se there might be parallel threads (parfor local) that
 access the same object (in case it was created before the loop).
 If all threads export the same data object concurrently it results in errors
 because they all write to the same file. Efficiency for loops and parallel threads
 is achieved by checking if the in-memory block is dirty.
 
 NOTE: MB: we do not use dfs copy from local (evicted) to HDFS because this would ignore
 the output format and most importantly would bypass reblocking during write (wh