A Glossary of the Data Center

I'm serving as a subject matter expert for a panel studying the IT capabilities of the Food and Drug Administration. In preparing our report, the team recognized that many FDA stakeholders are not well versed in the terms used to describe data centers. Here's the glossary that the team developmented, which I thought you might find useful for your own reports and presentations.

Classification of Data Centers (Tier 1 – 4). The Telecommunication Industry Association (TIA) has published the TIA-942 standard for classification of data center capabilities.

Tier 1 – Basic: 99.671% Availability
Susceptible to disruptions from both planned and unplanned activity
Single path for power and cooling distribution, no redundant components (N)
May or may not have a raised floor, UPS, or generator
Takes 3 months to implement
Annual downtime of 28.8 hours
Must be shut down completely for perform preventive maintenance

Tier 2 – Redundant Components: 99.741% Availability
Less susceptible to disruption from both planned and unplanned activity
Single path for power and cooling direction, includes redundant components (N+1)
Includes raised floor, UPS, generator
Takes 3 to 6 months to implement
Annual downtime of 22.0 hours
Maintenance of power path and other parts of the infrastructure require a processing shutdown

Tier 3 – Concurrently Maintainable: 99.982% Availability
Enables planned activity without disrupting computer hardware operation, but unplanned events will still cause disruption
Multiple power and cooling distribution paths but with only one path active, includes redundant components (N+1)
Takes 15 to 20 months to implement
Annual downtime of 1.6 hours
Includes raised floor sufficient capacity and distribution to carry load on one path while performing maintenance on the other.

Tier 4 – Fault Tolerant: 99.995% Availability
Planned activity does not disrupt critical load and data center can sustain at least one worst-case unplanned event with no critical load impact
Multiple active power and cooling distribution paths, includes redundant components (2 (N+1), i.e. 2 UPS each with N+1 redundancy)
Takes 15 to 20 months to implement
Annual downtime of 0.4 hours


Cloud Computing (and Storage). Cloud computing is a style of computing in which dynamically scalable and often virtualized resources are provided as a service over the Internet.

NAS (Network Attached Storage). The Network Attached Storage is file-level computer data storage connected to a computer network providing data access to heterogeneous network clients.

Reference Architecture. The reference architecture provides a proven template solution for an architecture for a particular domain. It also provides a common vocabulary with which to discuss implementations, often with the aim to stress commonality.

Reference Architecture can be defined as different levels of abstraction. A highly abstract one might show different pieces of equipment on a communications network, each providing different functions. A lower level one might demonstrate the interactions of procedures (or methods) within a computer program defined to perform a very specific task.

SAN (Storage Area Network). The Storage Area Network (SAN) is an architecture to attach remote computer storage devices (such as disk arrays, tape libraries, and optical jukeboxes) to servers in such a way that the devices appear as locally attached to the operating system. Although the cost and complexity of SANs are dropping, they are uncommon outside larger enterprises. Network attached storage (NAS), in contrast to SAN, uses file-based protocols where it is clear that the storage is remote, and computers request a portion of an abstract file rather than a disk block.

Virtualization (Server Virtualization). Virtualization is a method of partitioning a physical server computer into multiple servers such that each has the appearance and capabilities of running on its own dedicated machine. Each virtual server can run its own full-fledged operating system, and each server can be independently rebooted. (Best practice for reducing cost and increasing performance in large enterprises).