Availability is a measure of a system's operational time. A 99.9% availability requirement implies an allowed downtime of 0.1% (100% - 99.9%). To calculate the total allowed downtime in hours per year, we first determine the total hours in a standard year: 365 days × 24 hours/day = 8760 hours.

The allowed downtime is then calculated as:

8760 hours/year × 0.1% = 8760 × 0.001 = 8.76 hours.

Rounding 8.76 hours to the nearest tenth gives 8.8 hours, which corresponds to option D.

A. 6.8 hours: This value is mathematically incorrect and does not correspond to a 99.9% availability level.

B. 3.4 hours: This is significantly less than the calculated 8.76 hours of allowed downtime for 99.9% availability.

C. 8.6 hours: While close, this is a less accurate rounding of the calculated 8.76 hours compared to 8.8 hours.

1. HPE. (2019). HPE NonStop for Dummies

2nd HPE Special Edition. John Wiley & Sons

Inc. In Chapter 2

"Defining High Availability

" a table shows that 99.9 percent availability equates to approximately 8.77 hours of downtime per year (p. 16). This value rounds to 8.8 hours.

2. Patterson

D. A.

& Hennessy

J. L. (2011). Computer Organization and Design: The Hardware/Software Interface (4th ed.). Morgan Kaufmann. Appendix B

"Assemblers

Linkers

and the SPIM Simulator

" discusses dependability metrics

including the formula for Mean Time To Failure (MTTF) and availability. The standard calculation method used is foundational in computer architecture and systems design courseware based on this text. The calculation is Availability = MTTF / (MTTF + MTTR)

which underpins the "nines" calculation.

3. Arpaci-Dusseau

R. H.

& Arpaci-Dusseau

A. C. (2018). Operating Systems: Three Easy Pieces. Arpaci-Dusseau Books. This university-level textbook

used in many computer science curricula

discusses availability concepts in the context of distributed systems (e.g.

Chapter 50)

reinforcing the standard mathematical approach to calculating downtime from a given availability percentage.