How to do grub-install on a (non loading) virtual guest

[Ehud Karni]

I had problems with repeated virtual guest crashing and the grub boot
loader destroyed.  I found a way of doing grub-install to the virtual
guest (a file, not a partition) on Centos5 host.

I hope it may help anyone with a similar problem.


0) Ensure the virtual machine is not working (use `xm list' to check).

1) Mount the virtual disk (cooked file) on loop device. I use the
   `loop7' device because other loop devices may be used by working
   virtual guests.
       losetup /dev/loop7 <virtual-guest-file>

2) Add all the partitions to the device mapper:
       kpartx -a /dev/loop7

3) Mount the virtual root partition:
       mkdir -p /mnt/virt
       mount /dev/mapper/loop7p1 /mnt/virt

4) Check the Major/Minor values of the mounted devices
       ls -l /dev/loop7* /dev/mapper/loop7*

5) Work in as root partition:
       chroot /mnt/virt

6) create /dev/hda and /dev/hda1 devices:
       mknod dev/hda  b 7 7        - Major/minor of /dev/loop7
       mknod dev/hda1 b 253 0      - Major/Minor of /dev/mapper/loop7p1

7) Fix the grub by re-install:
       grub-install /dev/hda

8) Exit the `chroot' jail
       exit

9) Unmount the virtual root partition:
       umount /mnt/virt
       rmdir  /mnt/virt

10) Clear the /dev/mapper partitions:
       kpartx -d /dev/loop7

11) Release the loop device:
       losetup -d /dev/loop7

Now you can restart the virtual guest: xm create <virtual-guest-name>

Ehud


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