eXtremeDB事務日志版

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eXtremeDB事務日志版

　　事務日志功能
　　在嵌入式數(shù)據(jù)庫eXtremeDB2.3版本中，McObject通過引入事務日志功能，增強了其數(shù)據(jù)的一致性，事務日志就是當（通過事務）改變數(shù)據(jù)庫內(nèi)容時，將這個變化記錄下來的一個進程。當使用事務日志功能時，eXtremeDB捕獲數(shù)據(jù)庫的變化并將它們寫入到一個稱為事務日志的文件中。即使產(chǎn)生硬件或軟件錯誤，eXtremeDB也可以通過日志文件進行數(shù)據(jù)庫的恢復。日志記錄的實現(xiàn)是通過周期檢查點并將內(nèi)存數(shù)據(jù)庫的鏡像寫入非易失性存儲器中，而把這一改變數(shù)據(jù)庫的所有中間變化寫入日志文件中。通過這種方法，RAM數(shù)據(jù)庫保持了一致性。
　　事務日志功能并不改變eXtremeDB數(shù)據(jù)庫的純內(nèi)存架構(gòu)，這種架構(gòu)與基于磁盤的數(shù)據(jù)庫架構(gòu)相比擁有性能上的優(yōu)勢。事務日志功能不會影響讀取性能，寫入性能則會遠遠超出傳統(tǒng)的基于磁盤的數(shù)據(jù)庫。
　　產(chǎn)生這種情況的原因很簡單：eXtremeDB事務日志對于一個數(shù)據(jù)庫事務僅僅只需要一次文件系統(tǒng)的寫入。但是一個基于磁盤的數(shù)據(jù)庫對于每個事務會進行多個寫入（數(shù)據(jù)頁、索引頁、事務日志等），并且隨著事務規(guī)模的增大以及索引的增多，需要的寫入次數(shù)也會增加。

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