Tuesday, October 20, 2015

重复数据删除(De-duplication)技术研究 - 刘爱贵的专栏 - 博客频道 - CSDN.NET



重复数据删除(De-duplication)技术研究 - 刘爱贵的专栏 - 博客频道 - CSDN.NET

 存储系统的重复数据删除过程一般是这样的:首先将数据文件分割成一组数据块,为每个数据块计算指纹,然后以指纹为关键字进行Hash查找,匹配则表示该数据块为重复数据块,仅存储数据块索引号,否则则表示该数据块是一个新的唯一块,对数据块进行存储并创建相关元信息。这样,一个物理文件在存储系统就对应一个逻辑表示,由一组FP组成的元数据。当进行读取文件时,先读取逻辑文件,然后根据FP序列,从存储系统中取出相应数据块,还原物理文件副本。从如上过程中可以看出,Dedupe的关键技术主要包括文件数据块切分、数据块指纹计算和数据块检索。

 (1) 文件数据块切分

 Dedupe按照消重的粒度可以分为文件级和数据块级。文件级的dedupe技术也称为单一实例存储(SIS, Single Instance Store),数据块级的重复数据删除其消重粒度更小,可以达到4-24KB之间。显然,数据块级的可以提供更高的数据消重率,因此目前主流的dedupe产品都是数据块级的。数据分块算法主要有三种,即定长切分(fixed-size partition)、CDC切分(content-defined chunking)和滑动块(sliding block)切分。定长分块算法采用预先义好的块大小对文件进行切分,并进行弱校验值和md5强校验值。弱校验值主要是为了提升差异编码的性能,先计算弱校验值并进行hash查找,如果发现则计算md5强校验值并作进一步hash查找。由于弱校验值计算量要比md5小很多,因此可以有效提高编码性能。定长分块算法的优点是简单、性能高,但它对数据插入和删除非常敏感,处理十分低效,不能根据内容变化作调整和优化


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