docker image - merged/diff/work/LowerDir components of GraphDriver

Question

Below is the manifest file entry snippet(docker inspect image redis) of redis image

"GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/bd512eb256c8aa56cbe9243d440a311820712d1a245fe6f523d39d19cd6c862d/diff:/var/lib/docker/overlay2/7fa1e90f35c78fc83c3a
4b86e36e45d742383b394adf9ce4cf9b339d919c9cbe/diff:/var/lib/docker/overlay2/2c1869386b5b8542959da4f0173a5272b9703326d619f27258b4edff7a1dbbf9/diff:/var/lib/docker/overlay2
/23ba3955c5b72ec17b9c409bd5233a3d92cbd75543c7d144b364f8188765788e/diff:/var/lib/docker/overlay2/87d8a92919103e8ff723221200acb36e17c611fa499571ab183d0f51458e6f24/diff",
                "MergedDir": "/var/lib/docker/overlay2/e503ed41978e99fe9b71a4225763a40b7988e9a4f31d4c06ef1ec1af46b0b6ab/merged",
                "UpperDir": "/var/lib/docker/overlay2/e503ed41978e99fe9b71a4225763a40b7988e9a4f31d4c06ef1ec1af46b0b6ab/diff",
                "WorkDir": "/var/lib/docker/overlay2/e503ed41978e99fe9b71a4225763a40b7988e9a4f31d4c06ef1ec1af46b0b6ab/work"
            },
            "Name": "overlay2"
        },

---

where overlay2 filesystem is used by docker image and container.

Within GraphDriver entry of manifest,

what does LowerDir / MergedDir / UpperDir / WorkDir indicate?

Answer 1

LowerDir: these are the read-only layers of an overlay filesystem. For docker, these are the image layers assembled in order.

UpperDir: this is the read-write layer of an overlay filesystem. For docker, that is equivalent to the container specific layer which contains changes made by that container.

WorkDir: this is a required directory for overlay, it needs an empty directory for internal use.

MergedDir: this is the result of the overlay filesystem. Docker effectively chroot's into this directory when running the container.

For more on overlay filesystems (overlay2 is a newer release, but I don't believe there are any user visible changes), see the kernel docs: https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt