[plug] grub2 and the mystery of the moving hard drives

[Brad Campbell]

G'day all,

I'm in the process of building a new server. It has 24 drive bays and a 
peculiarity of the system firmware appears to be when I add or remove 
drives, the boot drive moves around in the BIOS order. This means that 
grub2 can rarely (if ever) find the drive its modules are stored on as 
its root directory is hardcoded at install time. Why it can't find it is 
a mystery to me given its stage 1 just loaded stage 1.5 from the boot 
track, but never mind.

After spending days fighting with this rotten thing booting of USB 
rescue disks to re-install grub, I stumbled across this little trick.

grub2 can have a config file embedded in its core image. Now, as is 
typical, how you get it in there is not particularly obvious.

Frustratingly grub-install has no way of letting you pass a config file 
to embed, however grub-mkimage does. grub-install has a way of letting 
you override which grub-mkimage it uses.

I created 2 files.

root at test:/root/grub2# cat grub.core.config
search.fs_uuid root 29f53f33-f452-4382-b598-ee307a65f360
set prefix=($root)/grub

root at test:/root/grub2# cat test
#!/bin/sh

grub-mkimage -c grub.core.config $@

Then installed with
grub-install --grub-mkimage=./test /dev/sde

Bingo. Grub now has its uuid search algorithm embedded along with the 
config file and the first thing it does is search all accessible 
partitions for the uuid and uses that to access its files. I can happily 
rearrange disks with gay abandon and the system boots every time.

Note I'm using a separate /boot partition that is plain ext2 and 
therefore the grub files are at /grub rather than /boot/grub.

I wish this was documented somewhere obvious.

Regards,
Brad