Re: Are linux-fs's drive-fault-tolerant by concept?

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On Sun, 20 Apr 2003 12:51:54 +0200, Helge Hafting said:

> > These don't address the problem - if the drive won't go "ready" because
> > of a blown server platter, your data won't get overwritten but it's still
> > readable (a number of companies make good money at this).
> > 
> I see.  Your data is so special that you expect people to pay for
> reconstruction hoping to find something that pays for all
> that trouble and more.

No, I don't consider my data that special - I just say "screw it" and
go looking for the backups.  However, my point is that it's not THAT
hard to salvage data off drives - it's easy enough that companies are
making a living doing it.

The question is regarding sites that are paranoid about shipping out
their data when such recovery is possible.

> Why would it be hard to reach the inner surfaces - the disks
> are not superconducting so the outer ones do not shield the
> inner ones from a strong magnetic field.  You should be fine
> as long as the field extend far enough to get the entire
> drive.  A high-frequency device might have trouble,
> but you don't need that - even a static field will do.

I didn't say it was impossible - I said you had to use a degausser that
was up to the task.  A degausser that is rated to bulk-erase tape media
is probably *NOT* sufficiently strong to do a multi-platter disk.  Most
tapes need 350 Oe or so to bulk, while disks are in the 1500 Oe range.

And you need a really big-ass magnet to make a 1500 Oe field big enough to
cover an entire disk (especially if it's an older disk of the 5.25 or
even larger variety - older IBM mainframe disks were up to 14" platters).

The Canadian government seems to agree - they recommend either getting the
degaussing wand between the platters (page 10) or using a cavity degausser
(page 12).  Note that they do *not* certify cavity degaussers for disk drives
over 3 1/2" with a coercivity over 1100Oe.

http://collection.nlc-bnc.ca/100/200/301/ cse-cst/cse_app-ef/itspl-02.pdf 

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