You can use the following methods to access HDFS metrics using the Java Management Extensions (JMX) APIs.
Use the HDFS Daemon Web Interface
You can access JMX metrics through the web interface of an HDFS daemon. This is the recommended method.
For example, use the following command format to access the NameNode JMX:
curl -i http://localhost:50070/jmx
You can use the qry parameter to fetch only a particular key:
curl -i http://localhost:50070/jmx?qry=Hadoop:service=NameNode,name=NameNodeInfo
Directly Access the JMX Remote Agent
This method requires that the JMX remote agent is enabled with a JVM option when starting HDFS services.
For example, the following JVM options in hadoop-env.sh are used to enable the JMX remote agent for the
NameNode. It listens on port 8004 with SSL disabled. The user name and password are saved in the mxremote.password file.
export HADOOP_NAMENODE_OPTS="-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.password.file=$HADOOP_CONF_DIR/jmxremote.password -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.port=8004 $HADOOP_NAMENODE_OPTS"
Details about related settings can be found here. You can also use the jmxquery tool to retrieve information through JMX.
Hadoop also has a built-in JMX query tool, jmxget. For example:
hdfs jmxget -server localhost -port 8004 -service NameNode
Note that jmxget requires that authentication be disabled, as it does not accept a user name and password.
Using JMX can be challenging for operations personnel who are not familiar with JMX setup, especially JMX with SSL and firewall tunnelling. Therefore, it is generally recommended that you collect JXM information through the web interface of HDFS daemons rather than directly accessing the JMX remote agent.
