write-ahead protocol (116)

Before DBWn can write a modified buffer, all redo records
associated with the changes to the buffer must be written to disk (the
write-ahead protocol). If DBWn finds that some redo records have not
been written, it signals LGWR to write the redo records to disk and
waits for LGWR to complete writing the redo log buffer before it can
write out the data buffers.

寫優先協議
當DBWn寫出modified buffer之前 , 日誌記錄中與buffer中相關的需要先寫入磁碟 ,
如果DBWn發現有日誌記錄還沒有寫出 , 它將通知LGWR將這些日誌寫入磁碟

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