Mitigate Performance Issues with PSA Replica Set缓解PSA副本集的性能问题
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Overview概述
In a three-member replica set with a primary-secondary-arbiter (PSA) architecture or a sharded cluster with three-member PSA shards, a data-bearing node that is down or lagged can lead to performance issues.在具有主-辅助仲裁器（PSA）架构的三成员副本集中，或具有三成员PSA分片的分片集群中，数据承载节点出现故障或滞后可能会导致性能问题。
If one data-bearing node goes down, the other node becomes the primary. Writes with 如果一个数据承载节点出现故障，另一个节点将成为主节点。w:1 continue to succeed in this state but writes with write concern "majority" cannot succeed and the commit point starts to lag. w:1的写入在此状态下继续成功，但写入关注"majority"的写入无法成功，并且提交点开始滞后。If your PSA replica set contains a lagged secondary and your replica set requires two nodes to majority commit a change, your commit point also lags.如果您的PSA副本集包含一个滞后的次要副本，并且您的副本集需要两个节点才能多数提交更改，那么您的提交点也会滞后。
With a lagged commit point, two things can affect your cluster performance:对于滞后提交点，有两件事会影响集群性能：
The storage engine keeps all changes that happen after the commit point on disk to retain a durable history.存储引擎将提交点之后发生的所有更改保存在磁盘上，以保留持久的历史记录。The extra I/O from these writes tends to increase over time. This can greatly impact write performance and increase cache pressure.这些写入的额外I/O往往会随着时间的推移而增加。这会极大地影响写入性能并增加缓存压力。MongoDB allows the oplog to grow past its configured size limit to avoid deleting theMongoDB允许oplog超过其配置的大小限制，以避免删除大多数提交点。majority commit point.
To reduce the cache pressure and increased write traffic, set 要降低缓存压力并增加写入流量，请为不可用或滞后的节点设置votes: 0 and priority: 0 for the node that is unavailable or lagging. votes: 0和priority: 0。For write operations issued with "majority", only voting members are considered to determine the number of nodes needed to perform a majority commit. 对于使用“多数”发出的写入操作，只考虑有投票权的成员来确定执行多数提交所需的节点数。Setting the configuration of the node to 将节点的配置设置为votes: 0 reduces the number of nodes required to commit a write with write concern "majority" from two to one and allows these writes to succeed.votes: 0将提交具有写入关注"majority"的写操作所需的节点数量从2个减少到1个，并允许这些写操作成功。
Once the secondary is caught up, you can use the 一旦二次投票被赶上，您可以使用rs.reconfigForPSASet() method to set votes back to 1.rs.reconfigForPSASet()方法将votes设置回1。
In earlier versions of MongoDB, 在MongoDB的早期版本中，enableMajorityReadConcern and --enableMajorityReadConcern were configurable allowing you to disable the default read concern "majority" which had a similar effect.enableMajorityReadConcern和--enableMajorityReadConcern是可配置的，允许您禁用具有类似效果的默认读取关注"majority"。
Procedure过程
To reduce the cache pressure and increased write traffic for a deployment with a three-member primary-secondary-arbiter (PSA) architecture, set 要为具有三成员主-辅助仲裁器（PSA）体系结构的部署降低缓存压力并增加写入流量，请为不可用或滞后的辅助仲裁器设置{ votes: 0, priority: 0 } for the secondary that is unavailable or lagging:{ votes: 0, priority: 0 }：
cfg = rs.conf();
cfg["members"][<array_index>]["votes"] = 0;
cfg["members"][<array_index>]["priority"] = 0;
rs.reconfig(cfg);
If you want to change the configuration of the secondary later, use the 如果以后要更改辅助的配置，请使用rs.reconfigForPSASet() method.rs.reconfigForPSASet()方法。