Formulae and Laws

Systems Availability

A = (F - (D + R)) / F

A = availability
F = mean time between failures
D = mean time to detect failure
R = mean time to repair failure

Systems Downtime

4 nines = 99.99% = <52 minutes downtime per year
5 nines = 99.999% = <5.26 minutes per year
6 nines = 99.9999% = < 32 seconds per year

Transactional equivalent --> 6 nines means one failed transaction in every million (4 nines = 1 in every 10,000).

Are 2 nines (99%) enough? Would result in...

20,000 articles of lost mail every hour
5,000 botched surgeries per week
4 or more accidents per day at major airports

(Note: can't remember the source for the stats - if you recognise them, let me know - circa 2002)

Moore's Law

The number of transistors on an integrated circuit will double roughly every 18 months... See also:

Repeal Denied: MST will help extend Moore's Law for another 15 years (Robert X. Cringely, Jan 2008) - explanation of Moore's law, why it is being sustained and what that means

Metcalfe's Law

The value of a network is proportional to the square of the number of endpoints

Links

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