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Shred 5.2.1 VS. Shred 6.10

From: Jeremy Moles
Subject: Shred 5.2.1 VS. Shred 6.10
Date: Fri, 29 Feb 2008 12:39:51 -0500

We've been utilizing an internal tool/userspace/stack/thing here for a
number of years that, along with performing other maintenance tasks,
uses shred version 5.2.1 (which was packaged with an old version of
Ubuntu, I believe) to wipe various drives in different circumstances.
Depending on the speed and size of the drive, this process can take
anywhere from 2 to 5 hours, and in our experience this is very
reasonable for a 7-pass wipe.

Recently, we decided to update our tool/userspace/stack/thing (what this
REALLY is is a PXE-booted Linux image of about 50 megs containing lots
of different scripts, system probing/gathering utilities, etc.) and
along with this shred was upgraded to 6.10. What we've noticed is that
shred is approximately 5-6x slower than before in every case, and I'm
not quite sure what has changed.

I've checked the ChangeLog and I see that some changes were made with
regards to the ISAAC code being divorced into a separate file, but I'm
not entirely sure I have the familiarity to really dive into the code
and track down what is going on, nor can I imagine any reason this
change would have any effect on the speed of shred.

My question is: is there anything that has changed substantially with
the way shred works between theses two versions (which I admit is a
4year span) to result in such a difference? Is it conceivable that the
old version was bugged, and working too quickly? Or perhaps the new
version is bugged, and working too slowly? Or perhaps the new version is
simply more sophisticated, generates better random numbers, and this is
just the way it is? :)

The difference, I'm sure, has to do with the way random numbers are
being generated in each version, as the uniform passes run at equal

Some data below:

A "real" case, using a full /dev/sda wipe:

        40GB PATA/SATA Hybrid Drive / 7 passes
                - Shred 5.2.1: 2.6hrs
                - Shred 6.10: 13.1hrs

A "contrived" case, using a small 20MB dd'd image:

        20MB (dd if=/dev/zero of=foo bs=1M count=20) / 7 passes:
                - Shred 5.2.1: 4.9s
                - Shred 6.10: 22.2s

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