S.M.A.R.T. - Background

Background

The purpose of S.M.A.R.T. is to warn a user of impending drive failure while there is still time to take action, such as copying the data to a replacement device.

Hard disk failures fall into one of two basic classes:

• Predictable failures: These failures result from slow processes such as mechanical wear and gradual degradation of storage surfaces. Monitoring can determine when such failures are becoming more likely.
• Unpredictable failures: These failures happen suddenly and without warning. They range from electronic components becoming defective to a sudden mechanical failure (perhaps due to improper handling).

Mechanical failures account for about 60% of all drive failures. While the eventual failure may be catastrophic, most mechanical failures result from gradual wear and there are usually certain indications that failure is imminent. These may include increased heat output, increased noise level, problems with reading and writing of data, or an increase in the number of damaged disk sectors.

Work at Google on over 100,000 drives found correlations between certain S.M.A.R.T. information and actual failure rates. In the 60 days following the first scan error on a drive, the drive was, on average, 39 times more likely to fail than it would have been if no such error occurred. First errors in reallocations, offline reallocations and probational counts were also strongly correlated to higher probabilities of failure. Conversely, little correlation was found for increased temperature and no correlation for usage level. However, a large proportion of the failed drives failed without giving any S.M.A.R.T. warnings at all, meaning that S.M.A.R.T. data alone was of limited usefulness in anticipating failures.

PCTechGuide's page on S.M.A.R.T. (2003) comments that the technology has gone through three phases:

"In its original incarnation SMART provided failure prediction by monitoring certain online hard drive activities. A subsequent version improved failure prediction by adding an automatic off-line read scan to monitor additional operations. The latest "SMART" technology not only monitors hard drive activities but adds failure prevention by attempting to detect and repair sector errors. Also, while earlier versions of the technology only monitored hard drive activity for data that was retrieved by the operating system, this latest SMART tests all data and all sectors of a drive by using "off-line data collection" to confirm the drive's health during periods of inactivity."