title: What's the usage and behaviour of w:majority on half cluster down?
body: <p>considering a replica set composed of 5 members :</p>
  
  <ul>
  <li>R1,R2,R3,R4 being replicas</li>
  <li>A1 being an arbiter</li>
  </ul>
  
  <p>If we use w:majority as write concern in our application :</p>
  
  <ul>
  <li>How many replicas will be used before ack write when all members are up and running ?</li>
  <li>If 2 replicas out of 4 crash, how is handled 'majority' requirement :
  <ul>
  <li>does it mean we still wait for write completion on the same number of replicas, meaning the write is blocked, waiting for a members to turn on.</li>
  <li>does arbiter as an effet on this particular writeConcern and downsize the number of replicas required to satisfy w:majority writeConcern ?</li>
  </ul></li>
  </ul>
  
  <p>I understand replication w: but majority seems to be a special case :
  <a href="http://docs.mongodb.org/manual/core/write-concern/#write-operations-write-concern" rel="nofollow">http://docs.mongodb.org/manual/core/write-concern/#write-operations-write-concern</a>
  "For the purposes of write concern, majority refers to a majority of the votes in the set. As a result, arbiters affect the definition of majority, in order to help prevent rollback."</p>
  
  <p>Thanks for your help.</p>