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From: grr@cbmvax.cbm.UUCP (George Robbins)
Newsgroups: net.unix-wizards
Subject: Re: "bitrot" on magnetic media: is there such a thing?
Message-ID: <588@cbmvax.cbmvax.cbm.UUCP>
Date: Thu, 7-Aug-86 20:16:42 EDT
Article-I.D.: cbmvax.588
Posted: Thu Aug  7 20:16:42 1986
Date-Received: Sat, 9-Aug-86 08:48:21 EDT
References: <826@PUCC.BITNET> <217@c3pe.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 36

In article <217@c3pe.UUCP> charles@c3pe.UUCP (Charles Green) writes:
>
>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?

Yes, there is such a thing as 'bitrot', but I'm sure you could get quite a
bit of argument going on the subject...

The following notions should be considered:

Magnetic storage devices work when a magnetic field from a read/write head
magnitizes the vast majority of magnetic particles in an area on the media
in the same orientation.  Not every particle particle cooperates, and some
are likely to change, as a function of time and temperature.  Also, the head
assembly may retain a slight magnetic field, and/or be subject to leakage
currents from the circuitry.  This can also encourage particles to change
orientation.  As the number of appropriately oriented particles diminish,
the signal to noise ratio seen by the read circuitry will decrease and you
may eventually see problems.

Note that there are other problems that can cause similar symptoms, such
as media wear, gradual change of drive speed, thermal effects and shifts
in positioner repeatability.  Drives that do not move the heads to a parking
postion may also suffer occasional glitches on power down.

The problem is not limited to 5.25" drives, I've had problems with 100MB
disk pack drives, where 'read-only' system packs had to be reformatted
maybe twice a year when random errors started to occur.  (note that ECC
and/or track substitution was not involved here).

Thoughtful comment appreciated...
--
George Robbins - now working with,      uucp: {ihnp4|seismo|caip}!cbmvax!grr
but no way officially representing      arpa: cbmvax!grr@seismo.css.GOV
Commodore, Engineering Department       fone: 215-431-9255 (only by moonlite)