ik had MTBF bekeken op IBM maar daar dat is een hoop theorie, waar ze wel gelijk in hebben maar moeilijk toetsbaar in de praktijk daarom ben ik blij met jullie persoonlijke ervaringen over de gemiddelde leeftijd van de schijven.
hierbij het schrijven van ibm over MTBF
IBM does not quote MTBF (Mean Time Before Failure) numbers for it's products or believe this type of information should be used as a meaningful description of the quality of IBM Storage Subsystems or component parts within those systems. MTBF is an older industry term that today has very
little value and is mostly misinterpreted and misunderstood. Following is a definition and example of why MTBF should be avoided....
MTBF is the point at which 63.2% of the population, (everything of that component built by a manufacturer), will fail. So for example... disks drive that claims 1,000,000 hour MTBF used 24x7 or 720 power on hours per month would take 115 years to reach the 63.2% mark. It does NOT mean that no failure should occur for 115 years. This is a total population statement from the manufacturers point of view not from the customer point of view. MTBF is a cumulative statistic. A customer can receive brand new product and have it fail the day of install and still meet the criteria of the 1,000,000 hour MTBF because that failure simply adds to the count on the way to the 63.2% marker...it is the rate of failure. If you take say a 5 year view then you would expect....1/1,000,000,000 X 720Power on Hours X 60 months or about a 4% fallout in 5 years. So if a customer has a statistically significant population one might assume that his population would reflect the same 4% in 5 year rate but even this is misleading. Since MTBF is from the manufacturer point of view, it reflects all of the product shipped out of the back dock. As a Manufacturer, we know that some of that product will fail but we have no idea which ones and we do not know the distribution of good or bad product shipped to any given customer so....a single customer may get more or less than his fair share of drives that may fail earlier than expected. Therefore any single customer may achieve better or worse results than in the 4% example and still meet the MTBF criteria.
Lastly there is no single industry standard as to how MTBF is calculated. All products, disk drives or toasters, have a life cycle curve that can contains 3 elements....early life, useful life, wear out period. Early life fallout occurs in the first 90days...useful life is the period where the product is stable until it reaches a point where it starts to wear out. Many manufacturers calculate MTBF using ONLY the Useful Life or center section of the curve ignoring early life and wear out periods. (This is how it is defined by MIL Spec 417). Others use the entire curve. This means that when comparing MTBF one needs to not only compare the number but also know and understand the methodology used to calculate it, As you can see MTBF can only lead to confusion and an inaccurate expectation of our quality doing damage to both customer and IBM. It is for those reasons that we are strongly against quoting MTBF numbers.
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