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<div class="moz-cite-prefix">Stephen mentioned one aspect - impact
if someone recreates the data. Another is the technical
capability of the hard drive recipient and anyone else that gets
the drive. Overwriting with 0's (or 1's) creates a regular
pattern that can be filtered out to retrieve the remnants of the
previous data, which is why the DOD standard is 7 passes. That
being said, I've never met anyone who had the technical capability
to retrieve data off drives once they've been overwritten.<br>
<br>
As fate would have it, I've developing a CD/USB-bootable image
whose sole purpose is DOD-wiping every drive in the system. It's
like DBAN (Derek's Boot and Nuke) on steroids.<br>
<br>
As far as bs (block size), in my experience, bs affects the speed
of the dd. Too small or too large and the time increases. For
the ATA drives I've dealt with, the sweet spot was 32K, but this
depends completely on drive type. Clearly YMMV and you might
experiment with different values. Wrap the dd command in a for
loop and time the execution of the dd command. Create a 1GB test
file and then run something like this:<br>
dd if=/dev/zero of=/tmp/testfile bs=1024 count=1024000 # close
enough<br>
while [ $BS -le 134217728 ]; do<br>
COUNT=$((1048576000/$BS))<br>
echo BS=$BS<br>
dd if=/dev/zero of=/tmp/testfile bs=$BS count=$COUNT<br>
BS=$(($BS+$BS))<br>
echo "-------"<br>
done<br>
<br>
When I ran this, I got speeds that varied by up to 50%. Finding
the right blocksize can save you several hours.<br>
<pre class="moz-signature" cols="72">Regards,
George Toft
</pre>
On 8/18/2013 8:19 PM, keith smith wrote:<br>
</div>
<blockquote
cite="mid:1376882395.5585.YahooMailNeo@web161202.mail.bf1.yahoo.com"
type="cite">
<div style="color:#000; background-color:#fff; font-family:times
new roman, new york, times, serif;font-size:12pt"><br>
<span></span>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>Hi All,</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>I have an old computer
that I am giving to a friend so I wanted to wipe the drives
in preparation for that.</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span><br>
</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>The master is 250GB</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>The slave is 1TB.</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>I read a couple
articles that suggested using a rescue disk and the shred
utility to take care of this. I also read that shred is not
necessary to just write all zero's to the drive.</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>The rescue disk I am
using is DVD disk one of CentOS 6.3.<br>
</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>I ran shred on the
fist drive. It took 4.5 hours to run 3 shred passes plus 1
that writes zeros to the entire drive.</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>Command : shred -zv
/dev/sda (this was on the master disk)</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>Then I ran : dd
if=/dev/zero of=/dev/sda bs=16M</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span><br>
</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>In one of the articles
it showed the above command with bs=1M</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>Does the size of "bs"
matter?</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>Also what about the
argument that shred is overkill?</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span><br>
</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>Thanks!!</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><span>Keith<br>
</span></div>
<div style="color: rgb(0, 0, 0); font-size: 16px; font-family:
times new roman,new york,times,serif; background-color:
transparent; font-style: normal;"><br>
<span></span></div>
<div>------------------------<br>
Keith Smith</div>
</div>
<br>
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