#linux

May 20, 2023

/usr Merge

While browsing lwn.net, I came across this article mentioning Debian’s decision to delay the implementation of the /usr merge. The article prompted me to dive deeper into the topic and explore its significance.

What is /usr merge?

The /usr merge (also known as /usr move) entails relocating specific directories, including /bin, /sbin, and /lib from the root file system ("/") to the /usr directory. This restructuring aims to streamline the system architecture and improve versatility.

To grasp the importance of the /usr merge, it is essential to examine its historical origins. The separation of the /usr directory from the root file system was initially implemented in the 1980s as a solution to optimize disk space usage. Back then, computer storage was relatively limited, and seperating user-related data from core system files proved beneficial. This helped in minimizing the size of the / partition, enabling the system to boot seamlessly from a single drive and to mount the /usr partition after start-up.

However, as time passed, the distinction between the root file system and the /usr directory became more intricate. While some critical directories like /bin and /sbin remained in the root file system, others gradually migrated to the /usr directory. This fragmented structure introduced complexities in system management and posed potential risks during system upgrades.

Benefits

The advantages of the /usr merge include:

  • Simplified Script/Program Compatibility: When scripts or programs written for other Unix or Linux distributions are ported to your specific distribution, there is no longer a need to modify file system paths for the called binaries. This eliminates a common source of frustration, and /usr/bin and /bin (or /usr/sbin and /sbin) become completely interchangeable.
  • Enhanced Compatibility with Other Unix Systems: The /usr merge improves compatibility with other Unix systems, particularly Solaris, in terms of appearance and overall functionality.
  • Better Compatibility with GNU Build Systems: The /usr merge enhances compatibility with GNU build systems, making it easier to integrate and build software projects using these systems.
  • Improved Alignment with Upstream Development: By adopting the /usr merge, the distribution becomes more compatible with current upstream development efforts, ensuring a smoother integration of updates and new features.

Implementation amongst distribution

Curious about the current status of the /usr merge of different distribution.

Fedora

Fedora was the second to implement the /usr merge after Oracle Solaris and the first Linux distribution to make the move 2012.

Arch Linux

$ ls -l /bin /sbin /lib /lib64 
lrwxrwxrwx 1 root root 7 Feb  1 00:51 /bin -> usr/bin
lrwxrwxrwx 1 root root 7 Feb  1 00:51 /lib -> usr/lib
lrwxrwxrwx 1 root root 7 Feb  1 00:51 /lib64 -> usr/lib
lrwxrwxrwx 1 root root 7 Feb  1 00:51 /sbin -> usr/bin

/bin, /sbin, and /usr/sbin are symbolic links to /usr/bin, and /lib and /lib64 are symbolic links to /usr/lib. Here discussing the /usr move.

Opensuse

UsrMerge is active in openSUSE Tumbleweed since 20210527

Gentoo

The /usr merge is optional in Gentoo. There is a script merge-user which perform the migration.

Other distrubutions

Other distribution which has /usr merge by default:

  • Mageia
  • Red Hat Enterprise Linux
  • Chakra
  • Void Linux
  • Debian
  • Ubuntu

Further Reading

The Case for the UsrMerge

References

http://lists.busybox.net/pipermail/busybox/2010-December/074114.html https://lwn.net/Articles/932158/ https://lwn.net/Articles/890219/ https://salsa.debian.org/md/usrmerge/raw/master/debian/README.Debian https://linux.fandom.com/wiki/Usr_move

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