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Apr 29 - Amazing Facts About Hard Drives

In last week's blog entry, we discussed some aspects of flash storage that are not commonly considered. This week we want to carry on that torch and further illuminate the minutiae of hard drives by discussing a few facts you probably didn’t know about hard drives. While these facts most likely won't affect your next NAS or rack JBOD related purchasing decisions, they do provide some interesting insight into the ingenious design of every IT manager’s favorite workhorse; the spinning hard drive.

The hard drive never touches the disk – If you’re not at all familiar with the way that a hard drive functions, it may help to picture a record player playing an LP. In this case, the “head” would be the needle and the actual “disk” containing your data would be the record. However, with hard drives, the head never physically touches the disk. Rather, the disk literally flies just above the head. This is possible because of the incredibly fast speeds at which most hard disks spin during operation. The intense air pressure created by a hard disk spinning creates an air pressure equilibrium that, with slight assistance from a magnetic coating on the disk, allows the disk to remain suspended during operation. To put this into perspective, the average non-enterprise disk speed is 7200 RPMs. That means that the disk makes 120 full rotations per second! At this speed, any sort of contact between the head and the disk could potentially destroy both forever and render the hard drive inoperable. Enterprise-level hard drives and helium drives spin even faster than this on average reaching speeds up to 10,000 RPMs and higher.

The gap between the head and the disk is approximately 40 atoms high – You read correctly, about 40 atoms of real space separate the head from the disk. That’s the equivalent to 3 nanometers, even smaller than your average bacteria or virus. That’s also just 1 nanometer more than the diameter of a DNA helix. That may seem impressive already, but to further push some perspective on the incredible engineering of the hard drive, consider that the head which reads the disk is a little under 100 nanometers in diameter. To make a comparison of the spinning disk in operation, that’s the equivalent of a passenger airliner traveling over 600,000 miles per hour, flying about 1 centimeter over the surface of the earth and counting every blade of grass within the state of Indiana with no more than 10 blades potentially missed.

Most hard drives are extremely sensitive to altitude – It is a common misconception that because hard drives are so sensitive to air particles and dirt, they are all hermetically sealed. However, because air is required for the hard drive to function, most hard drives have special holes that allow air in and out, usually labeled with a warning sticker so they don’t get covered. As mentioned above, these disks spin at incredibly fast speeds, so slight changes in air pressure change the resistance that is created by the contact of the disk and the air. Because of this, if you look closely, you’ll see that manufacturers also place stickers on their hard drives that list the altitude requirements for safe operation. Typically safe operation is between sea level and 10,000 feet. Anything above that will increase the hard drives chances of failing exponentially. Newer enterprise helium drives, however, are sealed tight because the low-resistance nature of helium makes it so that the drives do not require air holes and are not very sensitive to outside changes in pressure.