OverlayFS Driver: Examining Mounts and Layering Structure

$ ls /var/lib/docker/overlay2/3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/diff bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var ``` The second-lowest layer, and each higher layer, contain a file called `lower`, which denotes its parent, and a directory called `diff` which contains its contents. It also contains a `merged` directory, which contains the unified contents of its parent layer and itself, and a `work` directory which is used internally by OverlayFS. ```console $ ls /var/lib/docker/overlay2/223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7 diff link lower merged work $ cat /var/lib/docker/overlay2/223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7/lower l/6Y5IM2XC7TSNIJZZFLJCS6I4I4 $ ls /var/lib/docker/overlay2/223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7/diff/ etc sbin usr var ``` To view the mounts which exist when you use the `overlay` storage driver with Docker, use the `mount` command. The output below is truncated for readability. ```console $ mount | grep overlay overlay on /var/lib/docker/overlay2/9186877cdf386d0a3b016149cf30c208f326dca307529e646afce5b3f83f5304/merged type overlay (rw,relatime, lowerdir=l/DJA75GUWHWG7EWICFYX54FIOVT:l/B3WWEFKBG3PLLV737KZFIASSW7:l/JEYMODZYFCZFYSDABYXD5MF6YO:l/UL2MW33MSE3Q5VYIKBRN4ZAGQP:l/NFYKDW6APBCCUCTOUSYDH4DXAT:l/6Y5IM2XC7TSNIJZZFLJCS6I4I4, upperdir=9186877cdf386d0a3b016149cf30c208f326dca307529e646afce5b3f83f5304/diff, workdir=9186877cdf386d0a3b016149cf30c208f326dca307529e646afce5b3f83f5304/work) ``` The `rw` on the second line shows that the `overlay` mount is read-write. The following diagram shows how a Docker image and a Docker container are layered. The image layer is the `lowerdir` and the container layer is the `upperdir`. If the image has multiple layers, multiple `lowerdir` directories are used. The unified view is exposed through a directory called `merged` which is effectively the containers mount point.

This section explains how to view the mounts used by the `overlay2` storage driver using the `mount` command. It then details the layering structure of Docker images and containers, where the image layers act as `lowerdir` and the container layer as `upperdir`. The unified view is accessible through the `merged` directory, which serves as the container's mount point. A diagram illustrates this layering concept, showing how files from different layers are combined in the `merged` directory.