High availability
Monday, March 15, 2010
9:58 AM
High availability
High-availability sites are measured in sigmas. 
4-sigma: 99.99% up = about 1 hour of downtime a year.
6-sigma: 99.9999% up = about 1/2 minute downtime per year! 
Fact: each added sigma costs roughly an order of magnitude (10x) more than the previous one: cost is geometric in number of sigmas! 