May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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简介与概述
｜
17
据库发展受限的情况可能发生改变。NVM 存储减少甚至完全消除了（取决于某个确切的
技术）读和写的延迟之间的不对称，进一步提高了读写性能，并允许字节可寻址（byte-
addressable）访问。
基于内存存储的持久性
内存数据库系统在磁盘上维护备份，以提供持久性并防止易失性数据的丢失。有些数据
库只在内存中存储数据，没有任何持久性保证，但在本书的范围内我们不讨论它们。
数据库在认定操作完成之前，必须先将其结果写入一个顺序日志文件。我们将在 5.2 节
中更详细地讨论预写日志。为了避免在启动过程中或崩溃后重放完整的日志内容，内存
数据库维护了一个
备份副本
，该备份副本使用一个基于磁盘且已排序的数据结构，并且
对该结构的修改通常是异步（与客户端请求解耦）且分批处理的，这样可以减少 I/O 操
作的数量。在恢复过程中，数据库可以从备份和日志中还原数据库内容。
日志数据通常用于批量备份。在处理该批日志数据之后，备份将持有截止到这一特
定时间点的数据库
快照
，因此可以丢弃之前的日志内容。这个过程称为
生成检查点
（checkpointing）。它会更新磁盘上的数据库快照使其更接近最新的日志条目，从而缩短
恢复时间，且备份过程不会阻塞客户端请求。
一些说法认为内存数据库相当于一个具有巨大页缓存的磁盘数据库，这是不公
平 的 （见 5.1 节）。即使页
缓存
在内存里，序列化格式和数据布局也会使磁盘
数据库产生额外的开销，而不会达到与内存数据库相同的优化程度。
磁盘数据库使用专门的存储结构，针对磁盘访问进行了优化 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.