This is a blog post I would have definitely preferred not to write — it’s a topic that honestly does not touch me that much, for a few reasons I’ll explore in a moment, and at the same time it’s one that is quite controversial as it has quite a few meanings layered one on top of the other. Since I’m writing this, I would though first to make sure that the readers know who I am and why I’m probably going to just delete posts that tell me that I don’t care about compatibility with older systems and other operating systems.
My first project within Gentoo has been Gentoo/FreeBSD — I have a (sometimes insane) interest in portability with operating systems that are by far not mainstream. I’m a supporter of what I define “software biodiversity”, and I think that even crazy experiments have the right to exist, if anything to learn tricks and issues to avoid. So please don’t give me that kind of crap I noted above.
So, let’s see — I generally have little interest in keeping things around just for the sake of it, and as I wrote a long time ago I don’t use a separate /boot in most cases. I also generally dislike legacies for the sake of legacies. I guess it’s thus a good idea to start looking at which legacies bring us to the point of discussing whether /usr should be split. If it’s not to be split, there’s no point debating supporting split /usr, no?
The first legacy, which is specific to Gentoo is tied to the fact that our default portage tree is set to /usr/portage, and that both the ebuilds’ tree itself and the source files (distfiles), as well as the built binary packages, are stored there. This particular tree is hungry in both disk space itself and even more so in inodes. Since both the tree, and in general the open source projects we package, keep growing, the amount of these two resources we need increases as well, and since they are by default on /usr, it’s entirely possible that if this tree’s resources are allocated statically when partitioning, it’ll reach a point where there won’t be enough space, or inodes, to allocate anything in it. If /usr/portage resides in the root filesystem, it’s also very possible, if not even very likely, that the system would stop entirely to work because there is not enough space available on it.
One solution to this problem is to allocate /usr/portage on its own partition — I still don’t like it that much as an option, because /usr is supposed to be, standing to the FHS/LSB, for read-only data. Most other distributions you’ll find using subdirectories in /var as that’s what it’s designed to be used for. So why are we using /usr? Well, it turns out that this is something that was inspired from FreeBSD, where /usr is used for just about everything including temporary directories and other similar uses. Indeed, /usr/portage finds its peer in /usr/ports which is where Daniel seems to have been inspired to write Portage in the first place. It should be an easy legacy to overcome, but probably migrating it is going to be tricky enough that nobody has done so yet. Too bad.
Before somebody ask, yes, for a while ago splitting the whole /var – which is general considered much more sensible – was a pain in the neck, among other things because things were using /var/run and other similar paths before the partition could be mounted. The situation is now much better thanks to the fact that /run is now available much earlier in the boot process — this is not yet properly handled by all the init scripts out there but we’re reaching that point, slowly.
Okay so to the next issue: when do you want to split /usr at all? Well, this all depends on a number of factors, but I guess the first question is whether you’re installing a new system or maintaining an old one. If you install a new one, I really can’t think of any good reason to split /usr out — the only one that comes passing in my mind is if you want to have it in LVM and keep the rootfs as a standalone partition — and I don’t see why. I’d rather, at that point, put the rootfs in LVM as well, and just use an initrd to accomplish that — if it’s too difficult, well, that’s a reason to fix the way initrd or LVM are handled, not to keep insisting to split /usr! Interestingly enough, such a situation calls for the same /boot split I resented five years ago. I still use LVM without having the rootfs in it, and without needing to split /usr at all.
Speaking of which, most ready-to-install distributions only offer the option of using LVM — it makes sense, as you need to cater for as many systems as possible at once. This is why Gentoo Linux is generally disconnected to the rest: the power of doing things for what you want to use it for, makes it generally possible to skip the overgeneralization, and that’s why we’re virtually the only distribution out there able to work without an initrd.
Another point that came up often is with a system where the space in rootfs was badly allocated, and /usr is being split because there is not enough space. I’m sorry that this is a common issue, and I do know that it’s a pain to re-partition such a system as it involves at least a minimal downtime. But this is why we have workarounds, including the whole initrd thing. I mean, it’s not that difficult to manage, with the initrd, and yes I can understand that it’s more work than just having the whole system boot without /usr — but it’s a sensible way to handle it, in my opinion. It’s work or work, work for everybody under the sun to get split /usr working properly, or work for those who got the estimate wrong and now need the split /usr and you can guess who I prefer doing the work anyway (hint: like everybody in this line of business, I’m lazy).
Some people have said that /usr is often provided on NFS, and a very simple, lightweight rootfs is used in these circumstances — I understand this need, but the current solution to support split /usr is causing the rootfs to not be as simple and lightweight as before — the initrd route in that sense is probably the best option: you just get an initrd to be able to mount the root through NFS, and you’re done. The only problem with this solution is handling if /etc needs to be different from one system to the next, but I’m pretty sure it’s something that can be more easily fixed as well.
I have to be honest, there is one part of /usr that I end up splitting away very often: /usr/lib/debug — the reason is simple: it keeps increasing with the size of the sources, rather than with the size of the compiled code, and with the new versions of the compilers, which add more debug information. I got to a point where the debug files occupied four/five times the size of the rest of the rootfs. But this is quite the exception.
But why would it have to be that much of a problem to keep a split /usr? Well, it’s mostly a matter of what you’re supposed to be able to use without /usr being mounted. For many cases, udev was and is the only problem, as they really don’t want much in the matter of early-boot environment beside being able to start lvm and mount /usr, but the big problem happen if you want to be able to have even a single login with /usr not mounted — because the PAM chain has quite a few dependencies that wouldn’t be available until it’s mounted. Moving PAM itself is not much of an option, and this gets worse, because start-stop-daemon can technically also use chains that, partially, need /usr to be available, and if that happens, no init script using s-s-d would be able to run. And that’s bad.
So, do I like the collapsing of everything in /usr? Maybe not that much because it’s a lot of work to support multiple locations, and to migrate configurations. But at the same time I’m not going to bother, I’ll just keep the rootfs and /usr in the same partition for the time being, and if I have to split something out, it’ll be /var.