alone in a room for a few hours, and the room will become hot. Too hot, in fact, for the computer to keep operating. Advanced computers will sense this and shut themselves down; less advanced computers will suffer damage. So, in order to keep a computer running during a sustained power outage, there needs to be sufficient backup power not only for the computers, but also for the cooling. For various reasons, both electrical and economic, UPSs are not suitable for air-conditioning. The last step in the power supply, therefore, is a backup generator. With both UPS and generators in place, and a suitable fuel supply and re-fuelling
arrangements, mission-critical computers can be kept running indefinitely in the absence of mains power. Keeping cool Another common cause of environmental failure is air that is too hot, humid or dirty. While normal office air-conditioning goes some way on all of these fronts, the air filtration is often ineffective because the filters are never cleaned; the dehumidification is not controlled, and most office airconditioners run at a fixed amount of power rather than towards a fixed temperature. The resultant fluctuations in temperature can lead to condensation – and dripping water mixes very badly with electricity.
Quite apart from doing what you tell them rather than what you want them to do, or throwing away four hour’s work because the battery went flat before you could save that vital, needed-right-now spreadsheet, computers have a habit of breaking down when they are most needed.
The engineering solution to this is the Computer Room Air Conditioner, or CRAC. These are considerably more expensive than ordinary air-conditioning units, but they provide the steady temperature and humidity that servers need. They are also, in general, more efficient than smaller units, so have lower running costs. Add in the extended life time CRACs bring to expensive servers, and the Total Cost of Ownership is lower than cheap air-con. How much is enough? Air-conditioners and generator sets, as well as some UPSs, are mechanical things. Like all things mechanical, they sometimes break. Further reliability can be built into the environment for the servers by adding redundancy. The industry has therefore developed tiers of data centre, based on redundancy. A “Tier 2” design has no redundancy, a “Tier 3” design has one spare unit and therefore no single point of failure, and a “Tier 4” design has duplicate everything, so no multiple points of failure. Correspondingly, the unscheduled annual downtime is:
The Link
Issue 2/2014
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