May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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138
｜
第
7
章
数据文件
图
7-11
：
Bitcask
中
keydir
和数据文件之间的映射（实线表示从键到与其关联的最新值的指针，
遮蔽的键值对以浅灰色显示）
7.4.2 WiscKey
范围查询对于许多应用程序都很重要，如果有一种存储结构既有无序存储的写入和空间
优势，同时还支持执行范围扫描就好了。
WiscKey[LU16] 通过在 LSM 树中保存键排序并在称为
vLog
（值日志）的无序仅追加文
件中保存数据记录，来将排序与垃圾收集解耦。这种方法可以解决讨论 Bitcask 时提到
的两个问题：需要将所有键保留在内存中，以及需要在启动时重新构建哈希表。
图 7-12 展示了 WiscKey 的关键组件，以及键和日志文件之间的映射。vLog 文件保存无
序数据记录。键存储在排序的 LSM 树中，指向日志文件中最新的数据记录。
由于键通常比与其相关联的数据记录小得多，所以压实它们的效率要高得多。这种方法对
于更新和删除较少的场景特别有用，在这种情况下，垃圾收集不会释放太多的磁盘空间。
这里的主要挑战是，由于 vLog 数据是未排序的，所以范围扫描需要随机 I/O。WiscKey
在范围扫描时使用固态硬盘的内部并行性来并行预取数据块，以减少随机 I/O 的开销。
在数据块传输方面，其成本仍然很高：在范围扫描期间，要获取一条的数据记录，必须
读取该数据记录所在的整个页。
在压实过程中，vLog 文件的内容被顺序读取，并在合并后写入新的位置。指针（键
LSM 树中的值）被更新以指向这些新位置。为了避免扫描整个 vLog ...
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