From: ariel@world.std.com (Robert L Ullmann)
Subject: Re: Limits to the number of badspots a disk drive can have??
Organization: The World in Boston
Date: Sat, 9 Jan 1993 04:06:33 GMT

Hi Ed,

I don't know what they would say; but I can speak from other experience.
There are three kinds of bad spots:

1) manufacture and initial run-in defects (the first side of the "bathtub")

2) spots that develop during the operational lifetime (bathtub floor)

3) spots that develop as the entire surface eventually begins to
   degrade and fail (the other side of the bathtub).

(1) is no problem, even if lots of them exist, as long as it hasn't
affected the capacity of the disk or caused tracks to become unusable
because the overflow/replace positions are unusable (thus clobbering
capacity).

(2) is only a problem if the rate of new spots is too high for the
application (floor of the tub too high). It is reasonable in some
applications to insist that this rate be ZERO; and treat more than
one error as an indication that you have arrived at the upward slope
on the other side of the tub.

(3) means REPLACE THE DISK NOW! Indicated by any rate of dynamic
badspot detection (i.e. NEW spots) exceeding 0.005 per Mbyte-month
(i.e. more than 5/month on a Gbyte disk.) As I said, in some applications,
the appearance of the 2nd uncorrectible (data loss) bad spot after
the initial run-in may mean you should swap out the disk.

If you have an "event" (power failure, temperature domain excursion,
Halon(TM) system discharge, or similar) don't count any spots that
appear in the following 96 hours against the record. It helps to run
patterns that read the entire surface between the 48 and 96 hour marks
(assuming temperature stabilization is complete) as long as that
doesn't interfere with operations. (I don't know how to do that
on PRIMOS, on Unix you can copy the raw device to /dev/null a few
times; on VMS you can read the logical device with diag QIOs.)
This is assuming that the specific OS you have will deal with
problems found during such reads. Both VMS and (some) Unix provide tools
to do this properly without interfering with operations; PRIMOS
doesn't last I looked (FIX_DISK requires the disk be effectively
off-line, and read check doesn't read all the sectors).

Plan on replacing your disks every 4-5 years, regardless. (With
removable media, this is no problem; with fixed Winchester(TM) types
you probably can buy 4x the capacity every 5 years at the same
price with no difficulty, at least so far. ;-) Oh yes, removeable
media should be effectively "flag-free" (no errors after run-in);
dump it on the first new error, unless you have a non-critical
app. (I put "flag-free" in quotes; it used to mean NO bad sectors,
required by Certain Operating Systems that couldn't deal with
bad spots. Do you remember one of those? ;-)

Enough info? :-)

You want _real_ consulting, it'll cost ya.

Rob

--
Robert Ullmann		Ariel@World.STD.COM	+1 508 879 6994 x226
Quand Maigret poussa la porte du Tabac Fontaine, vers une heure et demie,
le patron du bar, qui venait de se lever, descendait lentement un escalier
en colimaon qui s'amorait dans l'arrire-salle. ... Arriv derrire le
comptoir, il repousa le garon d'un geste ngligent de la main, saisit
une bouteille de vin blanc, un verre, mlangea au vin de l'eau minrale et,
la tte renverse en arrire, se gargarisa.  -- Simenon

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