Re: Question about using grub on disk image files (as opposed to discs)

[John P. Looney]

On Tue, Dec 18, 2001 at 12:32:55PM +0100, Christoph Plattner mentioned:
> Interesting topic !
> 
> I play armound with this, with the further problem to have a cross
> build from Sparc (Solaris) to i386.
> 
> One quite simple way is, you create a stage1 + stage1_5 on a device of
> your choise with an normal (online) install. The dd the MBR+nr_of_blocks
> of stage1.5 into a file, let's call it bootldr.img.

 Ok, I did most of this. I plugged my flash card into my IDE reader, on
/dev/sda, with my workstations root disk as /dev/hda, and did;

# grub
grub> root (hd1,0)
 Filesystem type is ext2fs, partition type 0x83

grub> find /boot/grub/stage1
 (hd0,3)
 (hd1,0)

grub> setup (hd1)
 Checking if "/boot/grub/stage1" exists... yes
 Checking if "/boot/grub/stage2" exists... yes
 Checking if "/boot/grub/e2fs_stage1_5" exists... yes
 Running "embed /boot/grub/e2fs_stage1_5 (hd1)"...  22 sectors are
embedded.
succeeded
 Running "install /boot/grub/stage1 d (hd1) (hd1)1+22 p
(hd1,0)/boot/grub/stage
2 /boot/grub/grub.conf"... succeeded
Done.

grub> quit
# cat /mnt/rescue/boot/grub/grub.conf 
default=0
timeout=10
title Test Kernel 
    root (hd0,1)
    kernel /boot/bzImage
#

 Looks good. But, on booting a box with this flash chip, I got a grub
"hard disk error" of some description. I think it could be that grub is
telling init that the root filesystem is on hd1 (which it no longer is, on
the target system). This was just to test did the grub install work. On
the "dd" bit; for an ext2 filesystem, that'll be;

 round_up(10848/512) + 1 = 23 blocks

 yes ? Will this always be there ? Basically, I would really like to be
able to dd off those 23 blocks, and know that even if I write new files
into that partition, and move (non-grub) ones around, I won't have to
re-run grub. 

> Now you must know something about the geometry of the disk. Example:
> 4 heads, 20 sectors (only to have round values), 300 cylinders and
> ofcourse every sector has 512 bytes.
> 
> The layout is: The MBR is in the frist sector on the first cylinder
> on the first head. The rest of this track is kept empty and used
> for the stage1.5. So the first partition starts at lba sector 20,
> as 0..19 are reserved (MBR = stage1, stage1_5 13 to 16 sectors,
> rest is padding). So you need a padding file, to have fullfill
> the structure and you can create a disk image like
> 
> fdisk bootldr.img  to create partitions ...
> cat bootldr.img padding.img part1.img part2.img ...  > fulldisk.img

> Important, the 2nd, 3rd, etc.., partition then starts on 80 sector
> boundaries, always on complete cylinders.

 Hmm. Slightly different approach to mine; I was dding in the partitions
directly like;

# Make partition tables
/root/make_partition_tables $IMAGE_FILE

hda2_offset_bytes="2359296"
hda1_offset_bytes="16384"
hda1_offset_blocks="32"
hda2_offset_blocks="4608"

# Make, format & tune files to be used as templates for the image
dd if=/dev/zero of=/tmp/hda2 bs=512 count=121768
dd if=/dev/zero of=/tmp/hda1 bs=512 count=4526
mkfs -F -m0 /tmp/hda1
mkfs -F -m0 /tmp/hda2
tune2fs -c 0 /tmp/hda1
tune2fs -c 0 /tmp/hda2

# Put the two images into the IMAGE file, and mount them
dd if=/tmp/hda1 of=$IMAGE_FILE seek=$hda1_offset_blocks bs=512
dd if=/tmp/hda2 of=$IMAGE_FILE seek=$hda2_offset_blocks bs=512

 But, I assume this will work - "padding" will just be empty, as long as I
have the partition tables calculated properly.

> With giving the geometry to GRUB, it should also be possible to
> use GRUB directly for installing itself on the diskimage, but
> I have not tried it.

 Hmm. I do have all the geometry worked out, but I've no idea how to even
start at this. Are there grub config file examples anywhere ?

 Thanks for taking an interest in my private pain!

John

-- 
_______________________________________
John Looney             Chief Scientist
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