Mistake 1: treating “it can’t be down” as a design specification. Accepting an application owner’s statement without translating it into measurable availability requirements can produce an unnecessarily expensive HA/DR topology. section 13.2
When five nines means six seconds
A five-nines requirement means 99.999% availability, allowing only 6 seconds of downtime per week. A simple network blip could consume that entire allowance, while the associated HA/DR solution may cost more than the downtime itself. section 13.2
Use this decision sequence:
- Translate business expectations into a percentage of availability and acceptable downtime. section 13.2
- Build the topology that satisfies the justified requirement rather than assuming every application needs maximum availability. section 13.2
