F43 Change Proposal: Compress the initrd with zstd by default (self-contained)

[Aoife Moloney]

Wiki - https://fedoraproject.org/wiki/Changes/InitrdZstdDefault
Discussion thread
https://discussion.fedoraproject.org/t/f43-change-proposal-compress-the-init...

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 ==

Use zstd to compress the initrd for all Fedora variants by default
when built with dracut.

== Owner ==

* Name/Email: [[User:Hricky | Hristo Marinov]], [e-mail]
* Name/Email: [[User:Siosm | Timothée Ravier]], [e-mail]

== Detailed Description ==

Since https://github.com/dracut-ng/dracut-ng/pull/385, dracut is now
using zstd to compress the initrd by default. It does so only when the
zstd binary is available (and not the library as this is a shell
script), so right now on Fedora, most variants are still using xz
compressed initrds by default
(https://src.fedoraproject.org/rpms/dracut/blob/rawhide/f/dracut.spec#_100).
This changes however as soon as users install zstd on their systems.

The current zstd users are:
* Fedora CoreOS which explicitly added zstd and set a compression with
a specific ratio:
** https://github.com/coreos/fedora-coreos-config/blob/testing-devel/overlay.d/...
** https://github.com/coreos/fedora-coreos-config/pull/1844
* Some of the bootc images, which include zstd:
** https://gitlab.com/fedora/bootc/base-images/-/blob/main/standard/coreos-user...

Other related issues:
* https://gitlab.com/fedora/bootc/tracker/-/issues/17
* https://gitlab.com/fedora/ostree/sig/-/issues/34
* https://bugzilla.redhat.com/show_bug.cgi?id=1554931

For this change, we will replace the dependency in dracut from xz to
zstd and we might configure a different compression ratio, potentially
pushing that upstream, depending on feedback.

== Feedback ==

None so far.

We welcome testing for various compression ratios and comparing size
of the initrd and speed of decompression (which matters on boot) on
different hardware. See
https://github.com/coreos/fedora-coreos-tracker/issues/1247#issuecomment-117...
for an example.

== Benefit to Fedora ==

Smaller initrd and faster boots. See
https://github.com/coreos/fedora-coreos-tracker/issues/1247#issuecomment-117...
for some measurements. We did the change in Fedora CoreOS to reduce
the size of the initrd to save disk space in the `/boot` partition.

== Scope ==

* Proposal owners: Replace dependency on `xz` by `zstd` in `dracut` to
enable `zstd` by default
* Other developers: Make sure that their tools still work with the new
compression algorithm
* Release engineering: [https://pagure.io/releng/issues #Releng issue
number] TBD but nothing should be needed here
* Policies and guidelines: N/A (not needed for this Change)
* Trademark approval: N/A (not needed for this Change)
* Alignment with the Fedora Strategy: N/A, it's a general improvement

== Upgrade/compatibility impact ==

There should be no impact for most users as the decompression of the
initrd is handled by the kernel and it has had support for zstd for a
while now. Users of custom kernels should make sure this is supported
in their configs.

Existing installations will be updated to use zstd by default. No
manual action should be required.

Users that want to keep using xz for compressing the initrd can
configure it in their dracut config.

== Early Testing (Optional) ==

Do you require 'QA Blueprint' support? N

== How To Test ==

On package mode systems:

# Install zstd: `dnf install -y zstd`
# Rebuild the initrd: `dracut <path to initrd>` or `dracut
--regenerate-all` to do it for all kernel versions (make a backup to
compare sizes, etc.)
# Validate that it is zstd compressed: `LANG=C grep -aUPq
"\x28\xb5\x2f\xfd" <path to initrd> && echo "using zstd" || echo "not
using zstd"`
# Reboot

For ostree image systems, you should wait until the change lands in
Rawhide or build your own, including the configuration from Fedora
CoreOS as an example.

For bootable container image mode systems, you can install zstd and
rebuild the initrd in a Containerfile and switch/rebase your system to
it.

== User Experience ==

There should be no change in user experience beyond slightly faster boots.

== Dependencies ==

No other dependencies as far as the authors know right now.

== Contingency Plan ==

* Contingency mechanism: (What to do?  Who will do it?) Revert the change
* Contingency deadline: Final freeze
* Blocks release? No

== Documentation ==

To be written if accepted.

== Release Notes ==

To be written if accepted.

Discussion thread -

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.