fan_speed/README.md

# 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](https://github.com/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](https://git.izebra.net/izebra_projects/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.

A drive with a lower tolerance drives the fans even while a hotter drive does not.
With `iz-pve0`'s settings (floor 28%, ramp `limit-8` to `limit-5`):

| Drive | Limit | Ramp | Asks at 45c | at 50c | at 55c |
|---|---|---|---|---|---|
| Samsung 860 EVO | 70c | 62-65c | 28% | 28% | 28% |
| Toshiba / Seagate HDD | 60c | 52-55c | 28% | 28% | **50%** |
| a 55c-rated drive | 55c | 47-50c | 28% | **50%** | **50%** |

At 55c the SSD contributes nothing while the spinner asks for full speed, which is the
whole point: the same temperature means different things to different drives.

**Full cooling must arrive no later than the alarm.** `DISK_RAMP_HIGH_OFFSET` must be
`>=` `DISK_ALARM_OFFSET`, or the alarm mails you while there is still cooling left unused.
`check_ramp_ordering()` clamps this at startup and logs when it does — it clamps rather
than exits, because a fan controller that refuses to start leaves the fans wherever they
happened to be.

**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` | Temperatures climbing **and the fans cannot keep up** — see below |
| `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**, but a rising temperature is not by itself a
fault — it is what a busy machine looks like, and a multi-hour zpool migration will do it
all afternoon. What matters is whether the fans are answering. `trend_verdict()` returns
one of three things:

| Verdict | Meaning | Alarms |
|---|---|---|
| `no_headroom` | Climbing while the fans are already at `HIGH_FAN_SPEED` | yes — nothing left to give |
| `not_converging` | Climbing, fans ramping with it, and the rise is *not slowing down* | yes — cooling is losing ground |
| `quiet` | Anything else, including a large climb the fans absorbed and that has plateaued | no — that is just load |

It tells a plateau from a runaway by comparing the first half of the window against the
second: a normal load step decelerates once the fans catch up, a failing fan or a blocked
intake does not. A dying fan still gets caught long before an absolute threshold, without
mailing you every time the machine gets busy.

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.

### Ramp width controls how *bouncy* the fans are

Changing fan noise is more irritating than constant fan noise, and ramp width is the lever
for it. The span sets the sensitivity: 22 points of fan over a 7c ramp is 3.14%/degC, so
1.6c of CPU drift is enough to cross a 5% deadband. The same 22 points over 11c is
2%/degC, needing 2.5c.

Replaying 180 recorded samples from `iz-pve0` under load (CPU 57-69c) through candidate
curves, counting how often the fan would actually move:

| Ramp | Deadband | Fan changes | Distinct speeds |
|---|---|---|---|
| 62-69c | 5 | 19 | 28,31,34,37,40,43,46,50 |
| 62-69c | 8 | 6 | 28,37,46,50 |
| 62-73c | 5 | 5 | 28,34,40 |
| **62-73c** | **8** | **1** | 28,36 |

Confirmed live against a multi-hour zpool migration: **19 fan changes became 2** over a
comparable window, average fan speed dropped slightly (40% to 38%), and **CPU temperature
did not move** (64-68c against 64-69c before). The extra airflow had been buying nothing.

Widen the ramp and raise the deadband together. The cost is lag - a deadband of 8 at
2%/degC is about 4c of slack - which is affordable when you are 8-14c below any threshold.

## Calibrating a host

**The defaults are not portable and the fans will be wrong on an uncalibrated host.**
Airflow, bay layout and the PWM→RPM relationship are all chassis specific. Calibration is
a one-off, takes about five minutes, and is what keeps this from being louder than stock:

1. Hand the fans back to Dell and let them settle, then record what stock actually does:
   `ipmitool raw 0x30 0x30 0x01 0x01`, wait a minute, then read
   `ipmitool sdr type fan` and `ipmitool sdr type temperature`.
2. Take manual control (`ipmitool raw 0x30 0x30 0x01 0x00`) and sweep PWM to find the
   percentage that reproduces that RPM:
   `ipmitool raw 0x30 0x30 0x02 0xff 0x1c` (0x1c = 28%), waiting ~25s per step.
3. Set `LOW_FAN_SPEED` to that percentage. This is the floor the host idles at.
4. Set `CPU_RAMP_LOW_OFFSET` so the CPU ramp *starts a few degrees above the host's
   normal idle*, so idle sits on the floor rather than permanently part-way up a ramp.

Measured example — an R720xd (`iz-pve0`) whose CPUs idle at 60c and whose stock profile
holds 6240 RPM. Here 28% PWM == 6240 RPM, so `LOW_FAN_SPEED=28` and
`CPU_RAMP_LOW_OFFSET=15` (ramp starts at 62c). An R730xd (`iz-pve1`) idling at 48c is
fine on the 18/24 defaults. Put the values in the unit as `Environment=` lines.

### Put the disk ramp above the drives' normal working temperature

The single most common cause of a noisy host is a disk ramp that starts *inside* the
range the drives normally live in. Then every load spike pushes the fan up, the fan drops
back, and it cycles audibly forever.

Worked example, `iz-pve0`: `sda`/`sdh` are the rpool mirror in rear bays — busy root SSDs
that idle at 53-55c and touch 60-61c under load, rated to 70c. With the default ramp
starting at 52c they were permanently part-way up it, dragging the fans to 7920 RPM
(+27% over stock) to cool drives that were never in trouble. Starting the ramp at 60c
still cycled, because load spikes crossed it. Starting it at **62c**, just above their
working peak, holds 6240 RPM flat — and the drives settle at 53-54c anyway.

The rule: find the drives' normal peak, put `DISK_RAMP_LOW_OFFSET` a couple of degrees
above it, and let the alarm (`DISK_ALARM_OFFSET`, 5c below the limit) catch anything real.
Cooling a healthy drive from 57c to 53c is not worth 27% more fan.

### Expect to run above stock on some hosts, legitimately

Dell's profile does not look at drive temperature at all — only CPU, inlet and exhaust.
On `iz-pve0` two rear-bay SSDs sit in preheated exhaust air, and this was measured
directly: raising `DISK_RAMP_LOW_OFFSET` so those drives stopped voting dropped the fans
to 28% (exactly stock's 6240 RPM) and both SSDs climbed 54c → 61c within minutes.
Restoring their vote brought them back to 53-55c at 34% / 7080 RPM.

So on that host **~13% more RPM than stock is the price of keeping two SSDs 6-8c cooler**,
and it is the whole reason this script exists. If you would rather have stock noise and
hotter drives, raise `DISK_RAMP_LOW_OFFSET` until the drives stop asking — that is a
preference, not a bug. What is *not* acceptable is being louder than stock for no reason,
which is what an uncalibrated CPU curve does.

## 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

```sh
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 |