Comparison of Reliability of 162,299 HDDs

This report was written on January 26, 2021 by Andy Klein, director of compliance, on the blog of online backup company Backblaze, Inc.

Backblaze HDD Stats for 2020

In 2020, Backblaze added 39,792 HDDs and as of December 31, 2020 we had 165,530 drives under management. Of that number, there were 3,000 boot drives and 162,530 data drives. We will discuss the boot drives later in this report, but first we’ll focus on the HDD failure rates for the data drive models in operation in our data centers as of the end of December. In addition, we’ll welcome back Western Digital to the farm and get a look at our nascent 16TB and 18TB drives. Along the way, we’ll share observations and insights on the data presented and as always, we look forward to you doing the same in the comments.

2020 HDD Failure Rates
At the end of 2020, Backblaze was monitoring 162,530 HDDs used to store data. For our evaluation, we remove from consideration 231 drives which were used for testing purposes and those drive models for which we did not have at least 60 drives. This leaves us with 162,299 HDDs in 2020, as listed below.

Observations
The 231 drives not included in the list above were either used for testing or did not have at least 60 drives of the same model at any time during the year. The data for all drives, data drives, boot drives, etc., is available for download on the HDD Test Data webpage.

For drives which have less than 250,000 drive days, any conclusions about drive failure rates are not justified. There is not enough data over the year-long period to reach any conclusions. We present the models with less than 250,000 drive days for completeness only.

For drive models with over 250,000 drive days over the course of 2020, the Seagate 6TB drive (model: ST6000DX000) leads the way with a 0.23% annualized failure rate (AFR). This model was also the oldest, in average age, of all the drives listed. The 6TB Seagate model was followed closely by the perennial contenders from HGST: the 4TB drive (model: HMS5C4040ALE640) at 0.27%, the 4TB drive (model: HMS5C4040BLE640), at 0.27%, the 8TB drive (model: HUH728080ALE600) at 0.29%, and the 12TB drive (model: HUH721212ALE600) at 0.31%.

The AFR for 2020 for all drive models was 0.93%, which was less than half the AFR for 2019. We’ll discuss that later in this report.

What’s New for 2020
We had a goal at the beginning of 2020 to diversify the number of drive models we qualified for use in our data centers. To that end, we qualified 9 new drives models during the year, as shown below.

Actually, there were two additional HDD models which were new to our farm in 2020: the 16TB Seagate drive (model: ST16000NM005G) with 26 drives, and the 16TB Toshiba drive (model: MG08ACA16TA) with 40 drives. Each fell below our 60-drive threshold and were not listed.

Drive Diversity
The goal of qualifying additional drive models proved to be prophetic in 2020, as the effects of Covid-19 began to creep into the world economy in March 2020. By that time we were well on our way towards our goal and while being less of a creative solution than drive farming, drive model diversification was one of the tactics we used to manage our supply chain through the manufacturing and shipping delays prevalent in the first several months of the pandemic.

Western Digital Returns
The last time a Western Digital (WDC) drive model was listed in our report was 2Q19. There are still three 6TB WDC drives in service and 261 WDC boot drives, but neither are listed in our reports, so no WDC drives-until now. In 4Q20 a total of 6,002 of these 14TB drives (model: WUH721414ALE6L4) were installed and were operational as of December 31.

These drives obviously share their lineage with the HGST drives, but they report their manufacturer as WDC vs. HGST. The model numbers are similar with the first three characters changing from HUH to WUH and the last three characters changing from 604, for example, to 6L4. We don’t know the significance of that change, perhaps it is the factory location, a firmware version, or some other designation. As with all of the major drive manufacturers, the model number carries patterned information relating to each drive model and is not randomly generated, so the 6L4 string would appear to mean something useful.

WDC is back with a splash, as the AFR for this drive model is just 0.16% – that’s with 6,002 drives installed, but only for 1.7 months on average. Still, with only 1 failure during that time, they are off to a great start. We are looking forward to seeing how they perform over the coming months.

New Models From Seagate
There are 6 Seagate drive models that were new to our farm in 2020. Five of these models are listed in the table above and one model had only 26 drives, so it was not listed. These drives ranged in size from 12TB to 18TB and were used for both migration replacements as well as new storage. As a group, they totaled 13,596 drives and amassed 1,783,166 drive days with just 46 failures for an AFR of 0.94%.

Toshiba Delivers More Zeros
The new Toshiba 14TB drive (model: MG07ACA14TA) and the new Toshiba 16TB (model: MG08ACA16TEY) were introduced to our data centers in 2020 and they are putting up zeros, as in zero failures. While each drive model has only been installed for about two months, they are off to a great start.

Comparing HDD Stats for 2018, 2019, and 2020
The chart below compares the AFR for each of the last 3 years. The data for each year is inclusive of that year only and for the drive models present at the end of each year.

Annualized Failure Rate (AFR) for 2020 Is Way Down
The AFR for 2020 dropped below 1% down to 0.93%. In 2019, it stood at 1.89%. That’s over a 50% drop year over year. So why was the 2020 AFR so low? The answer: It was a group effort. To start, the older drives: 4TB, 6TB, 8TB, and 10TB drives as a group were better in 2020, decreasing from a 1.35% AFR in 2019 to a 0.96% AFR in 2020. At the other end of the size spectrum, we added over 30,000 larger drives: 14TB, 16TB, and 18TB, which as a group recorded an AFR of 0.89% for 2020. Finally, the 12TB drives as a group had a 2020 AFR of 0.98%. In other words, whether a drive was old or new, or big or small, they performed well in our environment in 2020.

Lifetime HDD Stats
The chart below shows the lifetime AFR of all of the drives models in production as of December 31, 2020.

AFR and Confidence Intervals
Confidence intervals give you a sense of the usefulness of the corresponding AFR value. A narrow confidence interval range is better than a wider range, with a very wide range meaning the corresponding AFR value is not statistically useful. For example, the confidence interval for the 18TB Seagate drives (model: ST18000NM000J) ranges from 1.5% to 45.8%. This is very wide and one should conclude that the corresponding 12.54% AFR is not a true measure of the failure rate of this drive model. More data is needed. On the other hand, when we look at the 14TB Toshiba drive (model: MG07ACA14TA), the range is from 0.7% to 1.1% which is fairly narrow, and our confidence in the 0.9% AFR is much more reasonable.

3,000 Boot Drives
We always exclude boot drives from our reports as their function is very different from a data drive. While it may not seem obvious, having 3,000 boot drives is a bit of a milestone. It means we have 3,000 Backblaze Storage Pods in operation as of December 31. All of these Storage Pods are organized into Backblaze Vaults of 20 Storage Pods each or 150 Backblaze Vaults.

Over the last year or so, we moved from using HDDs to SSDs as boot drives. We have a little over 1,200 SSDs acting as boot drives today. We are validating the SMART and failure data we are collecting on these SSD boot drives. We’ll keep you posted if we have anything worth publishing.