How long do disk drives last?
Written by Brian Beach / Courtesy of Backblaze.com
UPDATE: Click here to read an updated article on how long hard drives last.
How long do disk drives last? The short answer is: we don’t know yet, but it’s longer than you might guess.
Why does a company that keeps more than 25,000 disk drives spinning all the time not know how long they last? Backblaze has been providing reliable and unlimited online backup for over five years. For the past four years, we’ve had enough drives to provide good statistics, but 74% of the drives we buy are living longer than four years. So while 26% of drives fail in their first four years, and we have detailed information about the failure rates of drives in their first four years, we don’t yet know what will happen beyond that. So how long do drives last? Keep reading.
How Drives Are Used At Backblaze
Backblaze uses lots of hard drives for storing data. 45 drives are mounted in each Backblaze Storage Pod, and the Storage Pods are mounted in racks in our data centers. As new customers sign up, we buy more disk drives, test them, and deploy them. We are up to 75 petabytes of cloud storage now.
Before being deployed, each Backblaze Storage Pod is tested, including tests on all of the drives in it. Recently, Andy posted about Poor Stephen, a disk drive that failed this testing. His post describes the process Backblaze uses to set up, load test, and deploy a Storage Pod.
Types Of Hard Drives In The Analysis
Backblaze has standardized on “consumer-grade” hard drives. While hard drive companies say these drives are not designed to work in RAID arrays or the 24×7 workload of a data center environment, Backblaze uses software redundancy to protect data. In a future blog post we will delve into the statistics comparing “consumer” and “enterprise” hard drives.
By far the majority of these hard drives are “raw” or “internal” hard drives. However, because the Thailand Drive Crisis made it nearly impossible to find internal hard drives for sale at reasonable prices, Backblaze started to farm hard drives. Thus, approximately 6 petabytes of the drives in this analysis were originally “external” hard drives that were “shucked” out of their enclosures.
Number of Hard Drives
The chart below shows the age distribution of the drives in the Backblaze data centers. The shape of the chart is mostly a reflection of the growth of the company, and the addition of drives as the customer base grew. Overall, not that many drives fail.
Before diving into the data on failure rates, it’s worth spending a little time clarifying what exactly a failure rate means. At first glance, you might think that a failure rate of 100% is the worst possible. Every drive is failing! That’s not the whole story, though.
Imagine you have a disk drive supplier who provides drives that are 100% reliable for six months, but then all fail at that point. What’s the annual failure rate? If you have to keep 100 drives running at all times, you’ll have to replace the drive in every slot twice a year. That means that you’ll have to replace 200 drives each year, which makes your annual failure rate 200%. So, in theory at least, there is no worst possible failure rate. If every drive failed after one hour of use, the annual failure rate would be 876,000%. Fortunately, the drives that Backblaze gets are more reliable than that.
The Bathtub Curve
Reliability engineers use something called the Bathtub Curve to describe expected failure rates. The idea is that defects come from three factors: (1) factory defects, resulting in “infant mortality”, (2) random failures, and (3) parts that wear out, resulting in failures after much use. The chart below (adapted from Wikimedia Commons) shows how these three factors can be expected to produce a bathtub-shaped failure rate curve.
Continue Reading over at Backblaze.com