title: CephFS: mount fails with "can't read superblock"
body: <p>I have set up Ceph Jewel on 3 nodes equipped with Ubuntu Server 14.04. </p>
  
  <pre><code>ceph -v
  ceph version 10.2.1 (3a66dd4f30852819c1bdaa8ec23c795d4ad77269)
  </code></pre>
  
  <p>The cluster is up and running:</p>
  
  <pre><code>ceph -s
  cluster cb3537cf-05a0-4411-840d-ab5b8d855579
   health HEALTH_OK
   monmap e3: 3 mons at {ceph-monitor=192.168.0.173:6789/0,ceph-node01=192.168.0.171:6789/0,ceph-node02=192.168.0.172:6789/0}
          election epoch 28, quorum 0,1,2 ceph-node01,ceph-node02,ceph-monitor
    fsmap e25: 1/1/1 up {0=ceph-monitor=up:active}
   osdmap e55: 3 osds: 3 up, 3 in
          flags sortbitwise
    pgmap v302: 368 pgs, 9 pools, 3704 bytes data, 191 objects
          120 MB used, 15206 MB / 15326 MB avail
               368 active+clean
  </code></pre>
  
  <p>I have set up a separate client node. When I try to mount the filesystem</p>
  
  <pre><code>mount -t ceph 192.168.0.173:6789:/ /mnt/cephfs -o name=admin,secret=AQC3QjdXi1e3LBAAFzrYKXKz6oupPGXaKaW1cQ==
  </code></pre>
  
  <p>I get this output (after a long delay):</p>
  
  <pre><code>mount: 192.168.0.173:6789:/: can't read superblock
  </code></pre>
  
  <p>All services seem to be running:</p>
  
  <pre><code>ps -A | grep ceph
  1207 ?        00:00:00 ceph-mon
  1208 ?        00:00:00 ceph-mds
  1375 ?        00:00:00 ceph-osd
  </code></pre>
  
  <p>Has anybody an idea what is going on?</p>