containers-storage 1 August 2024

NAME
containers-storage-zstd-chunked - Information about zstd:chunked

DESCRIPTION
The traditional format for container image layers is application/vnd.oci.image.layer.v1.tar+gzip ⟨https://github.com/opencontainers/image-spec/blob/main/layer.md#gzip-media-types⟩. More recently, the standard was augmented with zstd: application/vnd.oci.image.layer.v1.tar+zstd ⟨https://github.com/opencontainers/image-spec/blob/main/layer.md#zstd-media-types⟩ which is a more modern and efficient compression format.

zstd:chunked is a variant of the application/vnd.oci.image.layer.v1.tar+zstd media type that uses zstd skippable frames ⟨https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#skippable-frames⟩ to include additional metadata (especially a "table of contents") that includes the SHA-256 and offsets of individual chunks of files. Additionally chunks are compressed separately. This allows a client to dynamically fetch only content which it doesn't already have using HTTP range requests.

At the time of this writing, support for this is enabled by default in the code.

You can explicitly enable or disable zstd:chunked with following changes to containers-storage.conf:
    [storage.overlay.pull_options]
    enable_partial_images = "true" | "false"

Note that the value of this field must be a "string bool", it cannot be a native TOML boolean.

IMPLEMENTATION
Each layer has an associated "big data" key called chunked-manifest-cache that is a custom binary format suitable for mmap() that contains index metadata for each layer with the full sha256 digest of each file plus its "chunks" (as computed by zstd:chunked).

When any image is pulled all existing other layers are scanned using chunked-manifest-cache to see if they contain a file with a matching digest. If one is found, the other file is hardlinked if use_hardlinks = "true", otherwise it is reflinked (if supported by the filesystem, or a full physical copy is made). There is a best-effort attempt to enable fsverity on the file if configured (see ⟨https://github.com/containers/storage/issues/2017⟩).

For more information, at the current time the file with the most information is pkg/chunked/internal/compression.go ⟨https://github.com/containers/storage/blob/39d469c34c96db67062e25954bc9d18f2bf6dae3/pkg/chunked/internal/compression.go⟩. The above is a permanent link for stability, but be sure to check to see if there are newer changes too.

STANDARDIZATION
At the current time the format is not officially standardized or documented beyond the comments and code in the reference implementation.

BUGS

•   https://github.com/containers/storage/issues?q=is%3Aissue+label%3Aarea%2Fzstd%3Achunked+is%3Aopen

FOOTNOTES
The Containers Storage project is committed to inclusivity, a core value of open source. The master and slave mount propagation terminology is used in this repository. This language is problematic and divisive, and should be changed. However, these terms are currently used within the Linux kernel and must be used as-is at this time. When the kernel maintainers rectify this usage, Containers Storage will follow suit immediately.