title: How does mapreduce work under HDFS replication mechanism, etc
body: <p>I am learning Hadoop and have three questions regarding to how dfs.replication and mapreduce work together, etc. Suppose replication = 1, so each block will be copied to two salves/data nodes (DN) to process data. </p>
  
  <p>Question1: how do master/name node (NN) and each DN know which DN should works on which block ( e.g. should DN1 processes block1 or DN2 do that?)</p>
  
  <p>Question2: if DN1 fails, NN will not know that immediately. It will know it after like 10 minutes, according a PPT I read. Then when NN reassigns the task to DN2, how does NN and DN2 know from which point should DN2 start to catch up?</p>
  
  <p>Question3: with the concept of clouding, DN can be anywhere in the world. and therefore it will lead to heavy traffics in order to send large amounts of blocks from a client to a DN if they are far from each other. will it be a big burden to the internet if many servers are doing this kind of work? </p>