官方地址

http://hadoop.apache.org/docs/r2.7.2/hadoop-project-dist/hadoop-hdfs/HdfsDesign.html#Data_Replication

For the common case, when the replication factor is three, HDFS’s placement policy is to put one replica on one node in the local rack, another on a different node in the local rack, and the last on a different node in a different rack. 

大致意思：

        对于常见的情况，当复制因子为3时，HDFS的放置策略是将一个副本放在本地机架的一个节点上，另一个放在本地机架的不同节点上，最后一个放在不同机架的不同节点上。