Hardware Reference
In-Depth Information
part of Seagate), Quantum (later merged with Maxtor), and Western Digital. The S.M.A.R.T.
specification produced by this group and placed in the public domain covers both ATA and SCSI
HDDs and can be found in most of the more recently produced drives on the market.
The S.M.A.R.T. design of attributes and thresholds is similar in ATA and SCSI environments, but the
reporting of information differs.
In an ATA environment, driver software on the system interprets the alarm signal from the drive
generated by the S.M.A.R.T.
report status
command. The driver polls the drive on a regular
basis to check the status of this command and, if it signals imminent failure, sends an alarm to the OS
where it is passed on via an error message to the end user. This structure also enables future
enhancements, which might allow reporting of information other than drive failure conditions. The
system can read and evaluate the attributes and alarms reported in addition to the basic
report
status
command.
SCSI drives with S.M.A.R.T. communicate a reliability condition only as good or failing. In a SCSI
environment, the failure decision occurs at the disk drive and the host notifies the user for action. The
SCSI specification provides for a sense bit to be flagged if the drive determines that a reliability
issue exists. The system then alerts the end user via a message.
Note that traditional disk diagnostics such as Scandisk work only on the data sectors of the disk
surface and do not monitor all the drive functions that are monitored by S.M.A.R.T. Most modern disk
drives keep spare sectors available to use as substitutes for sectors that have errors. When one of
these spares is reallocated, the drive reports the activity to the S.M.A.R.T. counter but still looks
completely defect-free to a surface analysis utility, such as Scandisk.
Drives with S.M.A.R.T. monitor a variety of attributes that vary from one manufacturer to another.
Attributes are selected by the device manufacturer based on their capability to contribute to the
prediction of degrading or fault conditions for that particular drive. Most drive manufacturers
consider the specific set of attributes being used and the identity of those attributes as vendor specific
and proprietary.
Some drives monitor the floating height of the head above the magnetic media. If this height changes
from a nominal figure, the drive could fail. Other drives can monitor different attributes, such as ECC
circuitry that indicates whether soft errors are occurring when reading or writing data. Some of the
attributes monitored on various drives include the following:
• Head floating height
• Data throughput performance
• Spin-up time
• Reallocated (spared) sector count
• Seek error rate
• Seek time performance
• Drive spin-up retry count
• Drive calibration retry count
Each attribute has a threshold limit that determines the existence of a degrading or fault condition.
These thresholds are set by the drive manufacturer, can vary among manufacturers and models, and
can't be changed.
