and the post.

     log file parallel write：

Writing redo records to the redo log files from the log buffer.

Wait Time: Time it takes for the I/Os to complete. Even though redo records are written in parallel, the parallel write is not complete until the last I/O is on disk.

      过程可以这样描述：

       1、进程接到提交请求

       2、进程通过LGWR写REdo数据到Redo文件

       3、LGWR写Redo到文件

       4、LGWR写完通知进程

       5、进程收到写完成通知，向用户端发送提交完成。

      从步骤1开始，进程开始等待log file sync，到步骤5结束。步骤3、步骤4，LGWR等待log file parallel write。

      如果log file sync比log file parallel write大很大，有两个原因，一是CPU紧张，二是同时提交的进程多。