The Ever-Changing Linux Filesystems: Merging Directoris into /usr

If you don’t like change, working in IT has to be a harrowing experience. That’s particularly true in open source, where few stand on tradition and things move at breakneck pace. The latest change that has a few folks excited? Fedora’s proposal to “move all to /usr.” On the face, this might seem like a shocking departure from standards – but look a bit deeper and it seems to make sense.

The proposal for Fedora comes from Harald Hoyer and Kay Sievers. The long-term plan is to “clean up the mess and confusion the current split of / vs. /usr has created. All tools will move back to /usr where they belong, and the rootfs will only contain compat-symlinks into /usr. Almost the entire system installed by packages will reside in /usr.”

But wait, isn’t the split there for a reason? Yes, but the reasons that /usr has been split off are rooted in the history of Unix and the limitations of the tiny (space-wise) hard drives that the original UNIX equipment came with. On the BusyBox list, Rob Landley noted that the /usr split came about when “the operating system grew too big to fit on the first RK05 disk pack (their root filesystem)” it was allowed to “leak out” on to the second one “which is where all the user home directories lived (which is why the mount was called /usr).

“They replicated all the OS directories under there (/bin, /sbin, /lib, /tmp…) and wrote files to those new directories because their original disk was out of space. When they got a third disk, they mounted it on /home and relocated all the user directories to there so the OS could consume all the space on both disks and grow to THREE WHOLE MEGABYTES (ooooh!).”

Landley goes on to say that the /usr split “stopped making any sense before Linux was ever invented” for several reasons. The early system startup is dealt with by initrd and initramfs. Shared libraries mean that you can’t upgrade /lib and /usr/bin independently anymore.

The size issue? Well, hard drives weren’t exactly huge when Linux was invented, but users had a lot more room to work by the time Linus Torvalds started working on Linux. These days? Even embedded devices have a lot more room than the dinky hard drives the early Unix guys had to work with. Now, when I say “dinky” I only mean storage capacity. Those things were huge, but they couldn’t even hold a copy of Dark Side of the Moon as a 128KB MP3. (Not least of which because the album wasn’t recorded until after the /usr split, and MP3 hadn’t been invented – but you get the idea.)

The Proposal

So the proposal in front of Fedora is to move directories /bin, /sbin, /lib, and /lib64 under /usr.

This will improve compatibility with other *nix type systems. Solaris started this some time ago, and Lennart Poettering notes that it was completed in Solaris 11.

Poettering also says that it will make porting packages much easier, and be more compatible with upstream development.

It also reduces complexity of the existing filesystem hierarchy, and makes sharing the /usr partition easier with multiple guest OSes or over NFS.

The transition is expected to start in Fedora 17. Users can test it out via a Yum update while testing Fedora 17 if they’re feeling brave.

Panic?

These types of things tend to be met with stiff resistance. Some folks see no reason to change, others feel concerned that system changes are going to lead to horrible system problems.

It’s entirely possible that there will be some glitches on the way to making the change, but overall it seems to be well thought-out and unlikely to lead to major catastrophes. Fedora may be the first to do this, but it isn’t going to be the last. I’d suspect that in a few years’ time, all of the major distros will have unified on the /usr merge.

Check out the freedesktop.org page with “myths and facts” too. There may be some slightly alarmist stuff out there about the merge, but be sure to check all the facts before getting worried.