What is a data centre?

You have driven past one without noticing. A plain building with few windows and a high fence. Large fans on the roof, and a row of generators in a yard at the side.

Think of the cold room behind a supermarket, and it hums day and night. If the power fails the meat spoils, so there is a generator outside and somebody checks the fuel.

A data centre is that same idea, built for computers. The machines must never stop, and the heat must always have somewhere to go.

What is inside

Rows of steel frames called racks. A rack is about as tall as a person, and it holds machines stacked one above another.

Space in a rack is counted in units. One unit is roughly four and a half centimetres of height. A thin server takes one unit, and a bigger one takes several.

Three kinds of machine fill those racks. Servers do the computing, and storage machines hold the data. Switches and routers move traffic between them and out to the world.

Cables run overhead or under a raised floor. In a good site they are labelled, tied and mapped, because somebody will have to find one at three in the morning.

You cannot see the software from the doorway. One physical server is usually cut into many virtual ones through virtualization, so a single machine serves several customers at once. Serverless computing hides the machine completely, even though the rack is still standing there.

The two things that must never stop

Power is the first. A site takes power from the grid, then guards it twice. Batteries carry the load for the first seconds, and generators start up and carry it for hours or days.

Why two guards? Because a generator needs a few seconds to start, and a computer will not wait a few seconds. Therefore, the batteries fill that gap.

Cooling is the second. Almost every watt of electricity a computer uses comes back out as heat. A hall full of machines is therefore a large heater, and the heat has to leave without pause.

The usual method is easy to picture. Machines are arranged so that they all breathe in from one aisle and blow out into the next. One aisle stays cold, and the next runs hot. The hot air is gathered up, cooled, and sent round again.

Newer machines, especially those doing AI work, run hotter than air can comfortably handle. So liquid cooling is coming back into fashion, piped right up to the chip.

The network

A data centre with one fibre is a data centre waiting for a digger. Serious sites bring fibre in along separate routes, from separate providers, entering the building at different corners.

Many sites also work as meeting points. Networks that would otherwise swap traffic in another country can plug into each other in the same room. That shortens the journey for everybody nearby.

Distance still rules, because a site close to its users answers faster than a site on another continent.

How a data centre is measured

Three numbers matter more than floor area.

The first is power capacity, given in megawatts. Ask how many computers a building can feed and you have really asked how much electricity it can bring in and cool.

The second is a ratio called PUE, and it compares the total power the building draws with the power that actually reaches the computers. A perfect score is 1.0, meaning nothing at all is spent on cooling and lights. Very efficient sites get close to 1.1. However, older ones often sit near 1.5 or above.

The third is redundancy. A basic site has one path for power and one for cooling, so any repair stops the service. However, a strong site has two of everything, and any part can be fixed while the machines keep running.

The main kinds

• Hyperscale. Enormous sites built by one company to run its own cloud services.
• Colocation. A landlord supplies the building, power and cooling, and you bring your own machines.
• Enterprise. A private room or building owned by one bank, hospital or government office.
• Edge. Small sites placed near users, inside towns rather than out in cheap countryside, to cut the delay.

Most organisations end up using more than one kind at the same time.

Power, water and the environment

Data centres use a noticeable share of the electricity the world produces, and that share has been climbing as AI work grows. Two things follow from that.

First, operators chase cheap and clean power. Cold countries, hydro power and large solar farms all pull new sites towards them.

Second, water matters. Some cooling systems evaporate water to shed heat, which is cheap on electricity and costly on water. In a dry region that trade becomes a public question rather than a technical one.

There is a good side, because waste heat is still heat. In some cold cities it is piped into homes and offices instead of being thrown into the air.

Where they are built, and why

Four things pull a site to a place. Cheap and steady power. A cool climate, or at least a dry one. Fibre routes passing nearby. And customers close enough to keep the delay low.

Law is the fifth pull. Some countries require certain records, such as health or banking data, to stay inside their borders. That rule alone has put sites in places the money would not have chosen.

This is why countries that once sent all their traffic abroad are now building sites of their own. They tend to go up near the point where undersea cables come ashore.

What to check

If you are choosing a site for your business, ask plain questions. How many separate power feeds enter the building? How long can the generators run without a fuel delivery? How many fibre routes, and do they leave in different directions? What is the PUE across a whole year, rather than on one cool morning?

If you are buying nothing, do this instead. Find out which data centre your main provider uses, and where it stands. Then ask what happens to your work when that building has a bad day. A DATA CENTRE IS A BUILDING, AND BUILDINGS FAIL.

Just Out Tech explains new research in plain language. This article was drafted with AI assistance and checked by a human against the original source.