@Override
    public void map(NullWritable key, Cell value, Context context)
      throws IOException, InterruptedException {
      context.write(new ImmutableBytesWritable(CellUtil.cloneRow(value)),
        new MapReduceExtendedCell(PrivateCellUtil.ensureExtendedCell(value)));
    }
A mapper that just writes out cells. This one can be used together with {@link CellSortReducer}
hbase-backup/src/main/java/org/apache/hadoop/hbase/backup/mapreduce/MapReduceHFileSplitterJob.java