Figure 6.1 is a listing of outages in an array of servers.
When dealing with the large scale of WSCs, it is important to balance cluster design and software architectures to achieve the required uptime without incurring significant costs. This question explores the implications of achieving availability through hardware only.

a. Assuming that an operator wishes to achieve 95% availability through server hardware improvements alone, how many events of each type would have to be reduced? For now, assume that individual server crashes are completely handled through redundant machines.

b. How does the answer to part (a) change if the individual server crashes are handled by redundancy 50% of the time? 20% of the time? None of the time?

c. Discuss the importance of software redundancy to achieving a high level of availability. If a WSC operator considered buying machines that were cheaper but 10%less reliable, what implications would this have on the software architecture? What are the challenges associated with software redundancy?

d. Discuss the importance of eventual consistency in how warehouse-scale computers can scale.

Figure 6.1:

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Approx. number events in 1st year 1 or 2 4 1000s 5000 Cause Power utility failures Cluster upgrades Hard-drive failures Slow disks Bad memories Misconfigured machines Flaky machines Individual server crashes Consequence Lose power to whole WSC; doesn't bring down WSC if UPS and generators work (generators work about 99% of time). Planned outage to upgrade infrastructure, many times for evolving networking needs such as recabling, to switch firmware upgrades, and so on. There are about nine planned cluster outages for every unplanned outage. 2%-10% annual disk failure rate (Pinheiro et al., 2007) Still operate, but run 10x to 20x more slowly One uncorrectable DRAM error per year (Schroeder et al., 2009) Configuration led to ~30% of service disruptions (Barroso and Hlzle, 2009) 1% of servers reboot more than once a week (Barroso and Hlzle, 2009) Machine reboot; typically takes about 5 min (caused by problems in software or hardware).