mapred.compress.map.output = true
map output files are compressed but when the in memory merger closes
on the reduce the on disk merger runs to reduce input files to <= io.sort.factor if needed.
when this happens it outputs files called intermediate.x files these
do not maintain compression setting the writer (o.a.h.mapred.Merger.class line 432)
passes the codec but I added some logging and its always null map output compression set true or false.
This causes task to fail if they can not hold the uncompressed size of the data of the reduce its holding
I thank this is just and oversight of the codec not getting set correctly for the on disk merges.
Just before the creation of the writer o.a.h.mapred.Merger.class line 432
and it outputs the second line above.
I have confirmed this with the logging and I have looked at the files on the disk of the tasktracker. I can read the data in
the intermediate files clearly telling me that there not compressed but I can not read the map.out files direct from the map output
telling me the compression is working on the map end but not on the on disk merge that produces the intermediate.
I can see no benefit for these not maintaining the compression setting and as it looks they where intended to maintain it.