"Five-nines" is a common industry term that refers to a system or service achieving 99.999% availability or uptime. This is a stringent standard typically required for mission-critical infrastructure. The remaining 0.001% represents the maximum allowable unplanned downtime over the service period. Calculated over a full year, 99.999% availability permits only about 5.26 minutes of total downtime. This metric is frequently specified in Service Level Agreements (SLAs) to guarantee a high degree of operational continuity.

A. PCI DSS is a security standard focused on protecting cardholder data; it does not define specific uptime percentages like "five-nines."

C. This describes a design methodology (redundancy) used to achieve high availability, not the availability metric itself.

D. This refers to the solution's expected lifespan and technological relevance, which is distinct from its operational uptime performance.

1. HPE Documentation: In discussions of high-availability solutions

HPE frequently references the goal of achieving "five-nines." For example

the HPE NonStop platform is designed for fault tolerance to deliver the highest levels of availability

often exceeding 99.999%.

Source: HPE NonStop Fundamentals (H4633S)

Module 1: Introduction to HPE NonStop

section on "Availability and Fault Tolerance." The courseware explains that NonStop systems are engineered for continuous availability

with "five-nines" being a benchmark for such critical systems.

2. Academic Courseware: University computer science and engineering courses on system reliability and design explicitly define this terminology.

Source: Patterson

D. A.

& Hennessy

J. L. (2017). Computer Organization and Design RISC-V Edition: The Hardware Software Interface. Morgan Kaufmann. In Chapter 6

"Parallel Processors from Client to Cloud

" the concept of availability is discussed

including a table that defines the "nines" and their corresponding annual downtime (e.g.

99.999% = 5.26 minutes/year).

3. HPE Technical White Paper: Documents detailing high-availability technologies like Aruba's Virtual Switching Extension (VSX) explain how these features help networks approach carrier-grade

five-nines availability.

Source: HPE ArubaOS-CX Switching High Availability Technical White Paper

Document Part Number: a00059622enw. The introduction and sections on VSX discuss the goal of minimizing downtime to achieve near-continuous network availability

a state often quantified as "five-nines."