3cpio

3cpio is a tool to manage initramfs cpio files for the Linux kernel. The Linux kernel's initramfs buffer format is based around the newc or crc cpio formats. Multiple cpio archives can be concatenated and the last archive can be compressed. Different compression algorithms can be used depending on what support was compiled into the Linux kernel. 3cpio is tailored to initramfs cpio files and will not gain support for other cpio formats.

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

Note: The Rust crate is named threecpio, because package names are not allowed to start with numbers.

Usage examples

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

$ 3cpio --examine /boot/initrd.img
0	cpio
77312	cpio
7286272	cpio
85523968	zstd

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

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

$ 3cpio --list /boot/initrd.img
.
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/AuthenticAMD.bin
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/.enuineIntel.align.0123456789abc
kernel/x86/microcode/GenuineIntel.bin
.
usr
usr/lib
usr/lib/firmware
usr/lib/firmware/3com
usr/lib/firmware/3com/typhoon.bin.zst
[...]

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

Extract the content of the initramfs cpio to the initrd subdirectory on an Ubuntu 24.04 system:

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

Benchmark results

Listing the content of the initrd

Runtime comparison measured with time over five runs on different initramfs cpios:

System	Kernel	Comp.	Size	Files	3cpio	lsinitramfs	lsinitrd
Ryzen 7 5700G	6.5.0-27-generic	zstd¹	102 MB	3496	0.052s	14.243s	–³
Ryzen 7 5700G VM	6.8.0-22-generic	zstd¹	63 MB	1934	0.042s	7.239s	–³
Ryzen 7 5700G VM	6.8.0-22-generic	zstd²	53 MB	1783	0.061s	0.452s	0.560s
RasPi Zero 2W	6.5.0-1012-raspi	zstd¹	24 MB	1538	0.647s	56.253s	–³
RasPi Zero 2W	6.5.0-1012-raspi	zstd²	30 MB	2028	1.141s	2.286s	6.118s
RasPi Zero 2W	6.8.0-1002-raspi	zstd¹	51 MB	2532	0.713s	164.575s	–³
RasPi Zero 2W	6.8.0-1002-raspi	zstd -1²	47 MB	2778	1.156s	2.842s	9.508s
RasPi Zero 2W	6.8.0-1002-raspi	xz²	41 MB	2778	6.922s	13.451s	35.184s

Legend:

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

Results:

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

Commands used:

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

Extracting the content of the initrd

Benchmarking the time to extraction initrd:

System	Distro	Kernel	Size	Files	3cpio	unmkinitramfs
Ryzen 7 5700G	noble	6.8.0-35-generic	70 MB	2097	0.107s	6.698s
Ryzen 7 5700G	jammy	6.8.0-35-generic	112 MB	3789	0.455s	2.217s
Ryzen 7 5700G	bookworm	6.1.0-21-amd64	62 MB	2935	0.268s	1.362s
RasPi Zero 2W	noble	6.8.0-1005-raspi	53 MB	2534	5.075s	173.847s

Raw measurements can be found in doc/Benchmarks.md.

Naming and alternatives

The tool is named 3cpio because it is the third cpio tool besides GNU cpio and bsdcpio provided by libarchive. 3cpio is also the third tool that can list the content of initramfs cpio archives besides lsinitramfs from initramfs-tools and lsinitrd from dracut.