fan_speed

Fan control for Dell PowerEdge servers over IPMI, driven by CPU, GPU and disk temperature, with trend tracking and email alarms.

Upstream (tigerblue77/Dell_iDRAC_fan_controller_Docker) sets fan speed from CPU and GPU only. On a chassis with two dozen drives that misses the thing most likely to be quietly cooking, so this fork adds disks to the curve, records a rolling temperature history, and mails when something looks wrong.

For the MD1200 disk shelf, see the separate md1200-fan-control repo — that enclosure has its own controller and its own serial protocol.

How the fan speed is decided

Each heat source asks for a fan speed and the loudest request wins:

speed = max( interpolate(hottest of CPU/GPU, 45 -> 75),
             per-drive interpolation against each drive's own limit )

Sources are never compared as raw temperatures — an 85c CPU and a 45c disk are both "fine", and a single max() over the numbers would be meaningless.

The result is then held unless it moves by FAN_SPEED_DEADBAND (5%). This matters more than it sounds. The CPU curve is roughly 1% of fan per degree and idle CPU noise is ±2c, so without a deadband the setpoint changes on almost every pass and the fans never settle — measurably worse than stock. Measured on iz-pve0: stock iDRAC held one speed for 5 minutes straight through 57→59c jitter, while this script made 7 distinct changes in 16 minutes off the same signal. A request for full speed is never held back, and the held value is re-pushed every FAN_REASSERT_INTERVAL in case the BMC forgets it.

Note that iDRAC's own profile regulates to a target RPM (closed loop, rock steady), while manual control sets a PWM percentage (open loop, so measured RPM still wanders a percent or two at a fixed setpoint). That residual is inherent to manual control, not something the deadband can remove.

Why disks are measured against their own limits

Every drive reports its own maximum operating temperature (SATA: Min/Max Temperature Limit; SAS: Drive Trip Temperature). Each drive's ramp is derived from that number:

ramp starts at  limit - DISK_RAMP_LOW_OFFSET   (default 18)
full speed at   limit - DISK_RAMP_HIGH_OFFSET  (default 8)

So a Samsung SSD rated to 70c ramps 52→62, and a Toshiba spinner rated to 60c ramps 42→52. The 50c SSD asks for less airflow than the 45c HDD, which is correct — it is further from its own limit.

This matters because temperature does not tell you what you think it does. On these servers the rear-bay SSDs idle 10c hotter than the front-bay spinners: they sit in preheated exhaust air. A shared threshold would peg the fans for drives that are fine and ignore the ones that are not. Measuring each drive against its own envelope makes the curve independent of both drive technology and bay position.

Reported limits are clamped by class. They are not uniformly trustworthy — Samsung and Kioxia report a real operating maximum (70), Toshiba and Seagate report 60, and WD Reds report 85, which is the SCT critical limit and not somewhere you want a drive living. HDD_LIMIT_CAP (60) and SSD_LIMIT_CAP (70) bound whatever the drive claims, and supply the value when a drive reports nothing.

Drive class comes from /sys/block/<dev>/queue/rotational. That is derived from the device's RPM flag, which a few SAS drives behind HBAs report incorrectly; if you hit one, SMART's Rotation Rate field is the fallback.

Alarms

Email via mail to ALERT_EMAIL, which the host's postfix relays. Four conditions:

Alarm	Fires when
disk_temp	A drive is within DISK_ALARM_OFFSET (5c) of its own limit
trend	CPU or hottest disk has climbed TREND_RISE_ALARM (8c) across the window while still under every threshold
smart	smartctl -H reports anything other than PASSED/OK
cpu / gpu	Threshold crossed; cpu also means fan control was handed back to Dell's profile
disk_count	Fewer drives answered than were present at startup

The trend alarm is the one worth having. A dying fan or a blocked intake shows up as a steady climb long before anything crosses a threshold — by the time an absolute alarm fires you have already been running hot for hours.

Every alarm is rate limited per key with a one hour cooldown, and sends a single recovery notice when it clears. On a 10s loop an un-throttled alarm sends 360 emails an hour, at which point the alarm is the outage.

smartctl -H across two dozen drives runs hourly, not per loop. Absolute and trend checks stay on the fast loop.

Trend history

Every pass appends to log/temps.csv:

epoch,cpu,gpu,hottest_disk,fan_speed

Trimmed to TREND_SAMPLES (90 = 15 minutes at a 10s interval). The trend check compares the newest sample to the oldest in that window, and reports zero until a full window has accumulated so a restart cannot alarm on a partial series. Flat file, tail to trim — no rrdtool, no database.

Usage

fan_speed.sh            # the service loop (default)
fan_speed.sh once       # a single pass, prints what it decided
fan_speed.sh disks      # every drive: temperature and the limit in use
fan_speed.sh selftest   # parsers, curve, trend detector, alert rate limiting

selftest touches no hardware and sends no mail — it runs the pure logic against canned smartctl output and a synthetic history file. Run it after any edit.

Install

install -m 755 fan_speed.sh functions.sh monitor.sh /root/fan_speed/
mkdir -p /root/fan_speed/log /root/fan_speed/state
install -m 644 fan_speed.service /etc/systemd/system/
systemctl daemon-reload
systemctl enable --now fan_speed.service

Tuning

Variable	Default
CHECK_INTERVAL	10	Seconds between passes
LOW_FAN_SPEED / HIGH_FAN_SPEED	18 / 50	Percent
FAN_SPEED_DEADBAND	5	Hold the current speed until the request moves this far
FAN_REASSERT_INTERVAL	300	Re-push the held speed this often regardless
LOW_TEMPERATURE_THRESHOLD	45	CPU/GPU ramp start
CPU_TEMPERATURE_THRESHOLD	90	Above this, Dell's profile takes over
GPU_TEMPERATURE_THRESHOLD	75
DISK_RAMP_LOW_OFFSET	18	Ramp starts this far below each drive's limit
DISK_RAMP_HIGH_OFFSET	8	Full speed this far below it
DISK_ALARM_OFFSET	5	Alarm this far below it
HDD_LIMIT_CAP / SSD_LIMIT_CAP	60 / 70	Ceiling on what a drive may claim
TREND_SAMPLES	90	Window length, in passes
TREND_RISE_ALARM	8	Degrees of climb that alarms
DISK_EXCLUDE_PATTERN	(empty)	by-path substring to skip, e.g. an external shelf's HBA
ALERT_EMAIL	Servers@ntfy1.izebra.xyz
ALERT_COOLDOWN	3600	Seconds between repeats of one alarm
SMART_CHECK_INTERVAL	3600	Seconds between SMART sweeps

DISK_RAMP_LOW_OFFSET is the knob to reach for first. At the default 18 an SSD rated to 70c starts ramping at 52c; raise the offset to react earlier and louder, lower it to stay quiet longer. It is set where it is because the hottest drive on iz-pve1 idles at 47c — close enough that a smaller offset would have the fans tracking normal daily drift, and you would lose the ability to tell "disks are warm" from "disks are fine".

Those defaults were sized against one chassis. Watch a day of log/temps.csv before trusting them anywhere else.

Notes

• Kernel device names are not stable — a shelf rescan renamed sdaa–sdai to sds–sdaa mid-session. Everything resolves through /dev/disk/by-path on every pass; never persist an sdX.
• An external shelf must be excluded by hand. Set DISK_EXCLUDE_PATTERN to its HBA's PCI address in the systemd unit. This cannot be inferred: iz-pve1's MD1200 sits behind its own HBA at pci-0000:04:00.0, while iz-pve0's internal drives sit behind a SAS expander at pci-0000:02:00.0 — so "behind an expander" identifies an external enclosure on one host and the internal backplane on the other. iz-pve1 sets it, iz-pve0 has no shelf and leaves it empty.
• Drive temperature limits are read once at startup. They do not change, and smartctl -x is far heavier than the -A used on the fast loop.
• smartctl -n standby throughout, so a sleeping drive is skipped rather than spun up just to be measured. That is also why disk_count alarms on "fewer drives answered" rather than on a device disappearing.

Files

File
fan_speed.sh	Main loop, fan speed decision, alarm conditions, selftest
monitor.sh	Disk reading, trend history, alarm delivery — all local code
functions.sh	Vendored upstream: IPMI, iDRAC, interpolation
fan_speed.service	systemd unit