title: Add replica set to already running mongo shard cluster
body: <p>Hi All i have set up a mongo cluster using thsi configuration</p>
  
  <ol>
  <li>I have 3 different machines with mongos running on them (all are physical machines with different IPs to ensure stability/ performance/ availability ).</li>
  <li>3 config servers on 3 diffrent VMs.</li>
  <li>Two Mongo shards running on high end machine( Two mongod instance running on two ports both mongod has separate db directories).</li>
  </ol>
  
  <p>This set is in place and already running.</p>
  
  <p>I want to add replica set to these two shard now means each shard will have two replica.
  My question is </p>
  
  <ol>
  <li>How can i add replica set to existing cluster when each shard is
  already</li>
  <li>Each Replica will carry the same db space as its corresponding shard
  having almost 2 TB data with them?</li>
  <li>Should i use different machine to host these replica to ensure fail over?</li>
  </ol>