Date: Thu, 28 Apr 2005 10:28:36 -0700 From: Brooks Davis <brooks@one-eyed-alien.net> To: Arne =?iso-8859-1?Q?W=F6rner?= <arne_woerner@yahoo.com> Cc: Robert Krten <root@parse.com> Subject: Re: Background block scrubbing Message-ID: <20050428172836.GA30626@odin.ac.hmc.edu> In-Reply-To: <20050428140249.88064.qmail@web41203.mail.yahoo.com> References: <200504281304.JAA02215@parse.com> <20050428140249.88064.qmail@web41203.mail.yahoo.com>
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--Qxx1br4bt0+wmkIi Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Apr 28, 2005 at 07:02:49AM -0700, Arne W=F6rner wrote: > --- Robert Krten <root@parse.com> wrote: > > Gotcha. I wasn't aware it was *35* :-) I was thinking/hoping > > more like 3 or 4 with random garbage. > >=20 > (citation from the above mentioned paper:) "Modern PRML/EPRML" > drives (whatever that might be; I think my hard discs both do > PRML) just need some random data passes... So you should first > check, which kind of drive you need a tool for. The paper is actually pretty clean that you do not need all 35 passes. Most of the patterns are for specific encodins your drive doesn't use. IIRC (it's been a few years since I read this and the really stupid DoD standard it refrences), for any drive other than a floppy random is the only thing that matters and for a floppy breaking the case, removing the floppy, and burning it is much faster and 100% effective. With HDDs there's the additional issue that bad block remapping happens below the surface so you can't actually know you've written to the only physical copy of the block. The issue the paper doesn't address that is relevent today is modern flash drives. There is clearly going to be resedue of the bits in the flash is it's there in DRAM, but you can't even get direct access to the raw bits due to wear balancing. What I'd like to see is a simple zeroing of blocks when they are freed so deletes actually remove the files from the hot disk. That would be a slight (though imperfect) improvement over the current situation for hot disks. You can then use gbde for cold disk protection. -- Brooks --=20 Any statement of the form "X is the one, true Y" is FALSE. PGP fingerprint 655D 519C 26A7 82E7 2529 9BF0 5D8E 8BE9 F238 1AD4 --Qxx1br4bt0+wmkIi Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (GNU/Linux) iD8DBQFCcR1DXY6L6fI4GtQRAgrPAKCqdu0Y7Ye35XinMK/gkkjMspKomwCeNqNk ZPq0aVSPueR8Enuf8gT0TYg= =mxOf -----END PGP SIGNATURE----- --Qxx1br4bt0+wmkIi--
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