Seagate Cheetah 18FC ST118202FC, Руководство по эксплуатации продукта

Страница: 24 / 76

14 Cheetah 18FC Product Manual, Rev. C
5.1.4 Seek errors
A seek error is defined as a failure of the drive to position the heads to the addressed track. If the error recov-
ery fails, a seek positioning error (15h) is reported with a Medium (03h) or Hardware Error (04h) reported in the
sense key. This is an unrecoverable seek error. Unrecoverable seek errors are classified as failures for MTBF
calculations.
5.2 S.M.A.R.T.
S.M.A.R.T. is an acronym for Self-Monitoring Analysis and Reporting Technology. This technology is intended
to recognize conditions that indicate imminent drive failure and is designed to provide sufficient warning of a
failure to allow you to back up the data before an actual failure occurs.
Note. The drive’s firmware monitors specific attributes for degradation over time but can’t predict instanta-
neous drive failures.
Each monitored attribute has been selected to monitor a specific set of failure conditions in the operating per-
formance of the drive and the thresholds are optimized to minimize “false” and “failed” predictions.
Controling 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 DEX-
CPT 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 two hours.
You can interrogate the drive through the host to determine the time remaining before the next scheduled mea-
surement 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 disc so that the events that caused a predictive failure can be recre-
ated. The drive measures and saves parameters once every two hours subject to an idle period on the FC-AL
bus. The process of measuring off-line attribute data and saving data to the disc is uninterruptable. The maxi-
mum on-line only (DEXCPT = 0; PERF = 1) processing delay is 55 msec. The maximum fully-enabled (DEX-
CPT = 0, PERF = 0) processing delay is 410 msec.
Reporting control
Reporting is controlled by the MRIE bits in the Informational Exceptions Control mode page (1Ch). Subject to
the reporting method, the firmware will issue to the host an 01-5Dxx sense code. 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 inter-
val and failure counters are reset and the process starts over.