-9s would have a very small impact to the site availability. A more interesting question might be whether it is beneficial to improve the physical cloud infra- structure layer from 4- to 5-9s. In this intermediate case, the answer would depend on the specific costs of achieving this improvement. Achieving high site availability at low cost comes down to understanding the tradeoffs between cost and availability for each layer and adjusting the "dials" Downtime 50 min 5 min 30 sec Downtime/yr Availability 3 9's 4 9's 5 9's 6 9's Application 53 min Virtualization (VM, Hypervisor) 53 min Bare Metal HW, 53r min Compute/Storage Infrastructure Fabric 30 sec Physical Cloud Infrastructure 53 min Downtime 11.5 hr 3.5 hr Overall Availability 99.87% 99.96% FIGURE 5.3 Bottleneck analysis. Building the Network of the Future Cost of Infrastructure Availability Grid only Tier 4 3-9's 4 9's 5 9's 6 9's Availability FIGURE 5.4 Infrastructure availability versus cost per watt. accordingly. For example, the power configuration at a site could greatly affect the availability and cost. Figure 5.4 shows an illustrative cost curve for providing power to a site, ranging from using Grid only to Tier 4 or better (ADC, 2006). Depending on the power configuration used (e.g., UPS, redundant Gensets, battery backup, etc.), the costs will change. Comparing the availability to cost ratio for each power configuration against the similar availability to cost ratios for other layers will help identify where the largest impact can be achieved at the lowest cost. By balancing these different layers, it is possible to minimize the cost of achieving a certain site availability (e.g., 4-9s). But it may be that certain VFs require higher availability (e.g., 5-9s), which may be cost prohibitive to achieve in a single site. In particular, with the introduction of commercial compute and storage, along with virtualization software, the availability of a single site generally would be no better than 3- to 4-9s. This is consistent with results reported elsewhe