SeqWriter.add is waiting 10 sec in a worse case before calling a flush. This was there to guarantee that the data has been written to HDFS before returning the ack.
It used to work well with 0.18 but in 0.20 the flush is a nohup so there's no point in waiting 10 sec for nothing.
The real flush will give us the ability to write redo logs directly to HDFS so the ack implementation will have to change anyway.
So I vote for removing the 10 sec ack from the add method.