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From: cprice@vianet.UUCP (Charlie Price)
Newsgroups: net.unix-wizards
Subject: Re: "bitrot" on magnetic media: is there such a thing?
Message-ID: <18@vianet.UUCP>
Date: Fri, 8-Aug-86 16:09:40 EDT
Article-I.D.: vianet.18
Posted: Fri Aug  8 16:09:40 1986
Date-Received: Sun, 10-Aug-86 20:03:33 EDT
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Organization: ViaNetix, Inc., Boulder, CO
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> -Charles Green at C3 Inc.	{{styx!seismo,cvl}!decuac,dolqci}!c3pe!charles
> But I'm beginning to wonder:  after the address marks are written on a disk
> during formatting, as the years go by, do they gradually "entrophy"
> (atrophy via entropy!), or melt into the noise?

The answer is -- YES.
Data (and "format info" is just data) *can* degrade on a magnetic disk;
though not just from some random evaporation into the air.

I used to work for Storage Technology Corp (in the very recent past)
and I'm familiar with at least one mechanism for gradually degrading recorded
data on the disk.  Of course, Storage Tek makes fairly big disks,
(2.5 Gbyte Head Disk Assembly using 14" disks) but the physics is the same.

In a winchester technology disk you have read/write heads flying
REALLY CLOSE to a disk.  What happens if there are any little particles
of gruk in the drive?  If it is the right kind of gruk and the right
sized particles the particle can either provide a material to rub
"under" the head or it can just bang the head around and cause it
to "bounce" and momentarily touch down on the surface.
If this is really bad, you have a crash in the making.
If it isn't too bad, the contact (in the disk business this is
called head-disk-interface) maybe knocks some more particles loose from
the disk surface and generatates a whole bunch of short-lived localized heat.
If you heat up a magnetized material above some particular temperature
for the material, called the curie point, the magnetic domains can move.
Since the media isn't in a strong field here, it will probably demagnetize.
If this happens repeatedly in the same area, the stored data can actually
degrade to the point it can't be read.
Though they believe it had always been going on, Storage Tech only noticed
this behavior with the most recent generation of drive technology
(very low-mass thin-film heads flying REALLY close to the media surface).
[A cleaner clean room eliminated the problem].

A typical cheap winchester is using technology that isn't as prone
to this sort of problem (head flight fairly far away from the disk).
If it weren't build-it-and-ship-it technology the drives would
be too expensive.

Gradually degrading behavior on a disk drive can indicate that it
is gradually getting dirtier (start-stop can kick loose particles).
Reformatting and/or rewriting all data CAN help but clearly doesn't
make the problem go away.

-- 
Charlie Price    {hao stcvax nbires}!vianet!cprice    (303) 440-0700
ViaNetix, Inc. / 2900 Center Green Ct. South / Boulder, CO   80301