Re: restoring GRUB loading after partition/device rename

[Jordan Uggla]

On Sat, Sep 18, 2010 at 4:07 PM, Gary S. Trujillo <address@hidden> wrote:
> Greetings.
>
> My machine, a Compaq notebook, has become unbootable - almost - due to
> a set of circumstances I'll describe later.  First, though, here where
> things now stand.
>
> The MBR of the internal drive has what I believe to be a good copy of
> GRUB2 (though it's not acting as I would expect - more on that subject
> also a bit later), and several partitions, including several for
> Windoze (which I'm using now to write this plea for help), and three of
> special interest:
>
>  /dev/sda6 (previously known as /dev/sda8): ext4 file system
>
>  /dev/sda7 (previously /dev/sda6): ext3 Linux /boot partition
>
>  /dev/sda8 (previously /dev/sda7): Linux swap partition
>
>  My problem is that GRUB knows about these file systems under their
>  previous names, not the new ones.
>  Here's what happened...
>
> After a recent install of Ubuntu, I began installing software using
>  apt-get.  However, I didn't know that the /boot partition would be
>  used during that process, so I had only allocated 80MB, which I
>  discovered in the process of updating the system is insufficient.
>  Since the /boot and swap partitions were immediately adjacent to one
>  another, I carved a bit off of swap to give to /boot, so it would
>  have 200MB, which seems sufficient.  That part worked fine - I backed
>  up everything in /boot, freed the space in both partitions, using
>  fdisk, re-partitioned and restored the previous contents of /boot. So
>  far, so good.  Then I tried rebooting, and found myself looking at a
>  GRUB rescue prompt.  After booting into Knoppix using a USB stick, I
>  saw that the partitions had been re-arranged as indicated above.
>  I've spent time poring over various descriptions of how to recover
>  from such a situation, including an article at:
>
>  https://help.ubuntu.com/community/Grub2
>
> which, though helpful in some general ways, leaves me somewhat
> confused.  The claim is made in the article that, though the grub2
> rescue mode has a reduced command set, it includes commands like
> "help," "cat," "linux" and "reboot," none of which are actually
> available.
>
> I was finally able to boot into Windoze using the advice in a GRUB2
> manual I got on this site - at http://gnu.org/software/grub/manual/
> using:
>
>  set prefix=(hd0,x)/grub
>  set root=(hd0,x)
>  insmod normal
>  normal
>
> ("x" being "7" in my case, to boot from /dev/sda7).
>
> I could select the partition containing Windoze, but not the Linux one,
> for reasons I can understand that have to do with incorrect device
> names being encoded into the "grub.cfg" file.
>
> Here's how the partition table looks, as fdisk sees it:
>
>   Device Boot      Start         End      Blocks   Id  System
> /dev/sda1   *           1          26      203776    7  HPFS/NTFS
> Partition 1 does not end on cylinder boundary.
> /dev/sda2              27       28622   229697339+   f  W95 Ext'd (LBA)
> /dev/sda3           28623       30389    14180352    7  HPFS/NTFS
> /dev/sda4           30389       30402      105656    c  W95 FAT32 (LBA)
> /dev/sda5              27       10406    83377318+   7  HPFS/NTFS
> /dev/sda6           11202       28622   139934151   83  Linux
> /dev/sda7           10407       10432      208813+  83  Linux
> /dev/sda8           10433       11201     6176961   82  Linux swap /
>   Solaris
>
> The /mnt/boot/grub/grub.cfg file is shown at
> http://pastebin.ca/1943856 .
>
> If I try to boot from the Linux partition, which is the first shown in
> grub.cfg, and go into edit mode, I see that the boot device is
> identified by means of a UUID designator, which is something I don't
> understand at present.  I tried replacing both the device name ("sda6"
> to "sda7") and the UUID name in that edit session and rebooting, but
> nothing useful happens (I don't recall at this point exactly what
> happened, except that it wasn't a boot into Linux). Here's a snippet
> from "grub.cfg": insmod ext2 set root='(hd0,6)' search --no-floppy
> --fs-uuid --set 23fdef48-e848-4c3f-9001-efe958b8cdf7 set
> locale_dir=($root)/grub/locale set lang=en insmod gettext
>
> Someone in an IRC channel suggested I try using "Super Grub
> 2" (http://www.supergrubdisk.org/).  Though it gives me some
> interesting information, I haven't yet figured out how to use it to
> solve my problem.  I intend to go back to it while waiting for replies
> to this trouble report, however.  (I found a note in the Wiki that
> serves as documentation for Super GRUB 2 that it can't deal with
> separate boot partitions (which I'm using because I hope to eventually
> be able to replace the file system I'm using with a KVM volume, so I
> can use encryption - however, for the time being, I've copied what's
> in /boot into the /boot directory on that file system, so it would be
> possible to boot from /dev/sda6).
>
> I should also mention that I understand, from the help.ubuntu.com
> article and elsewhere, that it is not possible to simply edit the
> "grub.cfg" file the way that was possible with respect to "menu.lst"
> under plain old GRUB.  However, the "update-grub" command seems to not
> take an argument, so it's seemingly not possible to edit the file while
> booted into another system, like the Knoppix live system I've been
> using to poke around has made possible (and Knoppix only has an old
> regular GRUB version of "update-grub" anyway).
>
> My questions at this point are:
>
>  1. Why don't I have access to all the GRUB 2 commands indicated in
>     the article at https://help.ubuntu.com/community/Grub2 in GRUB2
>     rescue mode?

Because that guide is inaccurate. I'll fix it (or replace that with a
link to the official manual) when I have time.

>
>  2. What's the easiest way to figure out what all I need to do to
>     get myself back into a bootable condition, and what's the best
>     procedure to use?

Follow this guide: http://grub.enbug.org/Grub2LiveCdInstallGuide

>
>  3. Are there any other good resources anyone can recommend?

The official manual, which you found already @
http://gnu.org/software/grub/manual/ , is probably the best resource.
There are still pieces left over from grub legacy that need to be
updated (as they don't apply to grub2 at all) but that will hopefully
be fixed shortly (at the very least before 1.99 is released).

>
> Thanks in advance for any advice or assistance!
>
> _______________________________________________
> Help-grub mailing list
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> http://lists.gnu.org/mailman/listinfo/help-grub
>

-- 
Jordan Uggla (Jordan_U on irc.freenode.net)