hadoop3.x：HDFS 写流程-数据复制和副本节点（DataNode）选择 作者：马育民 • 2021-04-01 16:23 • 阅读：10250 # 机架（机柜） [](https://www.malaoshi.top/upload/pic/hadoop/QQ20211101162254.png) # 说明 第一块副本：优先客户端本地，否则随机 第二块副本：不同于第一块副本的不同机架。 第三块副本：第二块副本相同机架不同机器。 [](https://www.malaoshi.top/upload/pic/hadoop/QQ20211101162319.png) # 出处 ### 官方文档 http://hadoop.apache.org/docs/r3.0.3/hadoop-project-dist/hadoop-hdfs/HdfsDesign.html#Replica_Placement:_The_First_Baby_Steps For the common case, when the replication factor is three, HDFS’s placement policy is to put one replica on the local machine if the writer is on a datanode, otherwise on a random datanode, another replica on a node in a different (remote) rack, and the last on a different node in the same remote rack. This policy cuts the inter-rack write traffic which generally improves write performance. The chance of rack failure is far less than that of node failure; this policy does not impact data reliability and availability guarantees. However, it does reduce the aggregate network bandwidth used when reading data since a block is placed in only two unique racks rather than three. With this policy, the replicas of a file do not evenly distribute across the racks. One third of replicas are on one node, two thirds of replicas are on one rack, and the other third are evenly distributed across the remaining racks. This policy improves write performance without compromising data reliability or read performance. ### 类 默认副本存储策略是由 `BlockPlacementPolicyDefault` 指定 [](https://www.malaoshi.top/upload/pic/hadoop/QQ20211101163657.png) 原文出处：/show_1IX29G4S51pN.html