An alert rule: threshold and duration

Not running Starting usually ~20 s

Step 6 of 6 · summary · 2 min

Summary

Prometheus checks only the rules from the files listed in rule_files; the package's empty rule_files: means "no rules".
promtool check rules and promtool check config catch a mistake before the reread, and a failed reload keeps the previous rules.
A rule expression is a filter: each series in the result becomes an alert, an empty result means no alert. The threshold is compared with what the expression computes.
for keeps an alert pending until the condition has held for the set time, then it is firing; as soon as the expression goes empty, the alert leaves.
DiskAlmostFull from the start of the lesson was silent for two reasons: Prometheus had not loaded its file, and the > 0.95 threshold sat on the free share.

Ubuntu 24.04