Arch Linux

idealeric

Member

Registered: 2009-03-11

Posts: 2

What's the purpose of the "Fallback" kernel image?

Hey there. I've been using Linux as a whole for a little under a year. I briefly used Ubuntu, made the migration to Arch soon after, and have used other distros here and there in VMs and as host OSs. One thing that I've never really understood is the purpose of the "Fallback" kernel. I know Arch is not the only distro to utilize it, so I'm assuming its function is pretty universal. I've of course had my fair share of issues, misconfiguration, etc, but never had I ran into a problem which I couldn't fix with the "normal" kernel. I originally thought it was a kernel that skimps out on loading modules and/or certain configurations (similar to "safe mode" in Windows or some other OS), but anything that bad, where I couldn't actually boot the default kernel or something, I likewise couldn't boot the fallback. If not that, what is it?

Lexion

Member

Registered: 2008-03-23

Posts: 510

Re: What's the purpose of the "Fallback" kernel image?

If you mess up the kernel image, it's there for you to use to boot linux and fix the problem.  You can mess up the kernel image with mkinitcpio.

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urxvtc / wmii / zsh / configs / onebluecat.net
Arch will not hold your hand

bender02

Member

From: UK

Registered: 2007-02-04

Posts: 1,328

Re: What's the purpose of the "Fallback" kernel image?

1) the kernel on arch is just one (ie there's no fallback kernel)
2) what's 'fallback' is the initramfs (the stuff that loads just after the kernel when you boot)
3) the difference is that the "normal" initramfs contains only the stuff that you configured to be there (/etc/mkinitcpio.conf) and the "fallback" contains the default selection of drivers (eg all filesystem drivers, etc..)

artiom

Member

From: France

Registered: 2008-07-29

Posts: 111

Re: What's the purpose of the "Fallback" kernel image?

The last kernel upgrade I made messed up both (here is the same problem http://bbs.archlinux.org/viewtopic.php?id=69849 ). So is there any tool to make a real fallback kernel ? I think the actual kernel is better than just default initramfs.

bender02

Member

From: UK

Registered: 2007-02-04

Posts: 1,328

Re: What's the purpose of the "Fallback" kernel image?

The last kernel upgrade I made messed up both (here is the same problem http://bbs.archlinux.org/viewtopic.php?id=69849 ). So is there any tool to make a real fallback kernel ? I think the actual kernel is better than just default initramfs.

You can compile your own kernel with all the required modules compiled in. Otherwise you'll always need to use some initramfs.

EDIT: what you can do instead of compiling your own kernel is to keep a backup of the working version of the kernel together with its initramfs - see for instance savekernel script.

Last edited by bender02 (2009-05-25 15:34:10)

artiom

Member

From: France

Registered: 2008-07-29

Posts: 111

Re: What's the purpose of the "Fallback" kernel image?

Its not what I mean. Is there any possibility in arch to have a previous kernel as "Fallback" ?

Edit: I posted my reply before your Edit

Ideally kernel upgrade should rename the actual kernel to something like Fallback to be used in case of upgrade problems.

Last edited by artiom (2009-05-25 15:38:40)

artiom

Member

From: France

Registered: 2008-07-29

Posts: 111

Re: What's the purpose of the "Fallback" kernel image?

I've looked at savekernel. It builds the old kernel from AUR... Not that I really need. I think I will make a kernleupgrade script to backup the current kernel before upgrade then upgrade.

Last edited by artiom (2009-05-25 15:51:45)

bender02

Member

From: UK

Registered: 2007-02-04

Posts: 1,328

Re: What's the purpose of the "Fallback" kernel image?

The trouble with just copying the old kernel and old modules somewhere is that it'll try to use the same modules directory (/lib/modules/2.6.XX-ARCH), so when you upgrade the kernel in that directory you'll have the modules which belong to the new kernel, so they might not work with your 'saved' one. Of course when there's a 2.6.XX to 2.6.YY upgrade this is not a problem, but on the small ones (2.6.XX.YY -> 2.6.XX.ZZ) it might be. Note that they *should* work but you can't be sure. It kind of defeats the purpose of a 'fallback' stuff.
You essentially have 2 options:
1) just keep the old kernel26-...-pkg.tar.gz package and have a boot cd/usb handy so that when something bad happens you can just boot it, chroot into your arch system and downgrade the kernel using this saved package
2) actually *build* one kernel yourself (eg using savekernel or any of the 100 PKGBUILDs from AUR or just the official PKGBUILD from ABS (but changing the name from -ARCH to something else)) and just have that one installed for the bad times.

Last edited by bender02 (2009-05-25 17:18:22)