Immutable OS
Also called: read-only root, immutable infrastructure
An immutable OS mounts its system partition read-only, so the running system cannot be modified in place and updates are applied as whole images.
An immutable OS makes the system partition read-only. Packages cannot be installed into the running system and configuration drift is impossible, because there is nothing writable to drift.
The gains are reproducibility and recoverability. Every device on a given version is byte-identical, so a bug reproduces everywhere and a fix lands everywhere. If an update fails you reboot into the previous image; there is no half-applied state.
The cost is a different mental model. You cannot apt install a missing tool. Changes go
through a rebuild, or through explicitly writable areas — usually a data partition and a
container runtime — carved out for the purpose.
This pairs naturally with A/B slots: the read-only design is what makes atomic rollback possible.