One interesting approach to visualise the effect of multiple load balanced hosts on uptime is to consider two identical physical hosts. Presume that they are in two separate data centres on diverse networks. Presume further that each host can achieve 99.0 percent uptime and the outages are random. Then, all else being equal, for the 1.0 percent that host A is down, host B will be up 99.0 percent of those same times. Therefore, outages of the two hosts simultaneously is:
1.0 percent * 1.0 percent = 0.01 percent
or
0.01 * 0.01 = 0.0001
and, more rigorously
0.01 - (0.01 * 0.99) = 0.0001
Conversely, at least one physical host is available for:
100 percent - 0.01 percent = 99.99 percent
when two hosts with 99.0 percent uptime are deployed.
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Instead of calculating your uptime, it's a bit easier to calculate the downtime.
The downtime calculation is something like this:-
D = Total downtime (in minutes)
T = Total time in the month (in minutes)
Downtime (in %) = D/T X 100
Example:
A site which has only 2 hours downtime in a 30-days month will have downtime % like the following :-
D = 2 X 60 minutes = 120 minutes
T = 30 days X 24 hours X 60 minutes = 43200 minutes
Downtime (in %)
= [120/43200] X 100
= 0.2777 %
Uptime (in %)
= 100 - 0.2777
= 99.7222 %
In other words, you have 99.722% of uptime throughout the month.
http://edgedirector.com/htm/9999.htm
http://tycoontalk.freelancer.com/web-hosting-forum/34463-uptime-calculation.html
http://easyuptimecalc.com/index.php
http://squobble.blogspot.com/2008/02/hosting-uptimedowntime-calculation.html
