Add temperature-aware MD1200 fan control
das_fanctl.sh reads drive temps via smartctl (falling back to the EMM backplane sensors), interpolates a fan percentage, and re-sends _shutup on a 5s keepalive so the shelf does not revert to full speed. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Zeb Hering
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# md1200-fan-control
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Temperature-aware fan control for a Dell PowerVault MD1200 disk shelf, driven over the
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EMM's serial debug console.
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Out of the box an MD1200 runs its fans at a fixed, very loud speed. The EMM firmware
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accepts a `_shutup <percent>` command that overrides it — but it forgets the setting
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after a few seconds, and it has no idea how hot your drives actually are. This repo
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reads drive temperatures on the host and re-sends an appropriate fan speed on a
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keepalive loop.
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## Hardware you need
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The MD1200 EMM exposes its debug console on the **top-left RJ45 port of the left EMM**
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(the one labelled for service use, not the SAS ports). It is not Ethernet — it is RS-232
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behind an RJ45 jack, so you need the Dell service cable:
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**Dell Password Reset / Service Cable — part `MN657`, also sold as `CT109`.**
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Same cable is listed for the MD1000 / MD1220 / MD3000 / MD3200 family, so search on any
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of those. It terminates in a DB9 serial connector; if your host has no physical serial
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port, add any USB-to-DB9 adapter and point the script at the resulting `/dev/ttyUSB*`.
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On the machine this was written for, the cable lands on an onboard UART at `/dev/ttyS1`.
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Serial settings: **38400 8-N-1**, no flow control.
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> Note: Dell's own KB articles list 115200 for MD3xxx/ME3xxx *controllers*. That is a
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> different port on different hardware. The MD1200 EMM debug console is 38400, which is
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> what this repo uses and what was verified against the real shelf.
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Once connected you get a prompt like:
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```
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BlueDress.106.000 >
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```
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## Debug commands
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There is no `help` — the firmware answers `unknown_cmd` to `help`, `_help` and `?`. The
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two commands this project relies on:
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| Command | Effect |
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|---|---|
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| `_shutup <0-100>` | Set fan speed to that percentage. Forgotten if not re-sent. |
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| `_temp_rd` | Print the enclosure temperature sensors. |
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`_temp_rd` output looks like:
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```
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BP_1[2] = 30c <- drive backplane
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BP_2[3] = 31c <- drive backplane
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SIM0[0] = 36c <- enclosure management module
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EXP0[4] = 59c <- SAS expander chip, runs hot by design
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AVG = 39c
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```
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Commands are terminated with a bare `\r` (not `\n`).
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## What this does
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`das_fanctl.sh` runs as a systemd service and loops:
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1. Read the temperature of every drive in the shelf with `smartctl`, take the hottest.
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2. Interpolate a fan percentage between the low and high thresholds.
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3. Send `_shutup <percent>`, and re-send it every 5s so the EMM doesn't revert.
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4. Re-read temperatures every 30s.
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**Drive temperature, not enclosure air.** The `BP_*` sensors read backplane intake air,
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which sat at 30c while the drives themselves were 32–39c — it lags what you actually
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care about. Drive temps come from `smartctl -n standby`, so a sleeping drive is skipped
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rather than spun up just to be measured. If no drive answers (all in standby, smartctl
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missing), it falls back to the hottest `BP_*` sensor with its own threshold pair.
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`EXP0` is deliberately ignored. The SAS expander idles around 59c by design; including
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it in the curve pegs the fans permanently.
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## Usage
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```
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das_fanctl.sh # the service loop (default)
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das_fanctl.sh once # one read + set, prints what it did
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das_fanctl.sh disks # per-drive temperatures and the hottest
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das_fanctl.sh temps # raw _temp_rd output from the shelf
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das_fanctl.sh selftest # asserts the parser, the curve, and that drives are readable
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```
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## Install
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The script sources `functions.sh` by absolute path, so it expects to live in
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`/root/fan_speed/`:
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```sh
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install -m 755 das_fanctl.sh functions.sh /root/fan_speed/
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mkdir -p /root/fan_speed/log
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install -m 644 fan_speed_das.service /etc/systemd/system/
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systemctl daemon-reload
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systemctl enable --now fan_speed_das.service
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```
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Check it: `das_fanctl.sh selftest`, then `tail -f /root/fan_speed/log/das_fan.log`.
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```
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2026-08-24 23:26:03 || DAS disk:39c | Fan Speed:10
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```
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## Tuning
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All knobs are at the top of `das_fanctl.sh`:
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| Variable | Default | Meaning |
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|---|---|---|
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| `PORT` | `/dev/ttyS1` | Serial device the service cable lands on |
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| `CHECK_INTERVAL` | `30` | Seconds between temperature readings |
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| `KEEPALIVE_INTERVAL` | `5` | How often to re-send `_shutup` |
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| `DISK_LOW_TEMPERATURE_THRESHOLD` | `40` | At/below this, run at `LOW_FAN_SPEED` |
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| `DISK_HIGH_TEMPERATURE_THRESHOLD` | `50` | At/above this, run at `HIGH_FAN_SPEED` |
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| `LOW_FAN_SPEED` | `10` | Percent |
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| `HIGH_FAN_SPEED` | `60` | Percent |
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| `BP_*_THRESHOLD` | `30` / `45` | Fallback curve, backplane air sensors |
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The defaults were sized against an idle spread of 32–39c across nine mixed SATA/SAS
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drives. Your airflow is not that airflow — watch a day of log before trusting them.
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`KEEPALIVE_INTERVAL` is inherited from the 5s cadence the original setup used, not
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measured. How fast the EMM actually reverts is unknown; if the fans audibly surge
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between keepalives, lower it.
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## Notes
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- Only one process may talk to the serial port. The script takes an exclusive `flock` on
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`/run/das_fanctl.lock` and exits rather than interleaving garbage with another writer.
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Detach `attachDAS.sh`'s `screen` session before starting the service.
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- The disk glob is `/dev/disk/by-path/*-sas-exp*-lun-0` — anything behind a SAS expander,
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i.e. the shelf. The host's own drives hang off `pci-…-scsi-*` and are not matched. A
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second enclosure would be lumped in with the first.
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## Files
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| File | |
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|---|---|
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| `das_fanctl.sh` | The fan controller |
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| `fan_speed_das.service` | systemd unit for it |
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| `functions.sh` | Only `calculate_interpolated_fan_speed` is used here — the rest is iDRAC/IPMI |
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| `attachDAS.sh` | Drop into an interactive `screen` session on the shelf console |
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| `get_das_temp.sh` | Superseded by `das_fanctl.sh temps` — it never read the reply back |
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| `log_rotate.txt` | logrotate snippet for the log directory |
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## Credit
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`functions.sh` comes from the
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[Dell iDRAC fan controller](https://github.com/tigerblue77/Dell_iDRAC_fan_controller_Docker)
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project; this repo only uses its `calculate_interpolated_fan_speed`. Prior art on the MD1200 console specifically:
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[tonybaltovski/md1200-reduce-fans-systemd](https://github.com/tonybaltovski/md1200-reduce-fans-systemd)
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and [iamjoshgilman/md1200-fan-controller](https://github.com/iamjoshgilman/md1200-fan-controller).
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