Inconsistent IO speed with dd command

Issue

• When I write to the disk and overwriting the file I get significant performance drop:

      # time sh -c "dd if=/dev/zero of=ddfile bs=8k count=100000 ; sync"
      100000+0 records in
      100000+0 records out
      819200000 bytes (819 MB) copied, 0.778416 s, 1.1 GB/s
  
      real    0m9.528s
      user    0m0.018s
      sys     0m0.594s
  
      # time sh -c "dd if=/dev/zero of=ddfile bs=8k count=100000 ; sync"
      100000+0 records in
      100000+0 records out
      819200000 bytes (819 MB) copied, 7.76798 s, 105 MB/s
  
      real    0m9.376s
      user    0m0.011s
      sys     0m1.084s