Date: Fri, 24 Mar 1995 08:35:53 -0500 (GMT-0500) From: "Serge A. Babkin" <babkin@hq.icb.chel.su> To: chuckr@Glue.umd.edu (Chuck Robey) Cc: freebsd-hackers@FreeBSD.org Subject: Re: Why IDE is bad Message-ID: <199503241335.IAA00423@hq.icb.chel.su> In-Reply-To: <Pine.SUN.3.91.950323151619.13833B-100000@cappuccino.eng.umd.edu> from "Chuck Robey" at Mar 23, 95 03:18:17 pm
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> On Thu, 23 Mar 1995, Joe Greco wrote:
>
> > > > 4) writing the disk more may wear it out faster.
> > >
> > > Now there's one for the books! :-)
> >
> > With an 800,000 hour MTBF, I think that enough years will
> > have passed that even if I reduce the lifetime of the
> > disk by 75%, I'll be too old and grey to care when it
> > dies. (yes I am sure the original comment was to see if
> > we were all awake :-) but it's a good point).
>
> I did once see someone wipe a disk clean, by doing continuous
> read/write/read cycles on it. The first disk went in 3 months, then
> three months later the second disk (and the programmer, by the way)
> went. So I think it can happen, but probably not with the kinda load we
> see.
Some information for everybody who thinks that MTBF is equal to the lifetime:
Take an experiment. We take a number of devices and begin to exploit it.
We remember when every unit dies until all of them are dead. Then we
plot a graph. This graph will be like this (plotted by *s):
% dead
^
100% | * 100%died
| *
| *
| *
/ --|----------------------------------------*
B _| | * |
| | * |
\ --|------* |
| * | |
| * | A |
| * | _______________|_______________ |
|* |/ \|
0 +------+---------------------------------+----------------->
| time of work | time
time of lifetime
manufacturer's tests
and free repair
MTBF is measured at time of work when the curve is near linear by
dividing A by B:
MTBF=A/B
Therefore MTBF 800,000 hours doesn't means that HDU will work 800,000 hours
before a failure, it only means that if lifetime is near 10,000 hours then
near 1/80 of all sold HDUs will die before reaching the lifetime.
I'm sorry if I had translated some terms to english wrong.
Serge Babkin
! (babkin@hq.icb.chel.su)
! Headquarter of Joint Stock Bank "Chelindbank"
! Chelyabinsk, Russia
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