How packaging systems should handle kernel updates

July 24, 2008

As a system administrator, I have some relatively strong opinions on how Linux distributions and packaging systems should handle kernel updates. Since I have just gone through the experience of yet another mass kernel update of our servers, I feel like writing them up:

  • you should be able to have multiple kernels installed at once.
  • it should be easy to tell which package's kernel you are running, and thus whether or not you are running the most current kernel.
  • installing a kernel update should never overwrite an existing kernel image; it should always install a new kernel.

    (The one time when it is marginally acceptable to overwrite an existing kernel image is when the package update has literally no changes in the kernel; all it does is fix some packaging mistake. But that should be vanishingly rare anyways.)

  • old kernels should get removed sooner or later, at a rate that is configurable but that has a sensible default. I do not need every kernel ever released for my distribution sitting around on my disks (especially if they have security holes).

    (Ideally there would be some sort of time-based minimum expiry, so I can say 'never remove a kernel until I haven't run it for a month'.)

  • you should be able to 'pin' any particular kernel so that it is never removed. You really want this if you have a 'last known good' kernel and you want to keep experimenting with kernel updates to see if your distribution got it fixed this time around.

  • the running kernel and its modules should never, ever get removed (or overwritten).

I don't know of any distribution that gets all of these right. Ubuntu fails spectacularly at never overwriting existing kernels; Red Hat has no good way to pin a particular kernel version so that it won't get removed (unless the yum versionlock plugin has very recently fixed its interaction with the 'keep the most recent N' plugin). No one has good kernel expiry.

Oh, and I prefer as much of this as possible to be in the kernel packages themselves, instead of in tools that are used to manage them, so that I cannot accidentally cause problems by using the wrong package management tool. This means that I somewhat prefer the Debian/Ubuntu approach of making different kernel versions have different package names to the Red Hat approach of making different kernel versions be different versions of the same package and putting the smarts in yum and other tools.

Written on 24 July 2008.
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Last modified: Thu Jul 24 22:31:51 2008
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