Definition
Plain language
A Linux filesystem that stacks layers on top of each other like transparent sheets, showing the top one when you look down.
As stated in the literature
The Linux union filesystem underlying Docker images, which presents a stack of read-only lower layers with a writable upper layer; extended in DeltaBox to allow runtime layer pivots without unmounting.
Why it matters: Layered filesystems make containers fast to start and cheap to store, and extensions like DeltaBox unlock live environment swaps.
For example, a Docker container reads from immutable base layers while any file you create lands in a writable top layer that's discarded on exit.
Heard on the show
“Linux already has OverlayFS — it's the thing that makes Docker images work.”Episode 068 — The OS Trick That Makes Tree Search Practical for Coding Agents