(IBM PC Server Technology and Selection Reference SG24-4760-02 Aug-1997 U.S.)
Calculating Availability - Examples
When establishing a goal for availibility, trying to get realistic figures is very important. Most Manufacturers (hardware and software) will not publish available figures, since they are dependent on the environment in which the machine operates. One figure you might find published, in the Mean Time Between Failure or MBTF value. The MBTF is defined as being the amount of time a piece of hardware can run, without breaking down. Since these MBTF are based on statistical models, the are theoretical. The theoretical MTBF is actually the average time a component would run without failing, tested on an enormous population, and without including some common breakdown reasons. It should be clear that the theoretical MTBF does not reflect the operational MTBF.
So, what can we do with this MTBF ? A first approximate figure we can calculate is the probability of survival . This figure gives the chance a component will still function after a certain amount of time, based on the MTBF. The formular used is:
R = e * (Useful Life / MTBF) = Probability of Survival
With Useful Life equal to the number of years the hardware is supposed to work.
Let's have an example:
The IBM Ultrastar ES 2.16 GB Ultra SCSI Hard Drive has a projected MTBF of 800.00 power-on hours. A first step to take is converting these power-on hours to actual years.
There are two possibilities:
(for example, a RAID-0 configuration), the following formula should be used:
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