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 and listing the content of the initramfs cpio.
Note: The Rust crate is named threecpio, because package names are not allowed to start with numbers.
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.
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:
- generated by initramfs-tools
- generated by
dracut --force --${compression}. On Raspberry Pi Zero 2W there is not enough memory for the defaultzstd -15. So using the default from initramfs-tools there:dracut --force --compress "zstd -1 -q -T0" - lsinitrd only reads the first two cpio archives of the file, but the initramfs consists of four cpios.
Results:
- 3cpio is 87 to 274 times faster than lsinitramfs for images generated by initramfs-tools.
- 3cpio is two to eight times faster than lsinitramfs for images generated by dracut.
- 3cpio five to nine times faster than lsinitrd for images generated by dracut.
Commands used:
3cpio -t /boot/initrd.img-${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
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.