title: Data destroy using shred agains ext4 filesystem
body: <p>I'm running shred against blockdevice with couple of etx4 filesystems on it. 
  The blockdevices are virtual drives - RAID-1 and RAID-5. Controller is PERC H710P. 
  command 
  shred -v /dev/sda; shred -v /dev/sdc ...</p>
  
  <p>I can understand from shred man(info) page that shred might be no effective on journal filesystems but only when shredding files. </p>
  
  <p>Anyone can please explain whether is shredding against blockdevice safe way to destruct all data on it?</p>