Self-Monitoring, Analysis, and Reporting Technology, or S.M.A.R.T. is a system to watch computer hard disks to help stop the risk of failures. It does this by watching reliability of the hard disks to try and see when a failure might happen and where.
Background
Hard drive failures fall into two categories: Predictable failures''' happen over a longer time period. Examples for this is mechanical wear, or degradation of the surface of the storage device.Unpredictable failures happen suddenly, in an unforeseen manner. Examples for this is the failure of an electronic component, or sudden mechanical failure, perhaps because of bad handling.
Predictable failures can be detected by certain monitoring devices. This is like a thermometer in a vehicle that can alert the driver to do something before serious damage occurs, for example because the engine is too hot.
About 60% of all drive failures are caused by failing mechanics. Most mechanical failures result from gradual wear. An eventual failure may be catastrophic. Before complete failure occurs, there are usually certain signs that failure is imminent. These may include increased heat output, a more noisy drive, problems with reading or writing data and a big increase in the number of damaged disk sectors.
The purpose of S.M.A.R.T. is to warn a user or system administrator that a drive is about to fail. At the time of the warning, there is usually still time to do certain things to prevent data loss, for example copying the data to a different drive. Approximately 30% of failures can be predicted by S.M.A.R.T. Work at Google on over 100,000 drives has shown little overall predictive value of S.M.A.R.T. status as a whole. The study suggests that certain sub-categories of information which some S.M.A.R.T. implementations track do correlate with actual failure rates. In the 60 days after the first scan error on a drive, the drive is 39 times more likely to fail on average than it would have been had no such error occurred. Also, first errors in reallocations, offline reallocations and probational counts are strongly correlated to higher probabilities of failure.
PCTechGuide's page on S.M.A.R.T. commented in 2003 that the technology had gone through three phases:
History and predecessors
The industry's first hard disk monitoring technology was introduced by IBM in 1992 in their IBM 9337 Disk Arrays for AS/400 servers using IBM 0662 SCSI-2 disk drives. Later it was named Predictive Failure Analysis (PFA) technology. It was measuring several key device health parameters and evaluating them within the drive firmware. Communications between the physical unit and the monitoring software were limited to a binary result – namely, either "device is OK" or "drive is likely to fail soon".
Later, another variant, which was named IntelliSafe, was created by computer manufacturer Compaq and disk drive manufacturers Seagate, Quantum, and Conner. The disk drives would measure the disk’s "health parameters", and the values would be transferred to the operating system and user-space monitoring software. Each disk drive vendor was free to decide which parameters were to be included for monitoring, and what their thresholds should be. The unification was at the protocol level with the host.
Compaq submitted their implementation to Small Form Committee for standardization in early 1995. It was supported by IBM, by Compaq's development partners Seagate, Quantum, and Conner, and by Western Digital, who did not have a failure prediction system at the time. The Committee chose IntelliSafe's approach, as it provided more flexibility. The resulting jointly developed standard was named S.M.A.R.T.
SMART Information
The technical documentation for SMART is in the AT Attachment (ATA) standard.
The most basic information that SMART provides is the SMART status. It provides only two values: "threshold not exceeded" and "threshold exceeded". Often these are represented as "drive OK" or "drive fail" respectively. A "threshold exceeded" value is intended to indicate that there is a relatively high probability that the drive will not be able to honour its specification in the future – that is, the drive is "about to fail". The predicted failure may be catastrophic or may be something as subtle as the inability to write to certain sectors, or perhaps slower performance than the manufacturer's declared minimum.