How to save only non empty reducers' output in HDFS

It is possible - see the documentation section on "Lazy Output":

import org.apache.hadoop.mapreduce.lib.output.LazyOutputFormat;LazyOutputFormat.setOutputFormatClass(job, TextOutputFormat.class);

hadoop reduce hdfs

If you're using the old API, you can use the NullOutputFormat class:

import org.apache.hadoop.mapred.lib.NullOutputFormat;conf.setOutputFormat(NullOutputFormat.class);

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