Changing the Log4j debug with `hadoop jar` in MapReduce jobs_modify mapreduce log to debug-优快云博客

本文介绍如何在启动时为Apache Hadoop 2.x中的MapReduce任务设置Log4j日志级别，包括调整Mapper、Reducer及Application Master的日志输出级别，无需修改代码或全局配置文件。

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Short Description:
The proper way to set the Log4j level on MapReduce jobs at launch time for Apache Hadoop 2.x.
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One of the most common questions I come across when trying to help debug MapReduce jobs is: “How do I change the Log4j level for my job?” Many times, a user has a JAR with a class that implements Tool that they invoke using the hadoop jar command. The desire is to change the log level without changing any code or global configuration files:

hadoop jar MyApplication.jar com.myorg.application.ApplicationJob


hadoop jar MyApplication.jar com.myorg.application.ApplicationJob -Dmapreduce.map.log.level=DEBUG <args ...>
To set the WARN Log4j level on Reducers:


hadoop jar MyApplication.jar com.myorg.application.ApplicationJob -Dmapreduce.reduce.log.level=WARN <args ...>
To set the DEBUG Log4j level on the MapReduce Application Master:

hadoop jar MyApplication.jar com.myorg.application.ApplicationJob -Dyarn.app.mapreduce.am.log.level=DEBUG <args ...>
And, of course, each of these options can be used with one another:


hadoop jar MyApplication.jar com.myorg.application.ApplicationJob -Dmapreduce.map.log.level=DEBUG -Dmapreduce.reduce.log.level=DEBUG -Dyarn.app.mapreduce.am.log.level=DEBUG <args ...>