Definition
Plain language
Swapping which layer of a stacked filesystem is the writable one without restarting anything.
As stated in the literature
A DeltaBox kernel-level operation that atomically freezes an OverlayFS writable upper layer into a read-only lower layer and inserts a fresh writable layer on top without unmounting.
Why it matters: It allows cheap, atomic checkpointing of an agent's filesystem state without ever interrupting the processes running inside it.
For example, after the agent finishes a successful step, the system freezes its writable changes into a new read-only layer and starts catching subsequent writes in a fresh layer above it.