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3cpio is a tool to manage initramfs cpio files for the Linux kernel. The Linuxkernel'sinitramfs buffer formatis based around thenewc orcrc cpio formats. Multiple cpio archives can beconcatenated and the last archive can be compressed. Different compressionalgorithms can be used depending on what support was compiled into the Linuxkernel. 3cpio is tailored to initramfs cpio files and will not gain support forother cpio formats.

As of now, 3cpio supports examining, listing, and extracting the content of theinitramfs cpio.

Note: The Rust crate is named threecpio, because package names are notallowed to start with numbers.

Examine the content of the initramfs cpio on an Ubuntu 24.04 system:

$ 3cpio --examine /boot/initrd.img0cpio77312cpio7286272cpio85523968zstd

This initramfs cpio consists of three uncompressed cpio archives followed by aZstandard-compressed cpio archive.

List the content of the initramfs cpio on an Ubuntu 24.04 system:

$ 3cpio --list /boot/initrd.img.kernelkernel/x86kernel/x86/microcodekernel/x86/microcode/AuthenticAMD.binkernelkernel/x86kernel/x86/microcodekernel/x86/microcode/.enuineIntel.align.0123456789abckernel/x86/microcode/GenuineIntel.bin.usrusr/libusr/lib/firmwareusr/lib/firmware/3comusr/lib/firmware/3com/typhoon.bin.zst[...]

The first cpio contains only the AMD microcode. The second cpio contains onlythe Intel microcode. The third cpio contains firmware files and kernel modules.

Extract the content of the initramfs cpio to theinitrd subdirectory on anUbuntu 24.04 system:

$ 3cpio --extract -C initrd /boot/initrd.img$ ls initrdbin   cryptroot  init    lib    lib.usr-is-merged  run   scripts  varconf  etc        kernel  lib64  libx32             sbin  usr

Runtime comparison measured withtime over five runs on different initramfscpios:

Legend:

1. generated by initramfs-tools
2. generated bydracut --force --${compression}. On Raspberry Pi Zero 2W thereis not enough memory for the defaultzstd -15. So using the default frominitramfs-tools there:dracut --force --compress "zstd -1 -q -T0"
3. lsinitrd only reads the first two cpio archives of the file, but theinitramfs consists of four cpios.

Results:

• 3cpio is 87 to 274 times faster than lsinitramfs for images generated byinitramfs-tools.
• 3cpio is two to eight times faster than lsinitramfs for images generatedby dracut.
• 3cpio five to nine times faster than lsinitrd for images generated by dracut.

Commands used:

3cpio -t /boot/initrd.img-${version} | wc -ltime 3cpio -t /boot/initrd.img-${version} > /dev/nulltime lsinitramfs /boot/initrd.img-${version} > /dev/nulltime lsinitrd /boot/initrd.img-${version} > /dev/null

List the content of single cpio archive that is not compressed (seedoc/Benchmarks.md for details) on a Raspberry Pi Zero 2W:

Benchmarking the time to extraction initrd:

Raw measurements can be found indoc/Benchmarks.md.

The tool is named 3cpio because it is the third cpio tool besidesGNU cpio andbsdcpio provided bylibarchive. 3cpio is also the third tool that canlist the content of initramfs cpio archives besideslsinitramfs frominitramfs-tools andlsinitrdfromdracut.