Cloud backup provider Backblaze has published more of its hard drive reliability data, giving a look at the company's experiences with its 56,224 hard disks in 2015.

In 2014, HGST was the standout performer, with all its models showing extremely good reliability. Some Seagate models, on the other hand, showed alarming unreliability and extremely high failure rates.

For 2015, HGST maintained its strong performance. Across all the HGST models that Backblaze used (one 2TB, two 3TB, three 4TB, and one 8TB), failure rates were low across the board. The HGST drives are some of the oldest in Backblaze's collection, with the 2TB units being almost five years old on average. Over the last two and a half years, only 1.55 percent of them have failed.

Seagate showed much stronger performance, too. Two of the models that performed so badly in 2014 are 1.5TB drives. One of those models has since been retired, for a lifetime cumulative failure rate of 23.86 percent. The other model, now averaging about five years old, still has a high failure rate of 10.16 percent. The third problematic Seagate model, a 3TB drive, has also been retired after showing a cumulative failure rate of 28.34 percent.

With these troublesome disks removed, the remaining Seagate disks, both 4TB and 6TB, fared much better. Although the failure rates are a bit higher than those of the HGST spindles, Backblaze has standardized on 4TB Seagate disks thanks to a combination of price and availability.

The weakest performance was from Western Digital. The standout here, for all the wrong reasons, is the company's 2TB units. Though Backblaze doesn't have many of these units (only 131), limiting the value of the data, the company found that almost 10 percent of the disks failed, even though their average age is just shy of nine months.

This data is striking because WD owns HGST, operating it as a subsidiary. So one company is building both the most and the least reliable new disks. The difference isn't readily attributable to different manufacturing, either; HGST's 3.5-inch disk manufacturing capability was sold to Toshiba. Backblaze has only a limited number of Toshiba drives, and thus far they're performing at about the same level as the Seagates.

Currently, Backblaze's storage is mainly made up of 3 and 4TB disks, but the company has started using the new 6 and 8TB models. At 6TB, Seagate has done very well, whereas WD has shown much worse reliability. Though the density of the 6TB disks is appealing, the company is currently sticking with 4TB as its main size. The price per gigabyte of the 6TB disks is about a third more than that of the 4TB disks, and power consumption is some 60 percent greater. As such, while the 6TB units make sense if you're strictly space constrained, the 4TB disks represent the better option for applications that are either power or cash constrained.

The latest innovation in hard disks is the use of helium-filled drives, with both HGST and Seagate having entrants in the field, and Backblaze has started experimenting with 8TB helium-filled HGST drives. These drives have some promise due to their greater density and lower power consumption, but their cost per gigabyte is currently almost 80 percent higher than that of the 4TB disks. As such, the company has filled only one of its storage pods with 45 of the drives, packing 360TB into a 4U system. Some drive failures have been experienced in this system, but the data set is too small to make any inferences from this data.

As ever, Backblaze's hard disk reliability data comes with a footnote of sorts. The company uses non-enterprise drives in a high density, (relatively) high vibration environment, with a 100 percent duty cycle. While Backblaze is now offering general purpose cloud storage that's comparable to Amazon's S3 or Microsoft's Azure storage, its main application is still backups, and this focus strongly influences access patterns. We aren't talking about hugely I/O intensive workloads, meaning that Backblaze's data may not be a good guide when compared to other usage patterns.

The study is still useful, though, not just because Backblaze actually names names and identifies the different models of drive that it uses rather than anonymizing them completely or grouping them by manufacturer. It doesn't seem too churlish to suggest that self-builders and home users should probably take a good hard look at the HGST drives as their track record is so strong. They don't match Backblaze's preference, but for home users, the cost seems insignificant, and the ability to easily source hundreds of drives at a time is irrelevant.