Background block scrubbing

Brooks Davis brooks at
Thu Apr 28 10:28:38 PDT 2005

On Thu, Apr 28, 2005 at 07:02:49AM -0700, Arne Wörner wrote:
> --- Robert Krten <root at> wrote:
> > Gotcha.  I wasn't aware it was *35* :-) I was thinking/hoping
> > more like 3 or 4 with random garbage.
> > 
> (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

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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: not available
Url :

More information about the freebsd-fs mailing list