29 Jun 2015

RAID: Everything you need to know

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RAID: Everything you need to know RAID: Everything you need to know Promise


RedShark Replay: Phil Rhodes explains the background and the (then) latest developments in the ever-evolving world of RAID ( first published in July 2013).

I can't wait until my computer contains no moving parts whatsoever. The first parts that fail on computers are fans, and the second are hard disks. They're also comparatively slow, heavy, power hungry, and they make an irritating noise. For all these reasons and more, everyone was very pleased when companies like OCZ and Micron started making noises about solid state disks in the late 2000s, although as I recall we weren't quite so pleased about the prices. Even in 2013, solid state storage is six or seven times the price per gigabyte of spinning disks, so people who need to store trivial little quantities of data such as stereographic feature-length productions at 4K resolution and 60 frames per second will need to look elsewhere for storage technology.

All of which is a complicated way of saying that we are, for the immediate future, still going to be storing things on traditional hard disks. These disks are neither large, fast nor reliable enough to store huge, valuable data, so we may have to keep using them in the form of arrays for some time to come. Most people are familiar with the idea of using several hard disks in cooperation to achieve better reliability or higher performance. This article is about exactly how we do that, and how some of the traditional approaches that are used by other industries don't necessarily suit film and TV work, even though they're widely used.

To evaluate these techniques, we really need to understand what makes hard disks fail. Disks store information on circular platters, often made of glass for stability, which are coated in a particulate magnetic medium. Information used to guide the magnetic pickups over the appropriate area of the disk, as well as to gauge its rotational speed, is factory-encoded onto the disk and referred to as Servo Information. User data is written among the servo information. Tracks are concentric circles (not a single spiral, as on a CD or LP) and are, of course, microscopically tiny. Tracks are broken up radially into chunks called sectors, each of which can usually store a few thousand bytes, and is the smallest individual part of the disk that can be read or written. Guiding the pickup over the right microscopically-tiny area of the disk, recognising with the pulses of magnetism thereon, and making sense of the signal received is reliant on extremely precise mechanical alignments. Most hard disks that aren't destroyed by abuse fail because of wear in the bearings that support the stack of platters, or the Read/Write heads. Mistreatment such as shock can cause the heads to actually contact the surface of the disk, damaging both. Heat exacerbates wear by spoiling mechanical tolerances with thermal expansion and causing lubricating oils to thin out. As with any high precision mechanical device, it's disturbingly easy to kill or contribute to the death of a hard disk while it's running.

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Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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