When they are properly set, alert
thresholds provide a valuable service—an alert—by indicating a performance
metric that is at an unexpected value. Unfortunately, in many cases the
expected value varies with the workload type, system load, time of day, or day
of the week. Baselines associated with certain workload types or days of the
week capture the metric values of that period. The baseline can then be used to
set the threshold values when similar conditions exist.
The statistics for baselines are
computed to place a minimal load on the system; statistics for static baselines
are manually computed. You can schedule statistics computation on the AWR
Baselines page. Statistics for the system moving window are automatically
computed according to the BSLN_MAINTAIN_STATS_SCHED schedule. By default, this
schedule starts the job every week at noon on Saturday.
Performance Monitoring and
Baselines
Metric statistics computed over a
baseline enable you to set thresholds that compare the baseline statistics to
the current activity. There are three methods of comparison:
- significance level
- percentage of maximum and
- fixed values
Thresholds based on significance
level use statistical relevance to determine which current values are
unusual. In simple terms, if the significance level is set to .99 for a
critical threshold, the threshold is set where 1% of the baseline values fall
outside this value and any current values that exceed this value trigger an
alert. A higher significance level of .999 or .9999 causes fewer alerts to be
triggered.
Thresholds based on percentage
of maximum are calculated based on the maximum value captured by the
baseline.
Threshold values based on fixed
values are set by the DBA. No baseline is required.
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