πŸ“ˆ Continuous Availability Tiers

Tier classifications define data center resilience levels.

TIER I

  • Small businesses
  • Single path for power and cooling
  • No redundancy
  • Availability: 99.671% – 99.749%
  • Downtime: 22.7–28.8 hours/year

TIER II

  • Medium-sized businesses
  • Some redundancy in power/cooling
  • Availability: 99.982%
  • Downtime: ~1.6 hours/year

TIER III

  • Larger enterprises
  • Multiple power/cooling paths (N+1)
  • Fault tolerant
  • Sustains 72-hour power outage
  • Availability: 99.995%
  • Downtime: ~25 minutes/year

TIER IV

  • Critical mission enterprises
  • Independent utility paths (2N+1)
  • Fully redundant systems
  • Sustains 96-hour outage
  • Availability: Implied β€œsix nines”

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