## Compress the initramfs with zstd by default on Fedora

Use `zstd` to compress the initramfs for all Fedora variants by default when built with `dracut`.
For more information about the change, see https://fedoraproject.org/wiki/Changes/InitrdZstdDefault.

I have tested the commands available below for `initramfs-6.15.4-200.fc42.x86_64.img` on several platforms.

For the Zstandard compression algorithm, the commands executed to compress images are:

sudo dracut --force --regenerate-all --parallel "--compress=zstd -3 --threads=0"
sudo dracut --force --regenerate-all --parallel "--compress=zstd -5 --threads=0"
sudo dracut --force --regenerate-all --parallel "--compress=zstd -10 --threads=0"
sudo dracut --force --regenerate-all --parallel "--compress=zstd -15 --threads=0"
sudo dracut --force --regenerate-all --parallel "--compress=zstd -19 --threads=0"

For the Gzip compression algorithm, the command executed to decompress the image is:

`gzip --decompress --stdout > /dev/null initramfs-6.15.4-200.fc42.x86_64.img`

For the Zstandard compression algorithm, the command executed to decompress the images is:

`unzstd --stdout > /dev/null initramfs-6.15.4-200.fc42.x86_64.img`

**Note:** The times listed in the "Mean [ms]", "Min. [ms]", and "Max. [ms]" columns in the following tables are for decompression.

### Fedora Cloud 42 on QEMU/KVM VM with 2 vCPUs and 4 GB RAM

| Size [MiB] | Tool | Algorithm | Level | Mean [ms] | Min [ms] | Max [ms] |
|:--- |:--- |:--- |:--- |---: |---: |---: |
| 48 |`gzip`| Gzip | 9 |467.8 ± 1.5| 465.8 | 471.3 |
| 47 |`zstd`| Zstandard | 3 |60.4 ± 1.2 | 58.8 | 67.1 |
| 46 |`zstd`| Zstandard | 5 |62.9 ± 1.7 | 60.6 | 69.3 |
| 45 |`zstd`| Zstandard | 10 |62.2 ± 1.4 | 60.2 | 67.3 |
| 45 |`zstd`| Zstandard | 15 |61.8 ± 1.2 | 59.5 | 66.1 |
| 43 |`zstd`| Zstandard | 19 |78.3 ± 1.2 | 76.8 | 82.8 |

### Fedora KDE 42 on bare metal machine with Intel Pentium CPU G4560 and 12 GB RAM

| Size [MiB] | Tool | Algorithm | Level | Mean [ms] | Min [ms] | Max [ms] |
|:--- |:--- |:--- |:--- |---: |---: |---: |
| 75 |`gzip`| Gzip | 9 |807.1 ± 2.0| 805.7 | 812.0 |
| 75 |`zstd`| Zstandard | 3 |134.6 ± 3.0| 131.0 | 141.4 |
| 74 |`zstd`| Zstandard | 5 |141.2 ± 2.2| 136.9 | 143.6 |
| 73 |`zstd`| Zstandard | 10 |146.8 ± 1.6| 144.4 | 148.9 |
| 73 |`zstd`| Zstandard | 15 |148.1 ± 2.7| 145.0 | 155.1 |
| 71 |`zstd`| Zstandard | 19 |177.0 ± 1.9| 174.1 | 179.5 |

---

### Testing initramfs with zstd on Fedora Atomic Desktops

**Note:** Fedora Atomic Desktops bootable container images are still considered experimental. It is recommended to use a discarded or virtual machine.

Install Fedora Kinoite and create the directory `~/initrd_with_zstd_by_default` and in it a `Containerfile` with the following content:

FROM quay.io/fedora-ostree-desktops/kinoite:42

RUN <<EORUN
    set -xeuo pipefail

    dnf --assumeyes --refresh install zstd
    kver=$(ls /usr/lib/modules)
    stock_arguments=$(lsinitrd "/lib/modules/${kver}/initramfs.img" |\
        grep --extended-regexp '^Arguments: ' |\
        sed 's/^Arguments: //')
    mkdir --parents /tmp/dracut /var/roothome
    bash <(/usr/bin/echo "dracut ${stock_arguments}")
    mv --verbose /boot/initramfs*.img "/lib/modules/${kver}/initramfs.img"

    dnf clean all
    rm --recursive --force /var/* /tmp/*
    bootc container lint --no-truncate
EORUN

Build a bootable container image:
`sudo podman image build --tag localhost/fedora-kinoite:42-initramfs-zstd --file ~/initrd_with_zstd_by_default/Containerfile`

Switch to the built container image and reboot the system:
`sudo bootc switch --apply --transport containers-storage localhost/fedora-kinoite:42-initramfs-zstd`

---

### Testing initramfs with zstd on Fedora Cloud

I modified [fedora-kiwi-descriptions](https://pagure.io/fedora-kiwi-descriptions) for the `Cloud-Base-Generic` image profile to include a local repository containing the RPMs from the x86_64 build of the [Draft: Add require on zstd to use it for initrd compression](https://src.fedoraproject.org/rpms/dracut/pull-request/85) PR, and the file `initramfs-compression-test.py`, the contents of which are at the bottom of this page.

---

### Testing initramfs with zstd on Fedora IoT

When I build a container image from the following Containerfile and a qcow2 image from it, the initramfs in the virtual machine is compressed using Zstandard. The `dracut-rpms` directory contains the x86_64 build of the [Draft: Add require on zstd to use it for initrd compression](https://src.fedoraproject.org/rpms/dracut/pull-request/85) PR, and the contents of `initramfs-compression-test.py` are at the bottom of this page.

FROM quay.io/fedora/fedora-iot:rawhide

COPY initramfs-compression-test.py /usr/libexec/
COPY dracut-rpms /tmp/dracut-rpms
RUN <<EORUN
    set -xeuo pipefail

    export DRACUT_NO_XATTR=1

    dnf --assumeyes reinstall --allowerasing /tmp/dracut-rpms/dracut*

    kver=$(ls /usr/lib/modules)
    stock_arguments=$(lsinitrd "/lib/modules/${kver}/initramfs.img" |\
        grep --extended-regexp '^Arguments: ' |\
        sed 's/^Arguments: //')
    mkdir --parents /tmp/dracut /var/roothome
    bash <(/usr/bin/echo "dracut ${stock_arguments}")
    mv --verbose /boot/initramfs*.img "/lib/modules/${kver}/initramfs.img"

    dnf clean all
    rm --recursive --force /var/* /tmp/* /boot/*
    bootc container lint --no-truncate
EORUN

---

The following Python script aims to simplify and automate to some extent the testing of initramfs compression on Fedora Linux.

#! /usr/bin/env python3

import os
from pathlib import Path
import re
import subprocess


def main():
    if os.getuid() != 0:
        raise SystemExit(f"\n Root access is required to test initramfs compression.\n")

    determine_initramfs_compression()


def get_initramfs_paths():
    initramfs_paths = []
    ostree_booted = Path("/run/ostree-booted")
    if ostree_booted.exists():
        print(f"\n Fedora Linux operates in image mode.\n")
        entries_dir = Path("/boot/loader/entries")
        for entry in entries_dir.iterdir():
            ostree_conf = entry.read_text()
            initrd_re_compile = re.compile(r"^initrd\b.+\.img.*", re.MULTILINE)
            initrd_line = re.search(initrd_re_compile, ostree_conf)
            initramfs_path_image_mode = ostree_conf[initrd_line.start():initrd_line.end():].replace("initrd ", "/boot")
            initramfs_paths.append(initramfs_path_image_mode)
    else:
        print(f"\n Fedora Linux operates in package mode.\n")
        initramfs_paths_package_mode = list(Path("/boot").glob("initramfs*"))
        for initramfs_path in initramfs_paths_package_mode:
            initramfs_paths.append(str(initramfs_path))

    return initramfs_paths


def determine_initramfs_compression():
    FILE_SIGNATURES = {
        "Gzip": r"\x1f\x8b\x08",
        "XZ": r"\xfd\x37\x7a\x58\x5a\x00",
        "Zstandard": r"\x28\xb5\x2f\xfd",
    }
    for initramfs_path in get_initramfs_paths():
        extract_initramfs = subprocess.run(
            ["/usr/lib/dracut/skipcpio", 
            initramfs_path], 
            capture_output=True, check=True)

        for compression, file_signature in FILE_SIGNATURES.items():
            if re.search(bytes(file_signature, encoding="UTF-8"), extract_initramfs.stdout[:8]):
                print(f" Initramfs: {Path(initramfs_path)}")
                print(f" Compression: {compression}\n")


if __name__ == "__main__":
    raise SystemExit(main())