除非重做日志缓冲区特别满，否则每个事务处理只需要一个同步写入。
Unless the redo log buffer is particularly full, only one synchronous writes is required per transaction. if piggybacking occurs, there can be less than one synchronous write per transaction.
日志读写器(LGWr)——从日志缓冲区写入项目，以便重做日志文件。
Log Writer (LGWr) - Writes entries from log buffer to redo the log files.
在重做日志记录达到缓冲区的三分之一。
The redo log buffer is one-third full or contains 1 MB of buffered data.
在重做日志记录达到缓冲区的三分之一。
The redo log buffer is one-third full or contains 1 MB of buffered data.
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