To Grasp the Cloud—Fundamental Concepts - Deploying and Managing a Cloud Infrastructure - page 31

TABLE 2.1 Availability and downtimes

Availability % Downtime/year Downtime/month Downtime/week

90% (one nine) 36.5 days 72 hours 16.8 hours

95% 18.25 days 36 hours 8.4 hours

97% 10.96 days 21.6 hours 5.04 hours

98% 7. 3 days 14.4 hours 3.36 hours

99% 3.65 days 7.2 hours 1.68 hours

99.5% 1.83 days 3.6 hours 50.4 minutes

99.95% 4.38 hours 21.56 minutes 5.04 minutes

99.99% 52.56 minutes 4.32 minutes 1.01 minutes

99.999% 5.26 minutes 25.9 seconds 6.05 seconds

This is not trivial for a web-scale business or social application serving hundreds of millions
of users across the globe 24 hours a day, especially transaction and payment gateway systems
Therefore, modern clouds are required to have failover support and mechanisms to ensure
maximum availability.
In 2011, Amazon's US-EAST data centers tripped for more than 24 hours. Hundreds
of applications serving hundreds of millions of users got affected. Even though Amazon
operates several massive data centers across North America, Europe, and Asia, a number
of very popular applications were completely taken down because of the outage. This
confirms that complete reliability on the cloud vendors for smooth failover support is not
advisable. If these applications (popular social app Reddit was among them) had imple-
mented their own failover mechanism upon the software stack on top of Amazon's EC2
service, they would have been able to get back live within an hour of the outage. Add
up the lost revenue and reputation and you realize why it is important to ensure failover
yourself, even if your cloud vendor guarantees it.

Virtualization and Scalability

Virtualization is the enabling technology for the cloud. This is what enables data centers to
populate tens of thousands of physical servers and then completely automate virtual server
creation, use, and deletion.