From: news@hrd769.brooks.af.mil (InterNet News)
Newsgroups: comp.periphs.scsi
Subject: Re: 500,000 hour MTBF? YES!!
Date: 11 Dec 1992 15:49:37 -0600
Organization: Armstrong Lab MIS, Brooks AFB TX
NNTP-Posting-Host: hrd769.brooks.af.mil

In article <1992Dec11.164025.17401@ugle.unit.no> oytor@teleserve.no (Oystein Torbjornsen) writes:
>It seems that there is some misconception about the term MTBF. Several posters
>on this newsgroup seems to mistake this term to mean "lifetime". My
>informal understanding of MTBF is:
>
>	average disk operating hours between failures for a large population
>	of disks in the bottom of the bathtub curve operating in an "friendly"
>	environment.
>
>With "friendly" environment I mean stationary in a cool computer room with few
>power up/down cycles.

  Isn't there usually a stipulation that the MTBF is computed for the complete
production run of ALL units produced.  From what I've been lead to understand,
this means that if 50,000 units are produced, and the MTBF is 500,000 hours, 
then one of those units will statistically fail every 10 hours.  Of course, the
infant mortality would drag the number down, so many companies only compute
MTBF for fieldable units (from the bottom of the bathtub).  What this means 
for the computer type on the street is that, for any particular disk drive
the 'death date' is not computable.  In fact, it really makes no guarantees
of length of service at all.

>

[lots of good stuff deleted]

>Since most manufacturers of disks give long guarantees, the difference between
>100,000h and 500,000h is of no interest for the average PC customer since the
>disk will be replaced if it fails anyway and becomes obsolete before it wears
>out.  (assuming that people take backup of their data). On the other hand, if
>the customer owns 1,000 disks he will observe a failure respectively each 100
>(4 days) and 500 hour (20 days). This means more work for the customer and
>perhaps more system unavailability.
>

  I don't know if the 100k or 500k MTBF figures should be of no interest.  One
thing is certain, in my experience; if it doesn't fail in the first 1000 hours 
of use, I don't expect to see it fail for a LONG time.  And of course, we 
haven't discussed the problems of changes in manufacturing that introduce
common mode failures, and operations in non-optimal conditions.  All of these
present their own perterbations of the mythical 500,000 hours, and should not 
be ignored by the traditional hobbyist.  A sample size of one or two can be
very misleading.
  I had some experience while I was stationed in Germany in 1986-88 that might 
be considered germaine (I had to, it was TOO easy).  We were using Zenith Z-150 
microcomputers with removeable 10Meg Hard drives.  The drives were rated at
50,000 hours MTBF.  Our experience with the 500 of them in Germany was that
we would have about 2 a week die.  That works out (not surprisingly) to about
42,000 MTBF.  The strange part was that it was always the same 50 computers 
that they were dying in.  With a few exceptions, we had virtually no failures
in machines that stood on sturdy, vibration free desks and ran on clean power.
The ones in rickety buildings, or in vibration areas, would die regularly.  
There were a few that would just quit, but not many.  Of course, we couldn't
convince the 50 folks whose computers were always down that the drives were
quite reliable, but the physical location of the drives was important.  
  I would be willing to bet that the drives were performing at better than
200,000 hours MTBF in optimal conditions, and less than 1000 hours MTBF in
the suboptimal conditions.

>-- 
>Oystein Torbjornsen                                  Phone: +47 7 596541
>Teleserve Transaction Technology AS                  Fax:   +47 7 596538
>N-7005 Trondheim, NORWAY                             Email: oytor@teleserve.no

TSgt Dave Burgess
NCOIC AL/MIS
Brooks AFB, TX
Newsgroups: comp.periphs.scsi