Daemonology.net Blog: Hacking the Amazon S3 SLA

[Juha-Matti Laurio <juha-matti.laurio () netti fi>]

"The Simple Storage Service (S3) provided by Amazon comes with a Service Level Agreement: If the Monthly Uptime 
Percentage is between 99% and 99.9%,
you get a 10% refund; if the Monthly Uptime Percentage is below 99%, you get a 25% refund.
The Monthly Uptime Percentage is computed in a fairly straightforward manner: 
Divide the month into 5-minute intervals and compute the Error Rate (failed requests divided by total requests, 
treating 0/0 as 0)
for each interval; compute the average Error Rate over all the 5-minute intervals in the month; and subtract this value 
from 100%.

If the probability of a request failing during the n th 5-minute interval is p(n), and the number of requests issued 
during the n th interval is determined solely by p(n),
the expected value of the Monthly Uptime Percentage is 100% minus the average value of p(n) over all the intervals;
put another way, you can't cheat by waiting for a high p(n) and then quickly running up the failure count by issuing 
lots and lots of requests. "
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More at
http://www.daemonology.net/blog/2008-10-23-hacking-the-amazon-s3-sla.html

Juha-Matti
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