π 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% β 99.749%
- Downtime: 22.7β28.8 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 βsix ninesβ
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