Generally to control output format from reducer you'd use multiple output class https://hadoop.apache.org/docs/r2.7.1/api/org/apache/hadoop/mapreduce/lib/output/MultipleOutputs.html You get best results by writing larger files, not sure what you benefit from splitting a dataset that fits your criteria into smaller chunks, job won't complete until all of the criteria is addressed and in fact I think you'll hurt performance by splitting what is by design a better approach.
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