Root cause: sum(rate(node_nfs_requests_total[5m])) == 0 was too fragile:
- rate() returns nothing after Prometheus restarts (needs 2 scrapes)
- Individual nodes show zero NFS rate during scrape gaps or low activity
- The sum() could hit zero during quiet hours + scrape gaps
New expression uses:
- changes() instead of rate() — works with a single scrape
- Per-instance aggregation: count nodes with any NFS counter change
- Threshold < 2 nodes: single-node restarts won't trigger, real NFS
outage (all nodes affected) will
- Prometheus startup guard: skip first 15m after restart to avoid
false positives from empty TSDB
- Wider 15m changes() window to smooth out scrape gaps
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