Changes/zstd:chunked

Default podman created images to zstd:chunked

This is a proposed Change for Fedora Linux.
This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee.

Summary

Modify podman push, buildah push and skopeo copy to default to creating container images compressed with zstd:chunked. Users can modify containers.conf to continue to use the legacy gzip image format.

Owner

• Name: Daniel J Walsh
• Email: dwalsh@redhat.com
• Name: Giuseppe Scrivano
• Email: gscrivan@redhat.com

Current status

• Targeted release: Fedora Linux 41
• Last updated: 2024-07-11
• [<will be assigned by the Wrangler> devel thread]
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

The goal is to speed up the pulling of container images, via using zstd:chunked. zstd:chunked format has shown itself to speed up the pulling of images by as much as 90%. This change has been waiting for change for over 3 years, waiting for Docker to support the format.

Feedback

It is not possible to directly push container images with both compression formats. See:

• https://github.com/moby/moby/issues/28394#issuecomment-1822741770
• https://github.com/opencontainers/image-spec/issues/803#issuecomment-745335277

Instead, it should be possible to create a manifest list with containers in both formats. See the discussion in https://discussion.fedoraproject.org/t/switch-fedora-container-images-to-support-zstd-chunked-format-by-default/123712 for more details.

This change was pushed by mistake to Fedora 41 and Fedora 40 with:

• Fedora 41: https://bodhi.fedoraproject.org/updates/FEDORA-2024-4918af16a3
• Fedora 40: https://bodhi.fedoraproject.org/updates/FEDORA-2024-ab42dd0ffb

This change has been reverted for Fedora 40 in:

• https://bodhi.fedoraproject.org/updates/FEDORA-2024-ebe5c816a1
• https://github.com/coreos/fedora-coreos-tracker/issues/1748

To avoid further disruptions, this change has not been reverted in Fedora 41 yet. It will be reverted if it is rejected.

This change also impacts all Bootable Container variants of Fedora. Initial (non-optimized) support for zstd:chunked images is available in rpm-ostree since v2024.6.

Benefit to Fedora

Fedora would be the first distribution to support building zstd:chunked format by default. Container engines (Podman, Buildah, Skopeo, CRI-O) which support the format natively would see a great speed up in pulling images. Other container engines including Docker and Containerd would see some speed up do the the compression algorithms of zstd, but not to the same degree as container engines with full support.

Scope

• Proposal owners: Dan Walsh, Giuseppe Scrivano
  • Switch the default to zstd:chunked
• Other developers:
  • Test if their containers still work with the new format.
• Release engineering: #Releng issue number (To Do)
  • Verify that the Fedora infra can use zstd:chunked container images.
• Policies and guidelines: N/A (not needed for this Change)
• Trademark approval: N/A (not needed for this Change)
• Alignment with Community Initiatives: N/A

Upgrade/compatibility impact

The following versions of each package have support for zstd:chunked:

• podman 5.1
• buildah 1.36
• skopeo 1.15

Moby doesn't support zstd:chunked but it handles the container image as a standard zstd layer since Moby 23.0. containerd supports zstd since 1.5.

This change is implemented by updating the /usr/share/containers/containers.conf configuration file to set compression_format = "zstd:chunked" by default.

Users that would like to keep using the existing format can enforce it with the following commands:

$ sudo install -dm 0755 -o 0 -g 0 /etc/containers/containers.conf.d/
$ echo 'compression_format = "gzip"' | sudo tee /etc/containers/containers.conf.d/compress-zstd-chunked.conf

How To Test

To test this change, you can create the following config file:

$ sudo install -dm 0755 -o 0 -g 0 /etc/containers/containers.conf.d/
$ echo 'compression_format = "zstd:chunked"' | sudo tee /etc/containers/containers.conf.d/compress-zstd-chunked.conf

See: https://github.com/containers/common/blob/main/docs/containers.conf.5.md

You can also use the --compression-format=zstd:chunked option with podman push or the --dest-compress-format=zstd:chunked with skopeo.

See: https://docs.podman.io/en/latest/markdown/podman-push.1.html

--compression-format=gzip | zstd | zstd:chunked
Specifies the compression format to use. Supported values are: gzip, zstd and zstd:chunked. The default is gzip unless overridden in the containers.conf file.

--compression-level=level
Specifies the compression level to use. The value is specific to the compression algorithm used, e.g. for zstd the accepted 
values are in the range 1-20 (inclusive) with a default of 3, while for gzip it is 1-9 (inclusive) and has a default of 5.

See: https://github.com/containers/skopeo/blob/main/docs/skopeo-copy.1.md

--dest-compress-format format

Specifies the compression format to use. Supported values are: gzip, zstd and zstd:chunked.

--dest-compress-level format

Specifies the compression level to use. The value is specific to the compression algorithm used, e.g. for zstd the accepted values are in the range 1-20 (inclusive), while for gzip it is 1-9 (inclusive).

Then:

• Build and push container images to container registries.
• Inspect them to check that their format is zstd:chunked.
• Pull the image back to the host.
• Now make a small change to the image and push it back to the registry.
• Remove the changed image leaving the original image in place.
• Now pull the updated image again.
• You should see a big change in download speed.
• Now attempt to pull the same image with Docker/Moby to make sure the image works with those container engines.

User Experience

Users should experience a nice improvement in download speeds of container images that were previously created with zstd:chunked.

Dependencies

N/A

Contingency Plan

If we find issues with zstd:chunked while turned on in Rawhide, we can easily reverse direction and go back to the original default gzip format.

All the container images and tags pushed with the zstd:chunked format will have to be pushed again with the gzip compression.

Documentation

N/A (not a System Wide Change)

Release Notes

Containers are pushed to container registries using the zstd:chunked format.