# md1200-fan-control

Temperature-aware fan control for a Dell PowerVault MD1200 disk shelf, driven over the
EMM's serial debug console.

Out of the box an MD1200 runs its fans at a fixed, very loud speed. The EMM firmware
accepts a `_shutup <percent>` command that overrides it — but it forgets the setting
after a few seconds, and it has no idea how hot your drives actually are. This repo
reads drive temperatures on the host and re-sends an appropriate fan speed on a
keepalive loop.

## Hardware you need

The MD1200 EMM exposes its debug console on the **top-left RJ45 port of the left EMM**
(the one labelled for service use, not the SAS ports). It is not Ethernet — it is RS-232
behind an RJ45 jack, so you need the Dell service cable:

**Dell Password Reset / Service Cable — part `MN657`, also sold as `CT109`.**

Same cable is listed for the MD1000 / MD1220 / MD3000 / MD3200 family, so search on any
of those. It terminates in a DB9 serial connector; if your host has no physical serial
port, add any USB-to-DB9 adapter and point the script at the resulting `/dev/ttyUSB*`.
On the machine this was written for, the cable lands on an onboard UART at `/dev/ttyS1`.

Serial settings: **38400 8-N-1**, no flow control.

> Note: Dell's own KB articles list 115200 for MD3xxx/ME3xxx *controllers*. That is a
> different port on different hardware. The MD1200 EMM debug console is 38400, which is
> what this repo uses and what was verified against the real shelf.

Once connected you get a prompt like:

```
BlueDress.106.000 >
```

## Debug commands

There is no `help` — the firmware answers `unknown_cmd` to `help`, `_help` and `?`. The
two commands this project relies on:

| Command | Effect |
|---|---|
| `_shutup <0-100>` | Set fan speed to that percentage. Forgotten if not re-sent. |
| `_temp_rd` | Print the enclosure temperature sensors. |

`_temp_rd` output looks like:

```
  BP_1[2] = 30c      <- drive backplane
  BP_2[3] = 31c      <- drive backplane
  SIM0[0] = 36c      <- enclosure management module
  EXP0[4] = 59c      <- SAS expander chip, runs hot by design

  AVG = 39c
```

Commands are terminated with a bare `\r` (not `\n`).

## What this does

`das_fanctl.sh` runs as a systemd service and loops:

1. Read the temperature of every drive in the shelf with `smartctl`, take the hottest.
2. Interpolate a fan percentage between the low and high thresholds.
3. Send `_shutup <percent>`, and re-send it every 5s so the EMM doesn't revert.
4. Re-read temperatures every 30s.

**Drive temperature, not enclosure air.** The `BP_*` sensors read backplane intake air,
which sat at 30c while the drives themselves were 32–39c — it lags what you actually
care about. Drive temps come from `smartctl -n standby`, so a sleeping drive is skipped
rather than spun up just to be measured. If no drive answers (all in standby, smartctl
missing), it falls back to the hottest `BP_*` sensor with its own threshold pair.

`EXP0` is deliberately ignored. The SAS expander idles around 59c by design; including
it in the curve pegs the fans permanently.

## Usage

```
das_fanctl.sh            # the service loop (default)
das_fanctl.sh once       # one read + set, prints what it did
das_fanctl.sh disks      # per-drive temperatures and the hottest
das_fanctl.sh temps      # raw _temp_rd output from the shelf
das_fanctl.sh selftest   # asserts the parser, the curve, and that drives are readable
```

## Install

The script sources `functions.sh` by absolute path, so it expects to live in
`/root/fan_speed/`:

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

Check it: `das_fanctl.sh selftest`, then `tail -f /root/fan_speed/log/das_fan.log`.

```
2026-08-24 23:26:03 || DAS disk:39c | Fan Speed:10
```

## Tuning

All knobs are at the top of `das_fanctl.sh`:

| Variable | Default | Meaning |
|---|---|---|
| `PORT` | `/dev/ttyS1` | Serial device the service cable lands on |
| `CHECK_INTERVAL` | `30` | Seconds between temperature readings |
| `KEEPALIVE_INTERVAL` | `5` | How often to re-send `_shutup` |
| `DISK_LOW_TEMPERATURE_THRESHOLD` | `40` | At/below this, run at `LOW_FAN_SPEED` |
| `DISK_HIGH_TEMPERATURE_THRESHOLD` | `50` | At/above this, run at `HIGH_FAN_SPEED` |
| `LOW_FAN_SPEED` | `10` | Percent |
| `HIGH_FAN_SPEED` | `60` | Percent |
| `BP_*_THRESHOLD` | `30` / `45` | Fallback curve, backplane air sensors |

The defaults were sized against an idle spread of 32–39c across nine mixed SATA/SAS
drives. Your airflow is not that airflow — watch a day of log before trusting them.

`KEEPALIVE_INTERVAL` is inherited from the 5s cadence the original setup used, not
measured. How fast the EMM actually reverts is unknown; if the fans audibly surge
between keepalives, lower it.

## Notes

- Only one process may talk to the serial port. The script takes an exclusive `flock` on
  `/run/das_fanctl.lock` and exits rather than interleaving garbage with another writer.
  Detach `attachDAS.sh`'s `screen` session before starting the service.
- The disk glob is `/dev/disk/by-path/*-sas-exp*-lun-0` — anything behind a SAS expander,
  i.e. the shelf. The host's own drives hang off `pci-…-scsi-*` and are not matched. A
  second enclosure would be lumped in with the first.

## Files

| File | |
|---|---|
| `das_fanctl.sh` | The fan controller |
| `fan_speed_das.service` | systemd unit for it |
| `functions.sh` | Only `calculate_interpolated_fan_speed` is used here — the rest is iDRAC/IPMI |
| `attachDAS.sh` | Drop into an interactive `screen` session on the shelf console |
| `get_das_temp.sh` | Superseded by `das_fanctl.sh temps` — it never read the reply back |
| `log_rotate.txt` | logrotate snippet for the log directory |

## Credit

`functions.sh` comes from the
[Dell iDRAC fan controller](https://github.com/tigerblue77/Dell_iDRAC_fan_controller_Docker)
project; this repo only uses its `calculate_interpolated_fan_speed`. Prior art on the MD1200 console specifically:
[tonybaltovski/md1200-reduce-fans-systemd](https://github.com/tonybaltovski/md1200-reduce-fans-systemd)
and [iamjoshgilman/md1200-fan-controller](https://github.com/iamjoshgilman/md1200-fan-controller).