Messages in this thread | | | Date | Wed, 9 Jan 2002 11:25:17 -0500 (EST) | From | "Richard B. Johnson" <> | Subject | Re: MTBF Was: Two hdds on one channel - why so slow? |
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On Wed, 9 Jan 2002, Andre Hedrick wrote:
> > Ricky,
[SNIPPED...] Don't think for an instant that MTBF has anything to do with the actual life-time of a device. The only correlation is that a longer MTBF may mean a longer life-time. MTBF is not equal to life-time at all.
MTBF is a numerical value obtained by using an agreed upon method of calculation or observation. MTBF will demonstrate that a machine that has no components will run forever. Of course it won't function.
Demonstrated MTBF is often (usually) obtained by taking a large number of components and subjecting them to short-term tests. This fails to produce any evidence of real life-time as the following example will show:
Suppose we have a timer chip that has a defective design in a stage which will short out and blow the device after 2 hours of operation. We want to measure the demonstrated MTBF so we take 10,000 chips and run them for an hour. None fail. We have now demonstrated 10,000 hr MTBF. Simple. This is not a joke. What is the demonstrated MTBF of a fuse? You have to destroy it to see if it worked -- at which time it has failed.
Marketing grabbed another buzz-word and used it as a ploy to attract customers when MTBF started appearing in consumer oriented data sheets.
Actual observation by many, of mechanical devices such as trucks, tractors, steel-roll-mills and disk drives shows that once started, then tend to run forever. However, they fail to restart if shut down after a long period of operation. A disk-drive that sits on a shelf often doesn't fare any better. It's like fruit that starts to decay after being picked from the "Disk Drive Tree".
Cheers, Dick Johnson
Penguin : Linux version 2.4.1 on an i686 machine (797.90 BogoMips).
I was going to compile a list of innovations that could be attributed to Microsoft. Once I realized that Ctrl-Alt-Del was handled in the BIOS, I found that there aren't any.
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