Self-Monitoring, Analysis, and Reporting Technology or S.M.A.R.T. is a monitoring system for computer hard disks to detect and report on various indicators of reliability, in the hope of anticipating failures.
Fundamentally hard drives can suffer one of two classes of failure:
- Predictable - Some failure modes, especially mechanical wear and aging, happen gradually over time. A monitoring device can detect these, much as a temperature dial on the dashboard of an automobile can warn a driver -- before serious damage occurs -- that the engine has started to overheat.
- Unpredictable - Other failures may occur suddenly and unpredictably, such as an electronic component burning out.
Monitoring a drive's behavior can predict approximately 60 percent of hard drive failures.
S.M.A.R.T. technology has the purpose of warning a user or system administrator of impending drive failure while time remains to take preventive action -- such as copying the data to a replacement device.
As an example, a WDC800JB-00CRA1 80-gigabyte hard drive can monitor the following S.M.A.R.T attributes:
- Raw read error rate
- Spin up time
- Start/stop count
- Reallocated sector count
- Seek error rate
- Power on hours count
- Spin retry count
- Recalibration retry count
- Power cycle count
- Reallocation event count
- Current pending sector count
- Uncorrectable sector count
- CRC error count
- Write error count
According to the documentation from Western Digital, "other drive manufacturers may have their own attribute definitions."
SMART attribute values can range from 1 to 253 with 1 being the worst case and 253 being the best case; depending on the manufacturer, either 100 or 200 will often be chosen as the "normal" value.
Note that this drive does not report drive temperature, because it lacks a temperature sensor. (Many other drives do include temperature sensors.) Values for these attributes are stored in a reserved area of the hard drive.
Compaq pioneered S.M.A.R.T., but most major hard-drive and motherboard vendors now support it. Most current motherboards will display a warning message when a disk drive approaches failure.
Various monitoring programs also exist that can keep closer track of a disk drive's health. Each disk-maker defines
a set of attributes which the controller monitors, and also defines threshold values for these attributes that normal operation should not exceed. Suitable software can keep track of these parameters over time, in order to extrapolate the current trends and predict when a "threshold exceeded condition" might occur. Also, the software can distinguish between gradual degradation over time(representing normal wear) and a sudden change which may indicate a problem.
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