Seagate Constellation ST4000NM0023 Product Manual page 19

Controlling S.M.A.R.T.
The operating mode of S.M.A.R.T. is controlled by the DEXCPT and PERF bits on the Informational Exceptions Control mode page
(1Ch). Use the DEXCPT bit to enable or disable the S.M.A.R.T. feature. Setting the DEXCPT bit disables all S.M.A.R.T. functions.
When enabled, S.M.A.R.T. collects on-line data as the drive performs normal read and write operations. When the PERF bit is set, the
drive is considered to be in "On-line Mode Only" and will not perform off-line functions.
You can measure off-line attributes and force the drive to save the data by using the Rezero Unit command. Forcing S.M.A.R.T. resets the
timer so that the next scheduled interrupt is in one hour.
You can interrogate the drive through the host to determine the time remaining before the next scheduled measurement and data logging
process occurs. To accomplish this, issue a Log Sense command to log page 0x3E. This allows you to control when S.M.A.R.T.
interruptions occur. Forcing S.M.A.R.T. with the RTZ command resets the timer.
Performance impact
S.M.A.R.T. attribute data is saved to the disk so that the events that caused a predictive failure can be recreated. The drive measures and
saves parameters once every one hour subject to an idle period on the drive interfaces. The process of measuring off-line attribute data
and saving data to the disk is interruptable. The maximum on-line only processing delay is summarized below:
Maximum processing delay
S.M.A.R.T. delay times
Reporting control
Reporting is controlled by the MRIE bits in the Informational Exceptions Control mode page (1Ch). An example, if the MRIE is set to
one, the firmware will issue to the host an 01-5D00 sense code. The FRU field contains the type of predictive failure that occurred. The
error code is preserved through bus resets and power cycles.
Determining rate
S.M.A.R.T. monitors the rate at which errors occur and signals a predictive failure if the rate of degraded errors increases to an
unacceptable level. To determine rate, error events are logged and compared to the number of total operations for a given attribute. The
interval defines the number of operations over which to measure the rate. The counter that keeps track of the current number of
operations is referred to as the Interval Counter.
S.M.A.R.T. measures error rates. All errors for each monitored attribute are recorded. A counter keeps track of the number of errors for
the current interval. This counter is referred to as the Failure Counter.
Error rate is the number of errors per operation. The algorithm that S.M.A.R.T. uses to record rates of error is to set thresholds for the
number of errors and their interval. If the number of errors exceeds the threshold before the interval expires, the error rate is considered to
be unacceptable. If the number of errors does not exceed the threshold before the interval expires, the error rate is considered to be
acceptable. In either case, the interval and failure counters are reset and the process starts over.
Predictive failures
S.M.A.R.T. signals predictive failures when the drive is performing unacceptably for a period of time. The firmware keeps a running
count of the number of times the error rate for each attribute is unacceptable. To accomplish this, a counter is incremented each time the
error rate is unacceptable and decremented (not to exceed zero) whenever the error rate is acceptable. If the counter continually
increments such that it reaches the predictive threshold, a predictive failure is signaled. This counter is referred to as the Failure History
Counter. There is a separate Failure History Counter for each attribute.
C ES.3 SAS P
ONSTELLATION
Fully-enabled delay
DEXCPT = 0
75 ms
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