View Source Frequently Asked Questions

This is a collection of questions that often come up as people are getting started with Nerves. If you tried to go through the Getting Started guide or some of the example projects and got stuck, hopefully one of the following answers will help. If not, please let us know in the #nerves channel on the Elixir-Lang Discord, or create an Issue or Pull Request to improve this documentation.

Where can persistent data be stored?

For most use cases, the /data partition is the right place to store data. It is initialized on first boot and is not overwritten when new firmware is pushed to the device.

The mix firmware.burn task clears it out so that partition is guaranteed to be empty when the device boots. This is useful to ensure that the device is known state. There's a pattern for implementing a "Reset to factory defaults" feature by erasing the partition and rebooting.

If you're updating firmware regularly by writing to a MicroSD card, try running mix firmware.burn --task upgrade. This won't reset the application data partition.

Some Elixir libraries write to their priv directory by default. This won't work since all code and the priv directories are stored in a read-only file partition. Usually there's a way to override this default choice and specify a path to /data for that library to use.

Factory calibration and other provisioning data is either stored in a custom file partition or in the U-Boot environment block. The latter is accessible via Nerves.Runtime.KV functions.

How can I apply a firmware update manually?

Assuming that you have already put a known good firmware inside "/data/known_good.fw" (perhaps with sftp) then you can run the following commands

iex> cmd("fwup -i /data/known_good.fw --apply --task upgrade " <>
  "--no-unmount -d #{Nerves.Runtime.KV.get("nerves_fw_devpath")}")
iex> reboot

How do I push firmware updates remotely?

SSH is a good default for local development and is enabled by default (via mix with (note: previously was enabled by default)

For production environments you might also want to look at (either hosted or self-hosted)

Change Behavior on BEAM Failure

Similar to the previous question, we have chosen to have the device default to halting on certain kinds of failures that cause the Erlang VM to crash. This allows you to more easily read the error and diagnose the problem during development.

For a production deployment, it's recommended that you change the behavior to restart on failure instead. That way, in the unlikely event that your application crashes, the entire device will reload in a known-good state and continue to operate.

This setting is also configured using the erlinit.config file described above. To have the device restart instead of hang on failure, make a copy of the erlinit.config file and make sure the --hang-on-exit option is commented out.

# Uncomment to hang the board rather than rebooting when Erlang exits

You can also have the device drop into a shell when the Erlang VM crashes, allowing you to troubleshoot at the Linux OS level.

# Optionally run a program if the Erlang VM exits
#--run-on-exit /bin/sh

Platform-Specific Hardware Support

Some target hardware has particular features that can be used from your application, but they're not covered in the general Nerves documentation. In general, platform-specific features will be documented in the target's system documentation. You may also find what you need by searching for libraries that use that feature.

If you still don't see what you're looking for, please let us know in the #nerves channel on the Elixir-Lang Discord, or create an Issue or Pull Request to the relevant nerves_system-<target> repository.