how to view off screen kernel messages during boot?

Charlie_Root · n00b Joined: 10 Mar 2005 Posts: 16

I'm having troubles getting a kernel to boot with a new system I'm doing and I'd like to see the kernel messages that fly by when you boot (they don't if you specify "quiet" to the kernel).

I know normally you could view most of this info with dmesg or by checking the log under /var but this kernel doesn't even get as far as mounting partitions.

On freebsd you can do this just by pressing the scroll lock key and using pg up/down or the arrows, but rather annoyingly this doesn't work with linux. Nor does pressing the pause key a few times while all the info scrolls by.

Reason I'm asking is I'm having problem booting off software raid but the messages on screen by the time it fails are useless, and I can't view older messages.

Kate Monster · Posted: Sun Mar 25, 2007 2:44 am Post subject:

Hi,

I believe what you're looking for is shift + page up/down.

Charlie_Root · n00b Joined: 10 Mar 2005 Posts: 16

Thanks, that's exactly what I was after. I've missed that functionality ever since I switched my desktops from freebsd to gentoo a few years back.

Does it not work with framebuffers though? or does it stop working at a certain point during booting?
Reason I ask is because it worked on the machine I'm having problems with but then I tried it on my desktop (which has a framebuffer, and is fully booted ready for login) and it doesn't. edit: answered in my last post in this thread

This is going off topic now, and isn't really that important, but is there a way to view the buffer on my desktop too? Last thing I see is "cleaning /tmp directory" and it would be nice to see beyond that, I've needed it in the past.

Kate Monster · Posted: Sun Mar 25, 2007 3:45 am Post subject:

I've had that problem too, with the framebuffer on my desktop(works fine on the laptop though, no framebuffer). I'm not really sure why it does this. I think there is a kernel option to set it, but I'm not sure since I'm using Windows right now.

Charlie_Root · n00b Joined: 10 Mar 2005 Posts: 16

Ah, I was kind of hoping the framebuffer was the reason, and not that it stops working after a certain point in the boot process.

I suspect I'll be fiddling with the kernel a lot over the next few days so I'll try find that option. If it exists and I find it, I'll add it here.

Anyway, thanks a lot. This knowledge will come in very handy

edit: just did a bit of research and...
"2.1.116 - scrollback buffer support added to vesafb." (pretty sure I use vesafb not vesafb-ng on the desktop I tried)
As well as talk of a "video=scrollback:" argument. I might try setting this and seeing if scrollback works, though I doubt it.

wynn · Advocate Joined: 01 Apr 2005 Posts: 2421 Location: UK

If you've got a second networked machine you can use How to enable logging of the init-process? (it logs the boot messages, not the init messages)
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widremann · Veteran Joined: 14 Mar 2005 Posts: 1314

dmesg guys...all that stuff shows up in the ring buffer log.

Charlie_Root · n00b Joined: 10 Mar 2005 Posts: 16

Thanks wynn, that looks handy too. Not at this moment but I can me using that in the future

widremann, yes dmesg is fine, as long as you can get as far as being able to run it. Also you can't get the init messages from it

late edit: shift + page up/down does work with framebuffers. The reason it doesn't work on my desktop is because the buffer seems to be reset once xdm starts. If you want to be able to use it just disable the xdm startup script (rc-update del xdm) and reboot... once you've got the info you need re-enable the startup script (rc-update add xdm default) and start xdm (/etc/init.d/xdm start)