Gluster users - Jun 2014 - why dd write fail, when nfs ha failover

Hi,all 
      I install two centos6.2 os,mount gluster  volume 
 ,and then expert path through Linux kernel  nfs ,I install ctdb for iP
failover,all is normal, when one node fail,another node takeover.
    But dd write Will fail, ,dd rewrite ok
    why write fail when takeover.


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?????????????