Date: Sun, 23 Nov 2003 00:16:31 -0800 From: Wes Peters <wes@softweyr.com> To: Stefan =?iso-8859-1?q?E=DFer?= <se@FreeBSD.org> Cc: freebsd-hackers@freebsd.org Subject: Re: "secure" file flag? Message-ID: <200311230016.31498.wes@softweyr.com> In-Reply-To: <20031121235607.GB16700@StefanEsser.FreeBSD.org> References: <20031119003133.18473.qmail@web11404.mail.yahoo.com> <200311211333.39520.wes@softweyr.com> <20031121235607.GB16700@StefanEsser.FreeBSD.org>
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On Friday 21 November 2003 03:56 pm, Stefan E=DFer wrote:
> On 2003-11-21 14:09 -0800, Wes Peters <wes@softweyr.com> wrote:
> > As for performance, you really need to flush the on-device cache on
> > each pass to make sure the bit patterns get written to the platter in
> > proper order. I don't see any clever way to coalesce the writing of
> > the various patterns to multiple blocks short of a kernel thread,
> > either, so performance would be abysmal. Imagine removing a large
> > file, overwriting each block in 37 (IIRC) passes, syncing all the way
> > through the on-disk cache after *every block.*
>
> I may be way off, but I do not think, that a special thread or
> a cache flush after each block is required:
>
> A simple algorithm could just mark each buffer with a special
> kind of dirty flag and a counter for the pass number (in fact,
> the existing dirty flag could be used, and a counter set to the
> number of passes required, with 0 indicating that the buffer is
> to be flushed to disk "as is" in the normal way).
Oh, but you're wrong, if you actually want to ERASE the data on the disk=20
platters. That's why I've referred people to the obliterate program in=20
ports several times. Read the references contained there, then come back=20
to this discussion.
If you just want to zero the blocks, that is a lot easier, but you're not=20
really protecting anything from anyone who can get their hands on the=20
disk.
=2D-=20
Where am I, and what am I doing in this handbasket?
Wes Peters wes@softweyr.com
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