Overlayfs

During development, many embedded Linux systems use a writable ext4 rootfs. It is convenient: copy missing files, edit configuration, and write logs under /var/log.

In a product, that convenience turns into risk:

• power loss can corrupt system files
• temporary files, logs, and databases get mixed into rootfs
• updates cannot easily tell user changes from system files
• factory reset has unclear boundaries
• the system partition becomes dirty, making field issues hard to reproduce

The point of a read-only rootfs is not simply to prevent changes. It is to make boundaries explicit: system files should be verifiable and recoverable; runtime state should be disposable; user data should have a clear home; updates and factory reset should not guess which files matter.