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How long do disk drives last? We asked that question several years ago, and at the time the answer was: We didn\u2019t know yet. Nevertheless, we did present the data we had up to that point and we made a few of predictions. Since that time, we\u2019ve gone to school on hard disk drive (HDD) and solid-state drive (SSD) failure rates<\/a>. Let\u2019s see what we\u2019ve learned.<\/p>\n The initial drive life study was done with 25,000 disk drives and about four years of data. Today’s study includes data from over 200,000 disk drives, many of which have survived six years and longer. This gives us more data to review and lets us extend our projections. For example, in our original report we reported that 78% of the drives we purchased were living longer than four years. Today, about 90% of the drives we own have lasted four years and 65% are living longer than six years. So how long do drives last? Keep reading.<\/p>\n Backblaze currently uses over 200,000 hard drives to store our customers\u2019 data. Drives range in size from 4TB to 18TB in size. When added together, we have over two exabytes<\/a> of hard drive space under management. Most of these drives are mounted in a storage server which accommodates 60 drives, plus a boot drive. There are also a handful of storage servers which use only 45 hard drives. The storage servers consist of Storage Pods<\/a> (our own homegrown storage servers) and storage servers from external manufacturers<\/a>. Twenty storage servers are grouped into a Backblaze Vault<\/a>, which utilizes our own Reed-Solomon erasure coding algorithm<\/a> to replicate and store customer data across the 20 servers in a Backblaze Vault.<\/p>\n The hard drives we use to store customer data are standard 3.5 inch drives you can buy online or in stores. The redundancy provided by the Backblave Vault software ensures the data is safe, while allowing us to use off-the-shelf drives from the three primary disk drive manufacturers: Seagate, Western Digital, and Toshiba. The following chart breaks down our current drive count by manufacturer. Note that HGST is now part of Western Digital, but the drives themselves report as HGST drives so they are listed separately in the chart.<\/p>\n Each of the storage servers also uses a boot drive. Besides the obvious function of booting the server, we also use these drives to store log files recording system access and activities which are used for analytics and compliance purposes. A boot drive can be either an HDD or an SSD. If you\u2019re interested, we\u2019ve compared the reliability of HDDs versus SSDs<\/a> as it relates to these boot drives.<\/p>\n As stated earlier, we currently have over 200,000 disk drives we manage and use for customer data storage. We use several different disk drive sizes as the table below shows, with over 60% of those drives being 12TB or 14TB in size.<\/p>\n Before diving into the data on failure rates, it\u2019s worth spending a little time clarifying what exactly a failure rate means. The term failure rate alone is not very useful as it is missing the notion of time. For example, if you bought a hard drive, what is the failure rate of a hard drive that failed one week after you purchased it? What about one year after you purchased it? Five years? They can\u2019t all be the same failure rate. What\u2019s missing is time. When we produce our quarterly and annual Drive Stats reports<\/a>, we calculate and publish the annualized failure rate (AFR). By using the AFR, all failure rates are translated to be annual so that regardless of the timeframe (e.g., one month, one year, three years) we can compare different cohorts of drives. Along with the reports, we include links to the drive data we use to calculate the stated failures rates.<\/p>\n 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 \u201cinfant mortality,\u201d (2) random failures, and (3) parts that wear out, resulting in failures after much use. The chart below (from Wikimedia Commons<\/a>) shows how these three factors can be expected to produce a bathtub-shaped failure rate curve.<\/p>\n When our initial drive life study was done, the Backblaze experience matched the bathtub curve theory. When we recently revisited the bathtub curve<\/a>, we found the bathtub to be leaking, as the left side of the Backblaze bathtub curve (decreasing failure rate) was much lower and more consistent with the constant failure rate. This can be seen in the chart below which covers the most recent six years worth of disk drive failure data.<\/p>\n The failure rate (the red line) is below 2% for the first three and a half years and then increases rapidly through year six. When we plot a trendline of the data (the blue dotted line, a second order polynomial) a parabolic curve emerges, but it is significantly lower on the left hand side, looking less like a bathtub and more like a shallow ladle or perhaps a hockey stick.<\/p>\n What\u2019s the life expectancy of a hard disk drive? To answer that question, we first need to decide what we mean by \u201clife expectancy.\u201d<\/p>\n When measuring the life expectancy of people, the usual measure is the average number of years remaining at a given age. For example, the World Health Organization<\/a> estimates that the life expectancy of all newborns in the world is currently 73 years. This means if we wait until all of those new people have lived out their lives in 120 or 130 years, the average of their lifespans will be 73.0.<\/p>\n For disk drives, it may be that all of them will wear out before they are 10 years old. Or it may be that some of them last 20 or 30 years. If some of them live a long, long time, it makes it hard to compute the average. Also, a few outliers can throw off the average and make it less useful.<\/p>\n The number that should be able to compute is the median lifespan of a new drive. That is the age at which half of the drives fail. Let\u2019s see how close we can get to predicting the median lifespan of a new drive given all the data we\u2019ve collected over the years.<\/p>\nHow Drives Are Used at Backblaze<\/strong><\/h2>\n
Types of Hard Drives in the Analysis<\/strong><\/h3>\n
![\"\"](\"https:\/\/www.backblaze.com\/blog\/wp-content\/uploads\/2013\/11\/hard-drives-by-manufacturer.jpg\")
<\/p>\nNumber of Hard Drives<\/strong><\/h3>\n
![\"\"](\"https:\/\/www.backblaze.com\/blog\/wp-content\/uploads\/2013\/11\/percentage-of-drives-by-size.jpg\")
<\/p>\nDrive Failure Rates<\/strong><\/h3>\n
The Bathtub Curve<\/strong><\/h2>\n
![\"\"](\"https:\/\/www.backblaze.com\/blog\/wp-content\/uploads\/2013\/11\/bathtub-curve-1024x723.png\")
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<\/p>\nCalculating Life Expectancy<\/strong><\/h2>\n
Disk Drive Survival Rates<\/strong><\/h3>\n
![\"\"](\"https:\/\/www.backblaze.com\/blog\/wp-content\/uploads\/2013\/11\/drive-survival-chart-.jpg\")
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