Understanding Docker’s AUFS and Image Layering Mechanism

Docker follows a client‑server architecture where the Docker client issues commands to the Docker daemon, which runs as a background service managing containers.

The Union File System (UFS), particularly AUFS, provides the layered file system that underpins Docker images, allowing read‑only and writable layers to be stacked and combined.

Images consist of a base (or parent) layer and optional additional layers; when an image is updated, only a new writable layer is added, enabling efficient storage and distribution.

AUFS supports per‑directory permissions (read‑only, read‑write, whiteout) and enables multiple containers to share the same read‑only image layers while writing to separate upper layers.

Docker supports several storage drivers such as AUFS, Btrfs, VFS, and DeviceMapper, each implementing the union‑mount mechanism for container file systems.

During container startup, Docker mounts a new writable layer atop the read‑only image layers, forming the container’s root file system; this layered approach facilitates rapid deployment, copy‑on‑write efficiency, and easy image updates.

The article concludes that using AUFS (or similar union file systems) gives Docker advantages like reduced storage usage, fast container provisioning, and the ability to update multiple containers by modifying a single base image.