开启HDFS的 dfs.client.read.shortcircuit 配置

１。在　hadoop的配置文件　hdfs-site.xml　添加如下配置：

        dfs.client.read.shortcircuit
        true


        dfs.domain.socket.path
        /home/yeshang/hdfs_sockets/dn_socket

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