Changing initrd contents and grub

[Richard Michael]

On Mon, Jun 23, 2008 at 10:39:35AM -0600, Phil Meyer wrote:
> Richard Michael wrote:
>> Hello list,
>>
>> I've changed my RAID and LVM configuration and need to modify the
>> respective commands in the /init of my initrd.
>>
>> I made those changes by decompressing and extracting the cpio archive,
>> editting the init script (add a couple lines for mdadm, changed the
>> activated volume group name), rebuilt a cpio archive (using the correct
>> "-c"/"-H newc" SVR4 format) and fed it back through gzip (max
>> compression), then I just moved aside the old initrd, replacing it with
>> my new one:
>>
>> mkdir /boot/tmp
>> cd !$
>> gzip -dc ../initrd-<version> | cpio -id
>> vi init
>> find . -depth -print | cpio -oc | gzip -9 > ../initrd-<version>.new cd ..
>> mv initrd-<version> initrd-<version>.orig
>> mv initrd-<version>.new initrd-<version>
>>
>> The kernel now panics (paraphrase) "can't find /init".
>>
>> It does not do this if I restore the original initrd.
>>
>> I have not changed the name of the initrd, filenames match grub.conf and
>> grub's boot menu, etc.  I have done this type of modification
>> successfully in the past, but only changing a single character in /init.
>>   
>
> Just a thought here, since I have also tried this several times with 
> limited success:
>
> The whole point of mkinitrd is to avoid these 'by hand' operations.
>
> After you make your changes, run mkinitrd to generate a new initrd.  It 
> will pick up changes in /etc/modprobe.conf and /etc/fstab and try to do the 
> right thing.  Besides that, mkinitrd will accept arguments that allow 
> additional drivers to be loaded, with arguments if needed, as well as many 
> other options.

I understand what you are saying, but I am of the opposite opinion.

I have a decent picture of what needs to change, and where to change it.
For example, those raid arrays which must be activated, and others not,
etc.  What I obviously need is a more detailed understanding of the
kernel boot process, the BIOS and grub.  (One problem here is
troubleshooting the kernel and boot is tricky when the output moves so
quickly on the screen!)

Automated tools often frustrate me because (a) I don't learn anything,
so I can't fix it when something goes wrong, and (b) something will go
wrong due to all the guesswork of an automated tool.  As I say, I've
fixed problems with initrd's on other systems this way before.  (In
fact, I had to change the uuid of an array that was being activated.  If
I hadn't know anything about the contents of an initrd and how to modify
it, etc. it would have been quite hard to fix with mkinitrd because the
system wouldn't boot!)

In this situation, mkinitrd is hard to employ because the system is
booted from the Fedora DVD in rescue mode.  I'm moving the entire system
to a new configuration (RAID1 on RAID5).  This means I can't run
mkinitrd on *the* system to have it autoprobe, etc., etc. (chrooting
from a rescue image always seems broken because the /dev entries never
exist in the newly root tree).  Moreover, I think nested RAID arrays
will really confuse any of the automated tools because that
configuration isn't (doesn't appear to be) supported (at least not at
install time).

Thanks though.

Regards,
Richard

> I am pretty sure that a modern mkinitrd will make almost all need for 
> manual edits of an initrd image unnecessary.
>
> Good luck!
>
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