**Q1: Characterize the following three cloud computing models.**

**Q2: Basic Cloud Calculus**

Consider the System Availability (A) of a server cluster in terms of three parameters:

namely the mean time to failure (MTTF), the mean time to repair (MTTR), and a regular maintenance time (RMT).

The MTTF reflects the average uptime between two adjacent natural failures. The MTTR is the downtime due to natural failure. The RMT refers to scheduled down time for hardware/software maintenance or updates.

- Given a cloud system with a demanded availability A = 98%. If the MTTF is known to be two years (or 365 × 24 × 2 = 17,520 hours) and the MTTR is known 24 hours, what is the value of RMT in hours per month that you can schedule for this cloud system?
- Consider a cloud cluster of three servers. The cluster is considered available (or acceptable with a satisfactory performance level), if at least k servers are operating normally where k ≤ 3. Assume that each server has an availability rate of p (or a failure rate of 1 − p). Derive a formula to calculate the total cluster availability A (i.e., the probability that the cluster is available satisfactorily). Note that A is a function of k and p.
- Given that each server has an availability p = 0.985, what is the largest minimum number of servers that must be available to achieve a total cluster availability A, which is higher than 95%? You have to check the effect of all possible values of k in part (2) in order to answer this question correctly.

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