Cymen Vig

I wanted to use Debian testing in a Xen DomU but after upgrading (including grub), “xm create “ no longer worked. It failed with the error:

Error: Boot loader didn't return any data!

I followed the suggestion of running “pygrub /path/to/xen/disk” but each of my DomU get a chunk of LVM disk. Within the LVM logical volume, the DomU OS partitions the disk however it wants. I need to get at one of the partitions inside the LVM from the Dom0 or Xen host OS. To do this, get kpartx and run it like so:

kpartx /dev/physical_volume_name/logical_volume_name

After running that, I found my disk partitions from the DomU logical volume at /dev/mapper/logical_volume_name1, 2, 3, etc. So I could then run:

pygrub /dev/mapper/logical_volume_name1

That error out again but it was line 68 of GrubConf.py:

` def set_disk(self, val): val = val.replace(“(“, “”).replace(“)”, “”) self._disk = int(val[2:])`

I added the Python lines to print out the value of val and found that it was getting set to an empty string. Then I simply mounted the now accessible root partition:

mount -t ext3 /dev/pv/lv1 /mnt/tmp

And editing the …/boot/grub/menu.lst revealed the debian upgraded and spewed some cruft into the file. Cleaning that up resulted in a working DomU. The upgrade probably mentioned this but I glazed over it. I suspect I’m not the only one so it’s worth a check.

Finding kpartx made it worthwhile…