May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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日志结构存储
｜
137
顺序的方式有所不同：B 树的页是原地更新的，FD 树的有序段是通过合并两个有序段来
创建的，而 SSTable 通过在内存中缓冲和排序数据记录而创建。
在本节中，我们将讨论按随机顺序存储数据记录的结构。无序存储一般不需要单独的日
志，并且允许我们按插入顺序存储数据记录以减少写入的开销。
7.4.1 Bitcask
Bitcask 是 Riak 中使用的存储引擎之一，它是一个无序的日志结构存储引擎 [SHEEHY10b]。
与此前讨论的日志结构存储实现不同，它不使用 memtable 进行缓冲，而是将数据记录
直接存储在日志文件中。
为了使值可被搜索，Bitcask 使用一种名为
keydir
（键目录）的数据结构，它保存与键相
应的
最新
数据记录的引用。旧的数据记录可能仍然在磁盘中，但不会被 keydir 引用，并
且它们会在压实过程中被垃圾收集。keydir 被实现为内存中的哈希表，并且在启动期间
必须从日志文件中重建。
在一次
写入
中，键和数据记录被顺序地追加到日志文件中，并且会在 keydir 中放入指向
新写入的数据记录位置的指针。
读取时会查看 keydir 以定位要搜索的键，并跟随指向对应日志文件的指针以定位数据记
录。由于在任何给定时刻都只能有一个值与 keydir 中的键相关联，所以点查询不必合并
来自多个数据源的数据。
图 7-11 展示了 Bitcask 的数据文件中键和记录之间的映射。日志文件保存数据记录，而
keydir 则指向与每个键相关联的最新
活动
数据记录。数据文件中被遮蔽的记录 ...
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