I'd like to understand how HDFS handle Datanode failure gracefully. Let's suppose a replication factor of 3 is used in HDFS for this discussion.
After 'DataStreamer' receives a list of Datanodes A, B, C for a block, it starts pulling data packets off the 'data queue' and putting it onto 'ack queue' after sending them off the wire to those Datanodes (using a pipeline mechansim Client -> A -> B -> C). If the Datanode B crashes during the writing, why the client need to put the data packets in the 'ack queue' back to the 'data queue'? (how can the client guarantee the order of resent packet on Datanode A after all?) I guess I have not fully understood the write failure handling mechanism yet. Can someone give a detailed explanation? Thanks, -- --Sean -- --Sean
