Consider the hierarchical network in Figure 6.30 and suppose that the data center needs to support e-mail and video distribution among other applications. Suppose four racks of servers are reserved For c-mail and four racks are reserved for video. For each of the applications, all four racks must lie below a single tier-2 switch since the tier-2 to tier-l links do not have sufficient bandwidth to support the intra- application traffic. For the e-mail application, suppose that for 99.9 percent of the time only three racks are used, arid that the video application has identical usage patterns.

a. For what fraction of time does the e-mail application need to use a fourth rack? How about for the video application?

b. Assuming e-mail usage and video usage are independent, for what fraction of time do (equivalently, what is the probability that) both applications need their fourth rack?

c. Suppose that it is acceptable for an application to have a shortage of servers for 0.001 percent of tine or less (causing rare periods of performance degradation for users). Discuss how the topology in Figure 6.31 can be used so that only seven racks are collectively assigned to the two applications (assuming that the topology can support ail the traffic).

Figure 6.30

Figure 6.31

Internet Border router Access router Load balancer Tier-1 switches Tier-2 switches TOR switches Server racks 1 2 3 4 5 6 7 8 Tier-1 switches Tier-2 switches TOR switches Server racks 1 2 3 4 5 6 7 8