control system

Means by which a set of variable quantities is held constant or caused to vary in a prescribed way. Control systems are intimately related to the concept of automation but have an ancient history. Roman engineers maintained water levels in aqueducts by means of floating valves that opened and closed at appropriate levels. James Watt's flyball governor (1769) regulated steam flow to a steam engine to maintain constant engine speed despite a changing load. In World War II, control-system theory was applied to anti-aircraft batteries and fire-control systems. The introduction of analog and digital computers opened the way for much greater complexity in automatic control theory. Seealso Jacquard loom, pneumatic device, servomechanism.

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Self-Monitoring, Analysis, and Reporting Technology, or S.M.A.R.T. (sometimes written SMART), 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-drive failures fall into one of two basic classes:

  • Predictable ones, which occur gradually over time – e.g., mechanical wear and gradual degradation of storage surfaces. A monitoring device can detect these, much as a temperature dial on a vehicle's dashboard can warn a driver, before serious damage has occurred, that the engine has started to overheat.
  • Unpredictable ones, which may occur suddenly and without warning, such as failure of an electronic component, or sudden mechanical failure (perhaps owing to mishandling).

Mechanical failures account for about 60 percent of all drive failures. Most mechanical failures result from gradual wear, although an eventual failure may be catastrophic. However, before complete failure occurs, 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, a marked increase in the number of damaged disk sectors, and so on.

The purpose of S.M.A.R.T. is to warn a user or system administrator of impending drive failure while there is still time to take preventative action, such as copying the data to a replacement device. 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, but suggests that certain sub-categories of information which some S.M.A.R.T. implementations track do correlate with actual failure rates – specifically, in the 60 days following the first scan error on a drive, the drive is, on average, 39 times more likely to fail 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. (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, whilst 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."

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.

The SMART status does not necessarily indicate the drive's past or present reliability. If a drive has already failed catastrophically, the SMART status may be inaccessible. Alternatively, if a drive has experienced problems in the past, but the sensors no longer detect such problems, the SMART status may, depending on the manufacturer's programming, suggest that the drive is now sound.

The inability to read some sectors is not always an indication that a drive is about to fail. One way that unreadable sectors may be created, even when the drive is functioning within specification, is through a sudden power failure while the drive is writing. In order to prevent this problem, modern hard drives will always finish writing at least the current sector immediately after the power fails (typically using rotational energy from the disk). Also, even if the physical disk is damaged at one location, such that a certain sector is unreadable, the disk may be able to use spare space to replace the bad area, so that the sector can be overwritten.

More detail on the health of the drive may be obtained by examining the SMART Attributes. SMART Attributes were included in some drafts of the ATA standard, but were removed before the standard became final. The meaning and interpretation of the attributes varies between manufacturers, and are sometimes considered a trade secret for one manufacturer or another. Attributes are further discussed below.

Drives with SMART may optionally support a number of 'logs'. The error log records information about the most recent errors that the drive has reported back to the host computer. Examining this log may help one to determine whether computer problems are disk-related or caused by something else.

A drive supporting SMART may optionally support a number of self-test or maintenance routines, and the results of the tests are kept in the self-test log. The self-test routines may be used to detect any unreadable sectors on the disk, so that they may be restored from back-up sources (for example, from other disks in a RAID). This helps to reduce the risk of incurring permanent loss of data.

Standards and implementation

Many motherboards will display a warning message when a disk drive is approaching failure. Although an industry standard among most major hard drive manufacturers, there are some remaining issues and much proprietary "secret knowledge" held by individual manufacturers as to their specific approach. As a result, S.M.A.R.T. is not always implemented correctly on many computer platforms, due to the absence of industry-wide software & hardware standards for S.M.A.R.T. data interchange.

From a legal perspective, the term "S.M.A.R.T." refers only to a signalling method between internal disk drive electromechanical sensors and the host computer. Hence, a drive may be claimed by its manufacturers to include S.M.A.R.T. support even if it does not include, say, a temperature sensor, which the customer might reasonably expect to be present. Moreover, in the most extreme case, a disk manufacturer could, in theory, produce a drive which includes a sensor for just one physical attribute, and then legally advertise the product as "S.M.A.R.T. compatible".

Depending on the type of interface being used, some S.M.A.R.T.-enabled motherboards and related software may not communicate with certain S.M.A.R.T.-capable drives. For example, few external drives connected via USB and Firewire correctly send S.M.A.R.T. data over those interfaces. With so many ways to connect a hard drive (SCSI, Fibre Channel, ATA, SATA, SAS, SSA, and so on), it is difficult to predict whether S.M.A.R.T. reports will function correctly in a given system.

Even on hard drives and interfaces that support it, S.M.A.R.T. information may not be reported correctly to the computer's operating system. Some disk controllers can duplicate all write operations on a secondary "back-up" drive in real time. This feature is known as "RAID mirroring". However, many programs which are designed to analyze changes in drive behaviour and relay S.M.A.R.T. alerts to the operator do not function properly when a computer system is configured for RAID support. Generally this is because, under normal RAID operational conditions, the computer is not permitted by the RAID subsystem to 'see' (or directly access) individual physical drives, but may access only logical volumes instead.

On the Windows platform, many programs designed to monitor and report S.M.A.R.T. information will function only under an administrator account. At present, S.M.A.R.T. is implemented individually by manufacturers, and while some aspects are standardized for compatibility, others are not.

ATA S.M.A.R.T. Attributes

Each drive manufacturer defines a set of attributes, and sets threshold values beyond which attributes should not pass under normal operation. Each attribute has a raw value, whose meaning is entirely up to the drive manufacturer (but often corresponds to counts or a physical unit, such degrees Celsius or seconds), and a normalized value, which ranges from 1 to 253 (with 1 representing the worst case and 253 representing the best). Depending on the manufacturer, a value of 100 or 200 will often be chosen as the "normal" value.

Manufacturers that have supported at least one S.M.A.R.T. attribute in various products include: Samsung, Seagate, IBM (Hitachi), Fujitsu, Maxtor, Toshiba, Western Digital and ExcelStor Technology.

Known ATA S.M.A.R.T. attributes

The following chart lists some S.M.A.R.T. attributes and the typical meaning of their raw values. Normalized values are always mapped so that higher values are better (with only very rare exceptions such as the "Temperature" attribute on certain Seagate drives), but higher raw attribute values may be better or worse depending on the attribute and manufacturer. For example, the "Reallocated Sectors Count" attribute's normalized value decreases as the number of reallocated sectors increases. In this case, the attribute's raw value will often indicate the actual number of sectors that were reallocated, although vendors are in no way required to adhere to this convention. As manufacturers do not necessarily agree on precise attribute definitions and measurement units, the following list of attributes should be regarded as a general guide only.

Higher raw value is better
Lower raw value is better
Potential indicators of imminent electromechanical failure

ID Hex Attribute name Better Description
01 01 Read Error Rate
Indicates the rate of hardware read errors that occurred when reading data from a disk surface. A non-zero value indicates a problem with either the disk surface or read/write heads. Note that Seagate drives often report a raw value that is very high even on new drives, and does not thereby indicate a failure.
02 02 Throughput Performance
Overall (general) throughput performance of a hard disk drive. If the value of this attribute is decreasing there is a high probability that there is a problem with the disk.
03 03 Spin-Up Time
Average time of spindle spin up (from zero RPM to fully operational [millisecs]).
04 04 Start/Stop Count A tally of spindle start/stop cycles.
05 05 Reallocated Sectors Count
Count of reallocated sectors. When the hard drive finds a read/write/verification error, it marks this sector as "reallocated" and transfers data to a special reserved area (spare area). This process is also known as remapping, and "reallocated" sectors are called remaps. This is why, on modern hard disks, "bad blocks" cannot be found while testing the surface – all bad blocks are hidden in reallocated sectors. However, as the number of reallocated sectors increases, the read/write speed tends to decrease. The raw value normally represents a count of the number of bad sectors that have been found and remapped. Thus, the higher the attribute value, the more sectors the drive has had to reallocate.
06 06 Read Channel Margin Margin of a channel while reading data. The function of this attribute is not specified.
07 07 Seek Error Rate
Rate of seek errors of the magnetic heads. If there is a partial failure in the mechanical positioning system, then seek errors will arise. Such a failure may be due to numerous factors, such as damage to a servo, or thermal widening of the hard disk. More seek errors indicates a worsening condition of a disk’s surface or the mechanical subsystem, or both. Note that Seagate drives often report a raw value that is very high, even on new drives, and this does not normally indicate a failure.
08 08 Seek Time Performance
Average performance of seek operations of the magnetic heads. If this attribute is decreasing, it is a sign of problems in the mechanical subsystem.
09 09 Power-On Hours (POH)
Count of hours in power-on state. The raw value of this attribute shows total count of hours (or minutes, or seconds, depending on manufacturer) in power-on state.
10 0A Spin Retry Count
Count of retry of spin start attempts. This attribute stores a total count of the spin start attempts to reach the fully operational speed (under the condition that the first attempt was unsuccessful). An increase of this attribute value is a sign of problems in the hard disk mechanical subsystem.
11 0B Recalibration Retries
This attribute indicates the number of times recalibration was requested (under the condition that the first attempt was unsuccessful). A decrease of this attribute value is a sign of problems in the hard disk mechanical subsystem.
12 0C Device Power Cycle Count This attribute indicates the count of full hard disk power on/off cycles.
13 0D Soft Read Error Rate
Uncorrected read errors reported to the operating system. If the value is non-zero, you should back up your data.
189 BD High Fly Writes (WDC)
Fly Height Monitor Improves Hard Drive Reliability. Western Digital's Fly Height Monitor protects write operations by detecting when a recording head is flying outside its normal operating range. If an unsafe fly height condition is encountered, the write process is stopped, and the information is rewritten or reallocated to a safe region of the hard drive. This constant monitoring process increases the reliability of write operations and reduces the probability of read errors. The new Fly Height Monitor is being implemented in Western Digital’s drives, beginning with the WD Enterprise WDE18300 and WDE9180 Ultra2 SCSI hard drives, and will be included on all future WD Enterprise products.(
190 BE Airflow Temperature (WDC)
Airflow temperature on Western Digital HDs (Same as temp. [C2], but current value is 50 less for some models. Marked as obsolete.)
190 BE Temperature Difference from 100
Value is equal to (100 – temp. °C), allowing manufacturer to set a minimum threshold which corresponds to a maximum temperature. (Seagate only?)
Seagate ST910021AS: Verified Present
Seagate ST9120823ASG: Verified Present under name "Airflow Temperature Cel" 2008-10-06
Seagate ST3802110A: Verified Present 2007-02-13
Seagate ST980825AS: Verified Present 2007-04-05
Seagate ST3320620AS: Verified Present 2007-04-23
Seagate ST3500641AS: Verified Present 2007-06-12
Seagate ST3250824AS: Verified Present 2007-08-07
Seagate ST31000340AS: Verified Present 2008-02-05
Seagate ST3160211AS: Verified Present 2008-06-12
Seagate ST3320620AS: Verified Present 2008-06-12
Seagate ST3400620AS: Verified Present 2008-06-12
Samsung HD501LJ: Verified Present under name "Airflow Temperature" 2008-03-02
Samsung HD753LJ: Verified Present under name "Airflow Temperature" 2008-07-15
191 BF G-sense error rate
Frequency of mistakes as a result of impact loads
192 C0 Power-off Retract Count
Number of times the heads are loaded off the media. Heads can be unloaded without actually powering off. (or Emergency Retract Cycle count – Fujitsu)
193 C1 Load/Unload Cycle
Count of load/unload cycles into head landing zone position.
194 C2 Temperature
Current internal temperature.
195 C3 Hardware ECC Recovered
Time between ECC-corrected errors.
196 C4 Reallocation Event Count
Count of remap operations. The raw value of this attribute shows the total number of attempts to transfer data from reallocated sectors to a spare area. Both successful & unsuccessful attempts are counted.
197 C5 Current Pending Sector Count
Number of "unstable" sectors (waiting to be remapped). If the unstable sector is subsequently written or read successfully, this value is decreased and the sector is not remapped. Read errors on the sector will not remap the sector, it will only be remapped on a failed write attempt. This can be problematic to test because cached writes will not remap the sector, only direct I/O writes to the disk.
198 C6 Uncorrectable Sector Count
The total number of uncorrectable errors when reading/writing a sector. A rise in the value of this attribute indicates defects of the disk surface and/or problems in the mechanical subsystem.
199 C7 UltraDMA CRC Error Count
The number of errors in data transfer via the interface cable as determined by ICRC (Interface Cyclic Redundancy Check).
200 C8 Write Error Rate /
Multi-Zone Error Rate
The total number of errors when writing a sector.
201 C9 Soft Read Error Rate
Number of off-track errors. If non-zero, make a backup.
202 CA Data Address Mark errors
Number of Data Address Mark errors (or vendor-specific).
203 CB Run Out Cancel
Number of ECC errors
204 CC Soft ECC Correction
Number of errors corrected by software ECC
205 CD Thermal Asperity Rate (TAR)
Number of thermal asperity errors.
206 CE Flying Height ? Height of heads above the disk surface.
207 CF Spin High Current ? Amount of high current used to spin up the drive.
208 D0 Spin Buzz ? Number of buzz routines to spin up the drive
209 D1 Offline Seek Performance ? Drive’s seek performance during offline operations
211 D3 Vibration During Write ? Vibration During Write
212 D4 Shock During Write ? Shock During Write
220 DC Disk Shift
Distance the disk has shifted relative to the spindle (usually due to shock). Unit of measure is unknown.
221 DD G-Sense Error Rate
The number of errors resulting from externally-induced shock & vibration.
222 DE Loaded Hours ? Time spent operating under data load (movement of magnetic head armature)
223 DF Load/Unload Retry Count ? Number of times head changes position.
224 E0 Load Friction
Resistance caused by friction in mechanical parts while operating.
225 E1 Load/Unload Cycle Count
Total number of load cycles
226 E2 Load 'In'-time ? Total time of loading on the magnetic heads actuator (time not spent in parking area).
227 E3 Torque Amplification Count
Number of attempts to compensate for platter speed variations
228 E4 Power-Off Retract Cycle
The number of times the magnetic armature was retracted automatically as a result of cutting power.
230 E6 GMR Head Amplitude ? Amplitude of "thrashing" (distance of repetitive forward/reverse head motion)
231 E7 Temperature
Drive Temperature
240 F0 Head Flying Hours ? Time while head is positioning
250 FA Read Error Retry Rate
Number of errors while reading from a disk
254 FE Free Fall Protection
Number of "Free Fall Events" detected

Threshold Exceeds Condition

Threshold Exceeds Condition (TEC) is a supposed date when a critical drive statistic attribute will reach its threshold value. When Drive Health software reports a "Nearest T.E.C.", it should be regarded as a "Failure date".

Prognosis of this date is based on the factor "Speed of attribute change"; how many points each month the value is decreasing/increasing. This factor is calculated automatically at any change of S.M.A.R.T. attributes for each attribute individually. Note that TEC dates are not guarantees; hard drives can and will either last much longer or fail much sooner than the date given by a TEC.

Comparison of S.M.A.R.T. tools

In the following some popular tools for reading S.M.A.R.T. data are listed.

smartmontools libatasmart HDAT2 DriveSitter HDD Health Active Smart SpeedFan SMARTReporter HDTune Norton System Doctor SMART Utility DiskCheckup Hard Disk Sentinel DiskChecker
Operating System Windows (native or Cygwin)
Darwin (Mac OS X)
Linux DOS Windows Windows Windows Windows Mac OS X Windows Windows Mac OS X Windows Windows, DOS, Linux Windows, .NET Framework 3.5
Price Open Source Open Source Freeware 29,69 $ Freeware 18,46 € Freeware Open Source Freeware proprietary 20.00 $ Freeware for personal use, 15.00 $ otherwise Windows: from 18,00 €, DOS/Linux: Freeware 19,00 $
Trial version can be used - - - 30 days - 21 days - - - - 30 days/5 launches - - 30 days
Target group
professionals professionals, programmers professionals advanced beginners to advanced beginners to advanced beginners to advanced beginners beginners to advanced beginners beginners to advanced beginners to advanced beginners to advanced beginners
User Interface
optional daemon or service
Command line,
Programming Library
text menu graphical graphical graphical graphical graphical graphical graphical graphical graphical Windows: graphical, DOS/Linux: commandline graphical
(S)ATA, SCSI, SAT (incl. some USB) (S)ATA, SAT (incl. some USB) (S)ATA (S)ATA (S)ATA (S)ATA, SCSI, SAT (incl. some USB) (S)ATA, SCSI (S)ATA (S)ATA (S)ATA, SCSI, SAT (incl. some USB) (S)ATA, SCSI, SAT (incl. some USB) (S)ATA, SCSI (S)ATA, SCSI, SAT (incl. some USB) (S)ATA
Reads hard discs on RAID controllers:
3ware (Linux, FreeBSD, Windows),
Compaq/HP (Linux, FreeBSD),
and HighPoint and Areca (only Linux)
- yes -
Areca and Software RAID - -
? 3ware,
OS X Software RAID
Software RAID only Software RAID only
Shows error log
yes no yes yes yes no no no no no yes yes yes no
Self testing
yes (also scheduled) yes yes yes yes no no no no no announced yes yes yes
Prediction of failure
no yes no yes yes yes yes no no no yes yes yes yes
Notification at
choosable parameter changes , threshold, temperature - - choosable parameter changes , threshold, temperature every parameter change, temperature threshold, temperature choosable parameter changes , threshold, temperature threshold, - threshold, (for every single medium) announced temperature temperature, parameter changes, threshold, new problem found, low disk space at startup, low health
Notification by window (only Windows), e-mail, system log, run a certain command - - window, sound, e-mail, network message, system log, run a certain command window, sound, e-mail, network message, system log window, sound, e-mail, network message window, sound, e-mail, run a certain command window, e-mail, run a certain command - taskbar symbol, sound, administrative message announced window, e-mail taskbar, window, e-mail, network message, sound alert (optionally repeating), run a certain command, run automatic backup projects, shutdown/hibernate window, task bar notification
smartmontools Lennart Poettering Lubomir Cabla Oliver Marr PANTERASoft Ariolic ATA / SCSI / USB Alfredo Milani Comparetti Julian Mayer EFD Software Symantec weblink Volitans Software Passmark Software H.D.S. Hungary Bapuli Online
Notes Possibility of AAM and further parameter, surface testing API for other programs highly scalable, can be set to activate hibernation mode on critical temperature can be set to activate hibernation mode on critical temperature offers online drive analysis , monitors PC temperatures benchmarks and surface testing individual configuration for every medium, Interface for Disc Doktor/chkdsk: surface testing, complete testing at restart based on smartmontools acoustic management, highly scalable scheduled and automatic projects upon failure, offers scheduled and manual disk testing, performance and logical disk information


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