Re: shred vs writing zeros to wipe a drive

Sparkfun to the rescue! https://www.sparkfun.com/tutorials/132


On Mon, Aug 19, 2013 at 1:40 PM, Matt Graham <mhgraham@crow202.org> wrote:

> On 2013-08-19 10:39, Carruth, Rusty wrote:
>
>> Don't' know if I need to see the code, but I'm certainly curious to know
>> how you made it slightly more efficient :-)
>>
>
> If you do "dd if=/dev/zero of=/dev/sdb bs=32k" , then dd is constantly
> reading from /dev/zero . Sure, reading from /dev/zero is fast, but
>
> zeroes=calloc(1,32768*sizeof(**char));
> /* later */
> fwrite(zeroes,32768,1,WRITE_**PTR);
>
> doesn't do *any* I/O to /dev/zero .
>
> When doing the "write random data" pass, randbuffer contains 32K of data
> read from /dev/urandom . After the data is written, randbuffer is only
> rewritten with new random data if rand() % 2 == 0. This means that there's
> 50% less I/O to /dev/urandom at the price of having the random data
> repeated for more than 1 block in a mostly non-deterministic way. Which
> means the program spends more time writing and less time reading.
>
> I figure /dev/urandom is at least one order of magnitude better
>> than /dev/zero (in some base or other) :-) But, yes, if you REALLY
>> want good random numbers use /dev/random and be prepared for a wait.
>>
>
> If you need more than about 256 bytes of stuff out of /dev/random, you'll
> be waiting for a looooong time. At least IME. If you have serious needs
> for lots of high-quality non-deterministic entropy, I guess you build an
> RNG made out of the beta emitters in old smoke detectors. (Fun for the
> whole family! :-) )
>
> --
> Crow202 Blog: http://crow202.org/wordpress
> There is no Darkness in Eternity
> But only Light too dim for us to see.
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