/* open a MapFileWriter for the specified key. This will automatically promoted to the index folder when the job is finished */
protected MapFileWriter openMapFileWriter(String keyName) throws IOException
{
final int metaKeyIndex = key2reverseOffset.get(keyName);
final int valueLength = key2valuelength.get(keyName);
keyFactory = new FixedSizeTextFactory(valueLength);
//logger.info("Opening MapFileWriter for key "+ keyName + " - index " + metaKeyIndex);
return FSOrderedMapFile.mapFileWrite(reduceTaskFileDestinations.toString() /*index.getPath()*/
+ "/" + index.getPrefix() + "."
+ jc.get("MetaIndexInputStreamRecordReader.structureName")
+ "-"+metaKeyIndex+FSOrderedMapFile.USUAL_EXTENSION