Scotland's Digital Future: Data Hosting and Data Centre Strategy for the Scottish Public Sector

The data hosting and data centre strategy sets the vision that Scotland’s public sector data hosting is cost-effective, carbon neutral and makes appropriate use of cloud technology, for the delivery of efficient and highly available ICT services.

Annex D - Tier Classification

The Uptime Institute who is recognised as the global data centre authority has a widely accepted tiered classification system. This system is an industry standard approach to data centre infrastructure functionality to address common benchmarking standard needs for data centres. It is a four tier system that provides a simple and effective means for identifying different data centre site infrastructure design topologies.

The four tiers, as classified by the Uptime Institute are as follows:

Tier 1 Tier 2
  • Susceptible to disruptions from both planned and unplanned activity
  • Less susceptible to disruption
  • Single path for power and cooling disruption
  • Single path for power and cooling disruption
  • no redundant components (N)
  • includes redundant components (N+1)
  • UPS or generator
  • Includes raised floor, UPS and generator
  • Annual downtime 28.8 hours
  • Annual downtime 22 hours
  • completely shut down for maintenance
  • Maintenance requires some processing downtime
  • Possible to provide 99.671% availability
  • Possible to provide 99.741% availability
Tier 3 Tier 4
  • planned activity without disrupting operation
  • Planned activity does not disrupt critical load
  • unplanned events will still cause disruption
  • data centre can sustain at least one worst case
  • Multiple power and cooling paths - one active
  • unplanned event causes no critical load impact
  • includes redundant components (N+1)
  • Multiple active power and cooling paths
  • Includes raised floor
  • includes redundant components ( 2(N+1))
  • Annual downtime 1.6 hours
  • Annual downtime of 0.4 hours
  • carry load on one path while
  • Possible to provide 99.995% availability
  • performing maintenance on the other
  • Possible to provide 99.982% availability

NOTE: The difference between 99.982% (T3) and 99.995% (T4), 0.013%, while seemingly nominal, it could be significant depending on the application. Looking at one year or 525,600 minutes, in expectation, T3 will be unavailable 94.608 minutes whereas T4 will only be unavailable 26.28 minutes. Therefore, T4 in a year will be available for an expected 68.328 more minutes than T3. Similarly, a T3 data centre would be expected to be 22.6 hrs. more available than a T2.


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