Customers are replacing disk drives at rates far higher than those suggested by the estimated mean time between failure (MTBF) supplied by drive vendors, according to a study of about 100,000 drives conducted by Carnegie Mellon University.

The study, presented last month at the 5th USENIX Conference on File and Storage Technologies in San Jose, also shows no evidence that Fibre Channel (FC) drives are any more reliable than less expensive but slower performing Serial ATA (SATA) drives.

That surprising comparison of FC and SATA reliability could speed the trend away from FC to SATA drives for applications such as near-line storage and backup, where storage capacity and cost are more important than sheer performance, analysts said.

At the same conference, another study of more than 100,000 drives in data centres run by Google indicated that temperature seems to have little effect on drive reliability, even as vendors and customers struggle to keep temperature down in their tightly packed data centres. Together, the results show how little information customers have to predict the reliability of disk drives in actual operating conditions and how to choose among various drive types.

Real world vs. data sheets

The Carnegie Mellon study examined large production systems, including high-performance computing sites and Internet services sites running SCSI, FC and SATA drives. The data sheets for those drives listed MTBF between 1 million to 1.5 million hours, which the study said should mean annual failure rates "of at most 0.88%." However, the study showed typical annual replacement rates of between 2% and 4%, "and up to 13% observed on some systems."

Garth Gibson, associate professor of computer science at Carnegie Mellon and co-author of the study, was careful to point out that the study didn't necessarily track actual drive failures, but cases in which a customer decided a drive had failed and needed replacement. He also said he has no vendor-specific failure information, and that his goal is not "choosing the best and the worst vendors" but to help them to improve drive design and testing.

He echoed storage vendors and analysts in pointing out that as many as half of the drives returned to vendors actually work fine and may have failed for any reason, such as a harsh environment at the customer site and intensive, random read/write operations that cause premature wear to the mechanical components in the drive.

Several drive vendors declined to be interviewed. "The conditions that surround true drive failures are complicated and require a detailed failure analysis to determine what the failure mechanisms were," said a spokesperson for Seagate Technology in Scotts Valley, Calif., in an e-mail. "It is important to not only understand the kind of drive being used, but the system or environment in which it was placed and its workload."

"Regarding various reliability rate questions, it's difficult to provide generalities," said a spokesperson for Hitachi Global Storage Technologies in San Jose, in an e-mail. "We work with each of our customers on an individual basis within their specific environments, and the resulting data is confidential."

Ashish Nadkarni, a principal consultant at GlassHouse Technologies, a storage services provider in Massachusetts, U.S., said he isn't surprised by the comparatively high replacement rates because of the difference between the "clean room" environment in which vendors test and the heat, dust, noise or vibrations in an actual data centre.

He also said he has seen overall drive quality falling over time as the result of price competition in the industry. He urged customers to begin tracking disk drive records "and to make a big noise with the vendor" to force them to review their testing processes.