Buildroot只读文件系统下，如何实现/etc目录的可持久化写入

Nice problem to solve—read-only root filesystems are great for stability on IoT devices, but balancing that with necessary writable configs is tricky. Let’s break down the best approaches for your Orange Pi Zero + Buildroot setup, focusing on avoiding full root FS remounts:

1. Bind Mount Individual Writable Files (Best for Your Exact Use Case) #

Since you only need to persist two specific files (/etc/wpa_supplicant.conf and your custom config file), this is the most lightweight and efficient method. Here's how to set it up:

• Partition your storage: Create a small, dedicated writable partition (e.g., ext4) on your SD card (say /dev/mmcblk0p2). This will hold your persistent files.
• Mount the writable partition: Add an entry to your /etc/fstab (or init script) to mount this partition at a fixed location like /mnt/rw on boot:

  /dev/mmcblk0p2 /mnt/rw ext4 defaults,noatime 0 2
  

• Prepare the persistent files: Copy the initial versions of wpa_supplicant.conf and your custom config file into /mnt/rw/etc/ (create the etc subdir if needed).
• Bind mount the files: Add these lines to your boot init script (like /etc/init.d/S99persist-config) to map the persistent files into /etc:

  mount --bind /mnt/rw/etc/wpa_supplicant.conf /etc/wpa_supplicant.conf
  mount --bind /mnt/rw/my_custom_config.conf /etc/my_custom_config.conf
  

Now, any writes to those two /etc files will actually go to the writable partition, no full root FS remount required. It’s simple, low-overhead, and exactly targeted to your needs.

2. Overlay Filesystem for /etc (More Flexible) #

If you ever need to modify other files in /etc later, an OverlayFS setup is better for scalability. It stacks a read-only lower layer (your original /etc) with a writable upper layer (on your dedicated partition):

• Enable OverlayFS in Buildroot: Make sure your kernel has CONFIG_OVERLAY_FS enabled (check Kernel Configuration in Buildroot), and that you have the overlay mount tool available (busybox includes it by default).
• Set up the mount: In your init script or fstab, mount OverlayFS over /etc:

  mount -t overlay overlay -o lowerdir=/etc,upperdir=/mnt/rw/etc,workdir=/mnt/rw/work /etc
  

  • lowerdir: The original read-only /etc from your Buildroot image
  • upperdir: The directory on your writable partition where modified files are stored
  • workdir: A temporary directory required by OverlayFS (must be on the same filesystem as upperdir)
    This way, all changes to /etc are persisted in the upperdir, while the root FS stays read-only. It’s more flexible than bind mounts but slightly more complex.

3. Avoid These Less Ideal Options #

• Remounting root FS read-write temporarily: This defeats the purpose of a read-only system and introduces stability risks (e.g., corruption if power is lost during writes).
• tmpfs with sync scripts: Using tmpfs for /etc and syncing specific files to disk on shutdown works, but you risk losing data if the device powers off unexpectedly. Only use this if persistence isn’t critical.

Buildroot Configuration Tips #

• When setting up your root filesystem, mark it as read-only in Buildroot’s System Configuration (set "Root filesystem read-only" to yes).
• Ensure your writable partition is properly formatted and included in your disk image—you can use Buildroot’s "Custom partition table" option to define it.

内容的提问来源于stack exchange，提问作者flip79