The passive heatsink that ASRock Rack ships with the ALTRAD8UD-1L2T is designed for server systems where noise is typically not a concern and so fans are running fast, pushing a large amount of air over the CPU.
For home systems, noise is a concern and there are two options for adequately cooling the SoC.
First is to use the passive heatsink but install some sort of shroud to direct airflow over the CPU: this allows far slower fans to be used but still achieve adequate airflow.
Second is to replace the passive heatsink with an active solution. In future (written in February 2024) this will include a HSF (such as the Noctua NH-D9 AMP-4926 4U https://noctua.at/en/nh-u14s-amp-4926/specification) - heatsink and fan combination - but for now I believe the only option is liquid cooling using an Alphacool Eisbaer AIO.
All of the fans on the ALTRAD8UD-1L2T are controlled through the BMC (OpenBMC or MegaRAC).
Some early boards shipped with MegaRAC. More recently they have OpenBMC installed.
Fan control on OpenBMC is controlled via the phosphor-pid-control service which runs swampd.
The configuration file is in /usr/share/swampd/config.json which is a symlink to /usr/share/swampd/config-asrr.json.
In that file you'll see definitions of the fans, the SoC temperature and then near the bottom the correlation between temperature and fan speed:
"reading": {
"0": 40,
"1": 55,
"2": 65,
"3": 75,
"4": 85
},
"output": {
"0": 30,
"1": 40,
"2": 50,
"3": 75,
"4": 100
}
This means that if the SoC temperature is 40°C the fans will run at 30%, 55°C at 40% and so on until 85°C at which point they'll run at 100%.
You can check the SoC temperature in the BMC sensors page.
TBD