incremental disk wipe?

[Colin Law]

On 7 March 2015 at 21:39, rikona <rikona at sonic.net> wrote:
> Hello Colin,
>
> Saturday, March 7, 2015, 7:01:03 AM, Colin wrote:
>
>> On 7 March 2015 at 06:09, rikona <rikona at sonic.net> wrote:
>>> I was checking the timing of securely wiping large disks - looks
>>> like it takes a looooong time. Is there a way to do this in smaller
>>> 'chunks', that might complete in an hour or so, eventually filling
>>> up the entire disk? I'm not hiding plots from NSA, so I don't need
>>> extreme security - just enough to discourage easy-tech snoops. :-))
>
>> Use dd with bs, skip and count options to write nulls over selected
>> areas. Increasing skip each time.
>
> AIUI [which is not very well :-) ] the concept is to write just a
> little each time so it doesn't take very long. I'm a bit confused by
> man dd, though, re 'BLOCKS' and increasing skip. Wouldn't I want to
> have bs small [512 bytes], vary count over a small range [say 1-5],
> have skip start with a good size number [to minimize time] but vary it
> up and down a bit on each run to make sure most of the disk is written
> to?

I meant seek of course, not skip. So if you do
dd if=/dev/zero of=/dev/sdX bs=1M count=1000
then my understanding is that it will write zeros to the first 1GB,
then if you do
dd if=/dev/zero of=/dev/sdX bs=1M seek=1000 count=1000
it will write to the second GB, then
dd if=/dev/zero of=/dev/sdX bs=1M seek=2000 count=1000
will write to the third GB, etc.  I have not tried it though.

Colin