An image layer is an immutable filesystem change set that contributes to a container image. Images stack layers: a base image supplies lower content, later layers add changes, and committing a container creates another layer. section 2.2.1
Podman stores layers through container/storage. A layered filesystem mounts each layer over the previous one, while a copy-on-write storage driver presents the assembled filesystem. Podman uses overlay by default; in Podman, this name refers to the newer overlay2 implementation. section 5.1.2
Image layers support several operational workflows:
- Inspect image ancestry with podman image tree, which displays layer identifiers, sizes, and the images contributing each layer. section 2.2.1
- Compare filesystem changes with podman image diff; entries are classified as changed (C), added (A), or deleted (D). section 2.2.1
- Pull layers as registry blobs, then let containers/storage untar and reassemble them into overlay mounts in local storage. section 6.2.1
Inspecting a committed image
After committing a container to an image named myimage, run podman image tree myimage to view its stacked layers. Run podman image diff myimage ubi8/httpd-24 to identify configuration files and certificates added or changed above the lower image. section 2.2.1
$ podman image tree myimage Image ID: 2c7e43d88038 Tags: [localhost/myimage:latest] Size: 461.7MB Image Layers ├── ID: e39c3abf0df9 Size: 233.6MB ├── ID: 42c81bd2b468 Size: 20.48kB Top Layer of: [registry.access.redhat.com/ubi8:latest] ├── ID: 51a7beaa0b88 Size: 57.43MB ├── ID: 519e681b5702 Size: 170.6MB Top Layer of: [registry.access.redhat.com/ubi8/httpd-24:latest] └── ID: bc3dcdefdac3 Size: 69.63kB Top Layer of: [localhost/myimage:latest localhost/myapp:latest]
Use the layer tree to inspect ancestry, sizes, and the top-layer relationship between ubi8, ubi8/httpd-24, and myimage. section 2.2.1
