title: Hadoop 2.0 data write operation acknowledgement
body: <p>I have a small query regarding hadoop data writes</p>
  
  <p>From Apache documentation</p>
  
  <blockquote>
    <p>For the common case, when the replication factor is three, HDFS’s placement policy is to put one replica on one node in the local rack, another on a node in a different (remote) rack, and the last on a different node in the same remote rack. This policy cuts the inter-rack write traffic which generally improves write performance. The chance of rack failure is far less than that of node failure; </p>
  </blockquote>
  
  <p>In below image, when the write acknowledge is treated as successful?</p>
  
  <p>1) Writing data to first data node?</p>
  
  <p>2) Writing data to first data node + 2 other data nodes?</p>
  
  <p><a href="https://i.stack.imgur.com/KSWwv.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KSWwv.jpg" alt="Hadoop data write"></a></p>
  
  <p>I am asking this question because, I have heard two conflicting statements in youtube videos. One video quoted that write is successful once data is written to one data node &amp; other video quoted that acknowledgement will be sent only after writing data to all three nodes.</p>