November 2021
Intermediate to advanced
410 pages
12h 1m
Chinese
Content preview from MongoDB权威指南（第3版）
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持久性
｜
333
从
MongoDB 4.0
开始，当应用程序对副本集执行写操作时，
MongoDB
会使用与
oplog
1
相同的格式创建日志条目。正如第
11
章所讨论的，
MongoDB
使用了一种基于操作日志
（
oplog
）的语句级
别的复制机制。
oplog
中的语句是对写操作影响的每个文档所做的实际更
改的表示。因此，
oplog
语句很容易应用于副本集的其他成员
，而无须考虑版本、硬件或
副本集成员之间的其他差异。此外，每个
oplog
语句都是幂等的
，这意味着它可以被应用
任意次数，而对数据库的更改结果总是相同的。
像大多数数据库一样，
MongoDB
同时维护了日志和数据库数据文件的内存视图
。默认情
况下，它每
50
毫秒会将日志条目
刷新到磁盘上，每
60
秒会将数据库文件刷新到磁盘上。
刷新数据文件的
60
秒间隔称为
检查点
（
checkpoint
）
。日志用于为自上一个检查点以来写入
的数据提供持久性。关于持久性的问题，如果服务器突然停止了，那么在其重新启动时，
可以使用日志重放在关闭前没有刷新到磁盘的所有写操作。
对于日志文件，
MongoDB
在
dbPath
目录下创建了一个名为
journal
的子目录
。
WiredTiger
（
MongoDB
的默认存储引擎）日志文件的名称格式为
W
iredTigerLog.<sequence>
，其中
<sequence>
是一个从
0 000 000 001
开始的零填充数字。除了非常小的日志记录，
MongoDB
会对写入日志的数据进行压缩。日志文件的最大大小限制大约为
100MB
。一旦日志文件超
过这个限制，
MongoDB ...
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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.