The internal components of a hard disk

Sound killed hard drive

1 February 2017

In today's world we rely on data storage more than ever. Dr Karl reveals the amazing engineering beneath the surface of modern hard drives and why data centres are going to extraordinary lengths to keep quiet in an emergency.

Dr Karl Kruszelnicki

In our complex world, the cure can sometimes be as bad as the original problem. For example, you would think that if you had a fire in a data centre, it would make sense to deprive the fire of oxygen by flooding the room with an inert gas. But what if the noise of the escaping gas is loud enough to kill the spinning hard drives – potentially causing more damage than the fire?

The very first spinning hard drives were introduced by IBM in 1956. Since then, the improvements have been enormous. The capacity has jumped from 3.75 MB to 10 TB – an improvement of 2.7 million to 1. Their volume has shrunk by a factor of 56,000, and the price per megabyte of data stored has plummeted by a factor of 300,000,000.

The modern spinning hard drive has amazing micro-engineering inside. It’s built at the scale of nanometres (where a nanometre is a billionth of a metre).

A hard drive has a spinning disc which is covered with a very thin layer of material (just 15 nanometres thick) that can be repeatedly magnetized or demagnetized. A tiny magnetic head sweeps across the spinning disc from the centre to the perimeter, and as it does so, it can read or write information.

The magnetic head that does the reading and writing is about 10 nanometres thick, and about 100 nanometres wide (about one-ten thousandth of a millimeter across). It hovers above a disc that is spinning at up to 15,000 RPM, and at a height of some 4 nanometres. It’s held up by a tiny bubble of air. If that magnetic head were to actually touch the spinning disc, at that speed it would spew up huge numbers of tiny particles, and cause immediate catastrophic failure of the hard drive.

There are about 10,000 parallel magnetic tracks in each millimeter of that spinning disc. If the head wanders off from the centre of a magnetic track by more than 25 nanometres, it cannot read and write data associated with that track.

Getting back to fires, the first known fire in a computer data centre happened on July 2, 1959 in a computer room operated by the United States Air Force inside the Pentagon. Very quickly, standards and procedures were brought into action to deal with a computer room fire. The trouble with using a water sprinkler system is that it can cause damage to the electronics.

The modern spinning hard drive has amazing micro-engineering inside. It’s built at the scale of nanometres.
Dr Karl Kruszelnicki

Today, we have the so-called Inert Gas Fire Suppression System. The inert gas is usually a mixture of nitrogen, argon and carbon dioxide. It’s stored inside gas cylinders at pressures up to 300 times greater than atmospheric pressure. It floods into the fire zone and dilutes the oxygen level from around 20% to a bit above 10%. That lower level of oxygen won’t support combustion, and so the fire goes out.

At first glance, putting out the fire with inert gas sounds like a good idea. Unfortunately, when the gas squirts out through the tiny nozzle, it can make a lot of noise. How much? Well, in some cases, as loud as a passenger jet taking off right next to you.

Can a sound that loud affect a modern spinning hard drive? Yes. In fact, even just shouting at a spinning hard drive can very significantly drop its performance. This was discovered back in 2008 by Brendan Gregg, an Australian computer engineer, while he was working for the computer company, Sun.

It turns out that because of the microscopic tolerances inside the modern spinning hard drive, loud noises can stop them operating, and even cause permanent failure. This is probably due to internal components vibrating. Fire extinguishing systems causing hard drive failures started appearing around 2009. There have been documented failures in Australia, France and Romania, when the noise of the fire suppression system killed hard drives in the data centres of big banks.

To put some numbers on it, 110 dB, about as loud as a petrol-driven chain saw at one metre, will degrade the performance of most hard drives. Sounds louder than 130 dB, similar to a trumpet blast at half-a-metre, will stop a hard drive from delivering data.

The cure is complex. The cylinders of inert gas are now being fitted with more nozzles, so there’s less gas moving through each spout. The nozzles themselves are being shifted further away from the hard drives, and are no longer aimed directly at them. The hard drive cabinets are being equipped with sound insulating doors. There’s also a shift to the more expensive Solid State Drives, which have no moving parts.

Maybe there’s a lesson in all of this. When you are frustrated, either by Life in General or a stubborn computer, don’t shout – just quietly whistle a cheerful song.

© Karl S. Kruszelnicki Pty Ltd 2017

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