[single *_grub_* partition for many distros]

Perry pwhite at bluewin.ch
Sun Sep 20 12:35:42 UTC 2009

Le Sunday 20 September 2009 08:05:31 Joe(theWordy)Philbrook, vous avez écrit :
> It would appear that on Sep 17, Perry did say:
> > Thanks Joe and Goh,
> >
> > Many interesting points in your mails.
> > I hope I will rememeber to pay attention to this "detail" in the future.
> You probably will Perry. You already seem to see the potential problem
> that has me maintaining a MANUALLY updated grub partition on /dev/sda3
> ..........................^^^^^^^^....................................
> I routinely install multiple linux distros. I always tell them to
> install grub on the superblock of the "root ( / )" partition rather
> than to the mbr. (Of course I always have to look for something like
> an "advanced" options button in the bootloader section of the linux
> installer to even know there is a choice. Each of the distros will put
> most of it's grub stuff in that distro's /boot/grub directory. But
> there is a very small, but critical piece, call it a bootstrap, that it
> will by default put in the MBR. The bootstrap's only real purpose is to
> transfer execution to the grub files in that distro's /boot/grub
> directory. By telling the installer to install grub to the superblock
> instead, I prevent the new grub from being the default boot loader.
> While still causing the installer to create a working grub loader.
> Confused yet? I would be... But bear with me and I will try to make it
> add up.
I'm not confused yet.

As I see it, the MBR is too small to contain a bootloader, instead it uses a 
pointer (bootstrap) to the bootloader's position.

As someone installs a new distro/version, most of grub will be copied  either 
in a 'boot' folder under '/' or in the '/boot' partition (if it is mounted).
Then the pointer shoud normally be copied/updated in the MBR but you make it 
instead in the superblock of your partition.
> It used to be that each time I installed another linux I started using
> a new grub set-up. But I didn't care for the fancy splash screens that
> most of them began using, so I learned how to set them up with a
> simpler looking menu again and again. Then I figured out that I could make
> a smallish partition and copy everything in the /boot & /boot/grub
> directories of whichever one I was using at the time to it.
> That is I copied "/boot" to "/" of the new partition, and "/boot/grub"
> to "/grub"...  And of course I had to make sure that I had copied the
> vmlinuz (kernel) files of each distro, as well as the initrd files to
> the "/" of the new partition.
If you mount this boot partition as you install the new Linux, files will be 
placed there.
*I guess the important point there is that you only do this copying after you 
have tested this new grub*
> Then I edited the "root lines" of the menu.lst (sometimes called
> grub.conf) file on the new partition from something like:
> root (hd0,6)
That means you installed on sda7, doesn't it?
> to say instead:
> root (hd0,2)
> Which in my case was the new boot partition. (see below)
Not quite clear, your are in the /boot/grub/ of your new Linux to tell the 
system it has to boot from sda3?
> Then from that distro, I used a root shell to mount the new partition
> right on top of the existing /boot
> # mount /dev/sda3 /boot
So you cover your boot folder with the boot partition, that contains just its 
copy. (*but you have tested it before*)
> And while it was so mounted, I created a symlink:
> # cd /boot
> # ln -s . boot
I don't understand, your boot partition will contain a 'link name', 
named 'boot', and that represents the eponymous '.' current directory.
> Next I used grub to set itself up in the mbr.
> I know of two methods to do this.
> There is a less well advised method that uses a linux command called
> Where I'd want to make very sure the new boot partition was mounted
> on /boot when I'd use the root shell to enter the command:
> # grub-install /dev/sda
> But I'm told it's better to use the grub shell to do this.
> Of course to do that I needed to understand that with grub the first hard
> drive isn't called "/dev/sda" but rather it's (hd0) [ zero because grub
> starts with zero rather than one] This means my grub partition on
> "/dev/sda3" is called (hd0,2)...
> I don't really think this method requires that I mount the partition
> because I explicitly tell grub which partition. But I've always
> mounted it anyway...
> So in any case, at the root shell prompt (with /dev/sda3 mounted on
> /boot) I started a grub shell with:
> # grub
> Then I typed three grub commands
> The first one tells grub which partition grub is rooted on...
> The second one tells grub  to setup in the mbr
> The third one tells grub I'm done
> root (hd0,2)
> setup(hd0)
> quit
OK, you place your bootstrap in the MBR to boot from your boot partition.
> Now, as long as I don't set up any linux to use /dev/sda3 { or (hd0,2) }
> and as long as don't let them use the mbr, this boot partition will
> remain stable no mater which linux I install something else on top of
> etc...
> The draw back is I have to make changes manually... If I upgrade a
> kernel, I need to copy it and it's new initrd file to the boot
> partition and edit that grub.conf [menu.1st] file to use the new one
> before it will work.
Normal, the MBR is set so.
> Except that if a linux automatically updated it's own grub, (the ones
> that I had the installer install grub on the superblock instead of the
> mbr) I can set my grub to boot that distro's grub.
> So that since my Kubuntu is on /dev/sda5 and is currently booted from
> this grub entry:
> title           Kubuntu kernel 2.6.28-11-genericKubuntu9.4jaunty
> root            (hd0,2)
> kernel          /boot/vmlinuz-2.6.28-11-genericJaunty root=/dev/sda5 ro
> vga=normal initrd          /boot/initrd.img-2.6.28-11-genericJaunty
> When I upgrade to karmic in such a way as to automatically configure the
> grub on /dev/sda5 to use the new kernel etc, I'll be able to test it
> before editing MY grub, with this existing grub entry:
Aha, very clever!
 and you do that before the other points (where I placed comments 
between "*...*")
> title chain-booting Kubuntu via it's loader on sda5
> rootnoverify  (hd0,4)
> chainloader +1
> It starts the grub on kubuntu's superblock...
> Hope this helps...
Very interresting in any case, as the following post from Goh

Thanks a lot, guys		Perry
> --
> |   ---   ___
> |   <0>   <->	   Joe (theWordy) Philbrook
> |	^		J(tWdy)P
> |    ~\___/~	     <<jtwdyp at ttlc.net>>

BOFH excuse #25: Decreasing electron flux

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