 Maximum VF availability versus Cloud site availability assuming 2 sites. two sites must be available for the VF to be available (i.e., have adequate capacity to serve the VF demand without performance degradation). In contrast, Figure 5.6 assumes that the VF is replicated across four sites, with 50% of the total required VF capacity provisioned at each site (again, 200% total). Thus, at least two of the four sites must now be available for the VF to be available. Assume the VF requires 5-9s availability as represented by the dotted line in each figure. From Figure 5.5, if the planned downtime is 2 weeks per site per year (bottom curve), then the individual site availability must be at least 99.99% (labeled 1) in order for the VF to potentially achieve 5-9s availability. With 1 week per site per year of planned downtime (middle curve), 5-9s VF availability can be achieved if single-site availability is 99.97% (labeled 2 and with no planned downtime (top curve), 5-9s VF availability can be achieved if single-site availability exceeds only 99.7%. In Figure 5.6 (four-site scenario), 5-9s VF availability can be achieved with 2 weeks per site per year of planned downtime when the single-site availability exceeds only 99.6% (labeled 3). Required single-site availability is further reduced to 99.4% with 1 week per site per year of planned downtime (green curve), and to 98.7% with 0 planned downtime "hitless" upgrades (blue curve). 100.000% 99.999% 99.998% 99.997% 99.996% 2 weeks per year per site 99.995% 99.994% 99.993% 200% capacity 2 of 4 sites must 99.992% be available 99.991% Network Cloud Site Availability 99.990% 98.0% 98.5% 99.0% 99.5% 99.9% FIGURE 5.6 Maximum VF availability versus Cloud site availability assuming 4 sites. Building the Network of the Future 3 sites Total Capacity 4 sites 5 sites 6 sites 7 sites 8 sites FIGURE 5.7 Capacity over-provisioning versus georedundancy tradeoff. Thus, spreading the same capacity across four sites greatly increases VF resiliency to planned upgrades in a 3-9s cloud environ