hadoop hdfs 机架感知（副本存储节点选择）

1.机架感知说明

http://hadoop.apache.org/docs/r3.1.3/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 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.

2. Hadoop3.1.3副本节点选择

   
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