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OCI images on FreeBSD

The goal of this document is to specify and document the format of OCI images on FreeBSD.

Let's start defining layers. A layer is a tarball with folders and files plus a list of folders and files to be removed.

Layers use checksums as identifiers. Layers are designed to be used in sequence. The first layer is called the root layer.

Example:

layer 0: f5a028e4d9dd2bec64d3adff150604f3bb1b7074
layer 1: 3d1d2978d7ce046741409f0ca4df9fe03a0456d4
layer 2: 035eb33f1f1d309cb159a8c6a986a9b6d7b19d3c

Theoretically, layers are independent from each other: the image contains the order in which they have to be applied.

The image will then contain various information: architectural specification (CPU architecture, operating system and version, etc), the ordered list of layers, and many other details. (Look at the OCI specification [2] for more details.)

Layers with ZFS

Following the Docker approach [1], ZFS can support layers via snapshot and clone.

The layer 0 is created as follows:

  • create an empty ZFS dataset
  • apply the tarball of layer 0
  • take a snapshot of layer 0

To create the layer 1, the system can:

  • clone the snapshot of layer 0
  • apply the tarball of layer 1
  • take a snapshot of the new layer 1

Images with ZFS

Images are clones of the top layer. For instance:

  • root layer
  • layer 1
  • layer 2

To use the image, the system can just clone the snapshot of layer 2.

How to create the layers

For the root layer, the tar ball is just a tarball of the whole file system.

For every consecutive layer, only the differences need to be archived. This tarball can be built using zfs diff. New and modified files will be added to the tarball. Deleted files will be managed with witheouts as specified in the OCI specification [2] .

Bibliography

[1] Docker on ZFS: https://docs.docker.com/storage/storagedriver/zfs-driver/#how-the-zfs-storage-driver-works [2] OCI image specification: https://github.com/opencontainers/image-spec/releases/tag/v1.0.1