Stabilize the previous commit.

 - Running more than 10 minutes exhibit the same problem, but in smaller
   scale.  Adding DD flag check seems to be effective as I confirm no
   more problem occurs after 20 minutes in the machine with this commit
   while the other machine shows the problem after 8 minutes without
   this commit.  Those two machines have identical hardware
   configuration (CPU, RAM, M/B and NICs).