Humidity, not heat, is a hard drive's biggest threat

It should be obvious, but moisture is really had for hard disks.

 Humidity, not heat, is a hard drive's biggest threat

Having been to Orlando in August, I know the meaning of the term "It's not the heat, it's the humidity." That was the first and only time I had my glasses fog up for just stepping outside.

And it turns out humidity is a greater threat to hard drive reliability than high temperatures, according to a study from Rutgers University in partnership with GoDaddy and Microsoft. In their paper titled "Environmental Conditions and Disk Reliability in Free-cooled Datacenters" (PDF), the team said the most notable result was that all other conditions aside, the effects on controllers and adapters were felt most as humidity levels rose.

The report notes that large data center operators today can report yearly Power Usage Effectiveness (PUE) of 1.1 to 1.2, meaning just 10% to 20% of energy goes into non-compute efforts, like cooling. That's a major improvement over seven to eight years ago, when PUEs were about 3.0. One of those ways to get the PUE down is called "direct-evaporative-cooling" or simply "free-cooled."

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Direct evaporative cooling lowers the temperature by using the latent heat of evaporation, which changes liquid water to water vapor. In the process, this can run the relative humidity of the data center as high as 90%. The data center then has to get rid of that humidity, lest it turn into a swamp. At a certain point, the humidity can't go any higher and evaporation would cease.

According to the study, disk failures accounted for 89% of component failures in a data center, with memory DIMMs coming second at 10%, CPUs at 5% and PSUs at just 2%. And the research found that as humidity levels rise, the rate of disk controller and connectivity failures increase.

The tests were carried out on more than a million drives in nine Microsoft data centers over a period of 1.5 to four years. The humidity-related failures were so high that you could tell which free-cooled data centers had humidity controls and which did not just by looking at the annualized failure rate (AFR) of the controllers.

One of the conclusions of the group is that free-cooled data centers in regions where high levels of naturally occurring humidity should be more aggressive with their humidity controls. They also found that positioning the drives in the hot region at the back of the server also improved reliability. The reason is that the heat is not as destructive as the humidity and the heat of that position in the server kept humidity at bay.

However, the researchers also found that in some cases, it's cheaper to replace the drives than shell out for expensive humidity control systems. So if you decide to just let them break and be replaced, there needs to be more aggressive management of data redundancy so you are ready when a drive goes up in smoke.

Data center management is an interesting and evolving science. Several years ago, Intel placed a data center in the Arizona desert and only used outside air for cooling, rather than turning the data center into a meat locker like is usually the case. It found no harm, no foul. Data centers can stand heat. They couldn't handle the dust, however.

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