Re: [gentoo-user] CD boot - dmesg buffer depth option?

[Mark Knecht]

On Mon, Jul 5, 2010 at 12:11 PM, Willie Wong <wwong@××××××××××××××.edu> wrote:
> On Mon, Jul 05, 2010 at 09:16:14AM -0700, Mark Knecht wrote:
>> I'm trying to capture the full boot log when booting from the Gentoo
>> install CD but it seems the buffer isn't deep enough to get the whole
>> thing. Is there by chance a command line option that will increase the
>> depth of what's captured by dmesg so that I can get all the way back
>> to the beginning?
>
> pass the following parameter to the kernel on GRUB/LILO:
>
> log_buf_len=n
>
> where n is a power of two. By default it is 16384. You can change it
> to 131072 (= 2^17) to get a much larger kernel log ring buffer. If you
> put in something that is not a power of two, the kernel will ignore
> the option.
>
> You may also have to pass an additional parameter to dmesg to read the
> early messages: look at 'man dmesg' and the -s flag.
>
> Cheers,
>
> W
> --

Hi Willie,
   Thanks for the ideas. I've been trying them but so far no luck.

   In my normal booting kernel (on the system hard drive) I did push
the length up to 18. With that setting dmesg prints all the way back
to the beginning. However booting the Live CD I hit F1, it shows the
kernels, so I type

gentoo log_buf_len=18

and then boot, but dmesg never goes back to the beginning. I've also
tried dmesg -s 300000 but still the results are cut off somewhere down
in the stack and I never see the first part of the buffer.

   I'm looking at this page for some info:

http://www.kernel.org/pub/linux/kernel/people/gregkh/lkn/lkn_pdf/ch09.pdf

Reading about log_buf_len I also see earlyprintk and time options. I
tried them at the Live CD command line but it didn't change what I'm
getting in dmesg like it did on my regular kernel.

   I'll continue to study.

Thanks,
Mark