# 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](https://hexdocs.pm/nerves/getting-started.html) or some of the [example projects](https://github.com/nerves-project/nerves_examples) and got stuck, hopefully one of the following answers will help. If not, please open a topic in the "Nerves" category on [https://elixirforum.com](https://elixirforum.com/c/nerves-forum/74) or [create an Issue or Pull Request to improve this documentation](https://github.com/nerves-project/nerves/tree/main/docs). ## 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 ```elixir 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 nerves.new`) with https://github.com/nerves-project/nerves_ssh (note: previously https://github.com/nerves-project/nerves_firmware_ssh was enabled by default) For production environments you might also want to look at https://www.nerves-hub.org/ (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. ```bash # Uncomment to hang the board rather than rebooting when Erlang exits #--hang-on-exit ``` You can also have the device drop into a shell when the Erlang VM crashes, allowing you to troubleshoot at the Linux OS level. ```bash # 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 [hex.pm](https://hex.pm) for libraries that use that feature. If you still don't see what you're looking for, please open a topic in the "Nerves" category on [https://elixirforum.com](https://elixirforum.com/c/nerves-forum/74), or create an Issue or Pull Request to the [relevant `nerves_system-` repository](https://github.com/nerves-project?query=nerves_system_).