１。在 hadoop的配置文件 hdfs-site.xml 添加如下配置：
<property>
<name>dfs.client.read.shortcircuit</name>
<value>true</value>
</property>
<property>
<name>dfs.domain.socket.path</name>
<value>/home/yeshang/hdfs_sockets/dn_socket</value>
</property>
２。进入本地文件系统：
mkdir -p /home/yeshang/hdfs_sockets/
chmod 0755 /home/yeshang/hdfs_sockets
dn_socket 是系统写入，不用手动创建，否则会出问题
３。如果该配置生效 datanode的log会有如下log:
例如：Hadoop/logs/hadoop-yeshang-datanode-cluster2.serversolution.sh.hxt.log
2019-01-04 14:44:38,092 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Balancing bandwith is 1048576 bytes/s
2019-01-04 14:44:38,092 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Number threads for balancing is 5
2019-01-04 14:44:38,092 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Listening on UNIX domain socket: /home/yeshang/hdfs_sockets/dn_socket
2019-01-04 14:44:38,222 INFO org.mortbay.log: Logging to org.slf4j.impl.Log4jLoggerAdapter(org.mortbay.log) via org.mortbay.log.Slf4jLog