infra/docs/plans/2026-06-04-pve-fan-control-design.md

PVE R730 presence-aware fan control — design

Date: 2026-06-04 Status: implemented Scripts: infra/scripts/fan-control.{sh,service,env.example}, test-fan-control.sh Runbook: infra/docs/runbooks/fan-control.md

Problem

The Dell R730 PVE host (192.168.1.127) runs its CPU at ~72–77°C under normal cluster load. That is safe (firmware warning at 88°C, critical 93°C) but the iDRAC's stock fan curve optimises for quiet, not cool — it pins the fans at the ~7080 RPM floor even at 72°C / load 30 and only ramps near ~80°C. We want the CPU to run cooler when it costs nothing (the box is in the garage, usually empty) while staying quiet when someone is physically in the garage.

Measured fan/temp relationship (manual IPMI sweep, 2026-06-04)

At a comparable CPU load (~45–53 % busy):

Fan setting	Fan RPM	CPU temp
Auto (floor)	7,080	71–72°C
50 %	9,360	65–66°C
70 %	12,800	60–61°C
100 %	17,000	55–56°C

Best °C-per-RPM is the first step; beyond ~70 % it is mostly noise. ~16°C of swing is available.

Power characterization (sweep 2026-06-05)

Averaged wall power (iDRAC DCMI) + temp at each fan setting:

Fan	RPM	Power	CPU	load
auto	7,080	296 W	68°C	21
20 %	4,800	281 W	73°C	20
30 %	6,360	288 W	72°C	19
50 %	9,360	299 W	65°C	18
60 %	11,040	303 W	61°C	17
70 %	12,720	324 W	59°C	16
100 %	16,920	378 W	59°C	17

The cooling-per-watt knee is ~60 %. Fan power follows ~RPM³: 60→70 % costs +21 W for −2°C; 70→100 % costs +54 W for 0°C (the CPU floors ~59°C at cluster load — more airflow does nothing). Full speed draws ~97 W (~850 kWh/yr) over the floor and buys nothing past 60 %.

Decision (2026-06-05): the COOL curve caps its normal band at 60 % (~303 W, ~61°C) — capturing essentially all achievable cooling while avoiding the wasteful 80–100 % zone, now reserved as a high-load safety ramp (≥73/79°C) before the 83°C ceiling. QUIET is unchanged (already at the low-power floor: 20 % / 4,800 RPM / 281 W). Verified live after re-tune: 63°C, 60 %, ~267 W.

Decisions

1. Custom bash daemon + systemd service, deployed to the PVE host the same way as apply-mbps-caps / daily-backup (source in infra/scripts/, scp to /usr/local/bin). It cannot be Terraform/k8s — it runs on the bare host where IPMI lives. (OSS tigerblue77/Dell-iDRAC-fan-controller was considered; rejected — it is a Docker container, off-pattern here, and unaware of our constraints.)

2. CPU temperature is the only control input. The Tesla T4 has its own always-on fan (owner-confirmed), so it self-cools and does not depend on chassis airflow — no GPU coupling needed.

3. Presence = the garage door, because the server is in the garage (memory id=1723); noise only matters to people physically there. Signal: ha-sofia sensor.garage_door_state_bg. Open now, or last changed within HOLD_SECS (15 min) ⇒ someone's around ⇒ QUIET; otherwise COOL. house_mode was rejected — it tracks apartment occupancy, irrelevant to garage noise.

4. Two curves, picked by presence (COOL power-tuned 2026-06-05 — see "Power characterization" below):

   CPU °C	COOL % (empty)	CPU °C	QUIET % (occupied)
   ≤54	30	≤72	20 (≈silent floor)
   55–63	50	73–77	40
   64–72	60 (knee)	78–81	65
   73–78	80	≥82	100
   ≥79	100

   3°C downward hysteresis prevents flapping at band edges (ramp up immediately, step down only once the curve still wants lower 3°C hotter).

Safety

Manual fan mode bypasses the iDRAC's own protection, so it is backstopped:

• Daemon exit/crash/stop → bash EXIT trap + systemd ExecStopPost both run ipmitool raw 0x30 0x30 0x01 0x01 (restore Dell auto). Restart=on-failure.
• CPU ≥ CEILING (83°C) → hand back to Dell auto until temp holds below RESUME_BELOW (75°C) for RESUME_STABLE (120 s), then resume manual.
• IPMI read failures ≥ MAX_IPMI_FAILS → restore Dell auto.
• ha-sofia unreachable → keep the last good presence decision; default COOL at cold start (thermally safe).

Observability

Pushes to the existing Pushgateway (http://10.0.20.100:30091, job fan_control): pve_fan_control_cpu_temp_celsius, _fan_percent, _mode (1 quiet / 2 cool / 0 fallback), _ha_reachable, _fallback. The existing CPU- temp alert is unaffected.

Testing

test-fan-control.sh sources the script (main is guarded by a BASH_SOURCE check) and unit-tests the pure functions: both curves, hysteresis up/down, presence open/recent/stale, temperature parsing, jq-free JSON field extraction, and percent→hex. 36 assertions, no hardware needed. The daemon also supports DRY_RUN=1 and RUN_ONCE=1 for integration checks.

Rollback

systemctl disable --now fan-control && ipmitool raw 0x30 0x30 0x01 0x01 on the host returns the box to stock firmware fan control. See the runbook.