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.
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.
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:
dd if=/dev/zero of=/tmp/testfile bs=1024 count=1024000 # close enough
while [ $BS -le 134217728 ]; do
COUNT=$((1048576000/$BS))
echo BS=$BS
dd if=/dev/zero of=/tmp/testfile bs=$BS count=$COUNT
BS=$(($BS+$BS))
echo "-------"
done
When I ran this, I got speeds that varied by up to 50%. Finding the
right blocksize can save you several hours.
Regards,
George Toft
On 8/18/2013 8:19 PM, keith smith wrote:
>
> Hi All,
>
> I have an old computer that I am giving to a friend so I wanted to
> wipe the drives in preparation for that.
>
> The master is 250GB
> The slave is 1TB.
>
> 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.
>
> The rescue disk I am using is DVD disk one of CentOS 6.3.
>
> 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.
>
> Command : shred -zv /dev/sda (this was on the master disk)
>
> Then I ran : dd if=/dev/zero of=/dev/sda bs=16M
>
> In one of the articles it showed the above command with bs=1M
>
> Does the size of "bs" matter?
>
> Also what about the argument that shred is overkill?
>
> Thanks!!
>
> Keith
>
> ------------------------
> Keith Smith
>
>
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