How Ubuntu and Fedora each do kernel packages

April 7, 2015

I feel the need to say things about the Ubuntu (and I believe Debian) kernel update process, but before I do that I want to write down how kernel packages look on Ubuntu and Fedora from a sysadmin's perspective because I think a number of people have only been exposed to one or the other. The Fedora approach to kernel packages is also used by Red Hat Enterprise Linux (and CentOS) and probably other Linux distributions that use yum and RPMs. I believe that the Ubuntu approach is also used by Debian, but maybe Debian does it a bit differently; I haven't run a real Debian system.

Both debs and RPMs have the core concepts of a package having a name, an upstream version number, and a distribution release number. For instance, Firefox on my Fedora 21 machine is currently firefox, upstream version 37.0, and release 2.fc21 (increasingly people embed the distribution version in the release number for reasons beyond the scope of this entry).

On Fedora you have some number of kernel-... RPMs installed at once. These are generally all instance of the kernel package (the package name); they differ only in their upstream version number and their release number. Yum normally keeps the most recent five of them for you, deleting the oldest when you add a new one via a 'yum upgrade' when a new version of the kernel package is available. This gives you a list of main kernel packages that looks like this:

kernel-3.18.8-201.fc21.x86_64
kernel-3.18.9-200.fc21.x86_64
kernel-3.19.1-201.fc21.x86_64
kernel-3.19.2-201.fc21.x86_64
kernel-3.19.3-200.fc21.x86_64

Here the kernel RPM with upstream version 3.19.3 and Fedora release version 200.fc21 is the most recent kernel I have installed (and this is a 64-bit machine as shown by the x86_64 architecture).

(This is a slight simplification. On Fedora 21, the kernel is actually split into three kernel packages: kernel, kernel-core, and kernel-modules. The kernel package for a specific version is just a meta-package that depends (through a bit of magic) on its associated kernel-core and kernel-modules packages. Yum knows how to manage all of this so you keep five copies not only of the kernel meta-package but also of the kernel-core and kernel-modules packages and so on. Mostly you can ignore the sub-packages in Fedora; I often forget about them. In RHEL up through RHEL 7, they don't exist and their contents are just part of the kernel package; the same was true of older Fedora versions.)

Ubuntu is more complicated. There is a single linux-image-generic (meta-)package installed on your system and then some number of packages with the package name of linux-image-<version>-<release>-generic for various <version> and <release> values. Each of these packages has a deb upstream version of <version> and a release version of <release>.<number>, where the number varies depending on how Ubuntu built things. Each specific linux-image-generic package version depends on a particular linux-image-<v>-<r>-generic package, so when you update to it it pulls in that specific kernel (at whatever the latest package release of it is).

Because of all of this, Ubuntu systems wind up with multiple kernels installed at once by the side effects of updating linux-image-generic. A new package version of l-i-g will depend on and pull in an entirely new linux-image-<v>-<r>-generic package, leaving the old linux-image-*-generic packages just sitting there. Unlike with yum, nothing in plain apt-get limits how many old kernels you have sitting around; if you leave your server alone, you'll wind up with copies of all kernel packages you've ever used. As far as the Ubuntu package system sees it, these are not multiple versions of the same thing but entirely separate packages, each of which you have only one version of.

This gives you a list of packages that looks like this (splitting apart the package name and the version plus Ubuntu release, what 'dpkg -l' calls Name and Version):

linux-image-3.13.0-24-generic   3.13.0-24.47
linux-image-3.13.0-45-generic   3.13.0-45.74
linux-image-3.13.0-46-generic   3.13.0-46.79
linux-image-3.13.0-48-generic   3.13.0-48.80

linux-image-generic             3.13.0.48.55

(I'm simplifying again; on Ubuntu 14.04 there are also linux-image-extra-<v>-<r>-generic packages.)

On this system, the current 3.13.0.48.55 version of linux-image-generic depends on and thus requires the linux-image-3.13.0-48-generic package, which is currently 'at' the nominal upstream version 3.13.0 and Ubuntu release 48.80. Past Ubuntu versions of linux-image-generic depended on the other linux-image-*-generic packages and caused them to be installed at the time.

I find the Fedora/RHEL approach to be much more straightforward than the Ubuntu approach. With Fedora, you just have N versions of the kernel package installed at once; done. With Ubuntu, you don't really have multiple versions of any given package installed; you just have a lot of confusingly named packages, each of which has one version installed, and these packages get installed on your system as a side effect of upgrading another package (linux-image-generic). As far as I know the Ubuntu package system doesn't know that all of these different named packages are variants of the same thing.

(A discussion of some unfortunate consequences of this Ubuntu decision is beyond the scope of this entry. See also.)

Sidebar: kernel variants

Both Ubuntu and Fedora have some variants of the kernel; for instance, Fedora has a PAE variant of their 32-bit x86 kernel. On Fedora, these get a different package name, kernel-pae, and everything else works in the same way as for normal kernels (and you have both PAE and regular kernels installed at the same time; yum will keep the most recent five of each).

On Ubuntu I believe these get a different meta-package that replaces linux-image-generic, for example linux-image-lowlatency, and versions of this package depend on specific kernel packages with different names, like linux-image-<v>-<r>-lowlatency. You can see the collection with 'apt-cache search linux-image'.

Both Fedora and Ubuntu have changed how they handled kernel variants over time; my memory is that Ubuntu had to change more in order to become more sane. Today their handling of variants strikes me as reasonably close to each other.

Written on 07 April 2015.
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Last modified: Tue Apr 7 01:24:19 2015
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