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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配置分片
｜
261
15.4
均衡器
均衡器
负责数据的迁移。它会定期检查分片之间是否存在不均衡，如果存在，就会对块进
行迁移。在
MongoDB 3.4
以上的版本中
，均衡器位于配置服务器副本集的主节点成员上，
而在
MongoDB 3.4
及之前的版本中，每个
mongos
会偶尔扮演“均衡器”的角色。
均衡器是配置服务器副本集主节点上的后台进程，它会监视每个分片上的块数量。只有当
一个分片的块数量达到特定迁移阈值时，均衡器才会被激活。
在
MongoDB 3.4
以上的版本中，可以并发执行的迁移数量增加到了每个分片
一个迁移，并发迁移的最大数量是分片总数的一半。而在早期版本中，只支
持全局进行一个并发迁移。
假设一些集合已经达到了阈值，则均衡器会开始对块进行迁移。它会从负载较大的分片中
选择一个块，并询问该分片是否应该在迁移之前对块进行拆分。在完成必要的拆分后，就
会将块迁移到具有较少块的机器上。
使用集群的应用程序不需要感知数据的迁移：所有读写请求都会被路由到旧的块上，直到
迁移完成。一旦元数据被更新，任何试图访问旧位置数据的
mongos
进程都会收到一个错
误。这个错误对客户端是不可见的：
mongos
会默默地处理这个错误并在新的分片上重试
此操作。
有时可能会在
mongos
日志中看到
“
unable to
setShardVersion
”的信息，这是一个常见的
错误。当
mongos
收到这种类型的错
误时，它会从配置服务器查找数据的新位置，并更新
块分布表，然后重新执行之前的请求。如果成功从新位置检索到数据，则会将数据返回给 ...
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I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
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.