Ever since Clayton Christensen published The Innovator's Dilemma it has been common knowledge that disk-drive cost per byte halves every two years. So you might argue that you don’t need to know how many copies you need to keep your data safe for the long-term, you just need to know how many you need to keep it safe for the next few years. After that, you can keep more copies.
In fact, what has been happening is that the capacity at constant cost has been doubling every two years, which isn’t quite the same thing. As long as this exponential grows faster than you generate new data, adding copies through time is a feasible strategy.
Alas, exponential curves can be deceiving. Moore's law has continued to deliver smaller and smaller transistors. A few years ago, however, it effectively ceased delivering faster and faster CPU clock rates. It turned out that, from a business perspective, there were more important things to spend the extra transistors on than making a single CPU faster. Like putting multiple CPUs on a chip. At a recent Library of Congress meeting, Dave Anderson of Seagate warned that something similar is about to happen to hard disks
. Technologies — HAMR (heat-assisted magnetic recording) and BPM (bit-patterned media) — are in place to deliver the 2013 disk generation (i.e., a consumer 3.5-inch drive holding 8 TB). But the business case for building it is weak. The cost of the transition to BPM in particular is daunting. Laptops, netbooks, and now tablets are destroying the market for the desktop boxes that 3.5-inch drives go into. And very few consumers fill up the 2009 2-TB disk generation, so what value does having an 8-TB drive add? Let alone the problem of how to back up an 8-TB drive on your desk!
What is likely to happen — indeed, is already happening — is that the consumer market will transition rather quickly to 2.5-inch drives. This will eliminate the high-capacity $100 3.5-inch drive, since it will no longer be produced in consumer quantities
. Consumers will still buy $100 drives, but they will be 2.5 inches and have perhaps one-third the capacity. For a while the $/byte curve will at best flatten, and more likely go up. The problem this poses is that large-scale disk farms are currently built from consumer 3.5-inch drives. The existing players in the market have bet heavily on the exponential cost decrease continuing; if they’re wrong, it will be disruptive.