It would be easier to debug some corner in tasks if we knew what was the input split processed by that task. Map reduce task tracker log should accommodate the same. Also in the jobdetails web UI, the split also should be displayed along with the Split Locations.
hdfs://myserver:9000/userdata/sampleapp/inputdir/file1.csv - <split no>/<offset from beginning of file>
This would be much beneficial to nail down some data quality issues in large data volume processing.