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 centers run by Google Inc. 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 centers. 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."