Definition
Plain language
A filesystem that stacks read-only and writable layers on top of each other like transparent sheets.
As stated in the literature
A union-mount filesystem (e.g., OverlayFS) that presents a stack of read-only lower layers with a writable upper layer; the basis of Docker images and of DeltaBox's DeltaFS.
Also called: union filesystem
Why it matters: It enables cheap snapshotting, sharing, and rollback of complex environments, which is foundational to sandboxing AI agents.
For example, a Docker container starts from an immutable base image layer and writes any new files into a thin writable layer stacked on top.
Heard on the show
“And the beautiful thing about this is they didn't invent the layered filesystem.”Episode 068 — The OS Trick That Makes Tree Search Practical for Coding Agents