Re: [Xen-API] How Pygrub work on VHD

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 25 Jan 2010, at 21:51, Ian Pratt wrote:

>>> That's correct. When a vhd-based VDI is attached to a domain,
>> blktap(kernel-space) + tapdisk(user-space) do the translation from raw
>> disk block accesses to vhd read/writes.
>> 
>> What you are talking about is how VM accesses vhd-based disk image. What
>> I want to know is how pygrub grabs kernel and initrd from vhd-based disk
>> image, pygrub is running on dom0, there is no /dev/xvda, which is inside
>> VM.
> 
> Before booting a VM, xapi will set up blktap for each of the VM's disks. One 
> side effect of this is that a block device is exposed in dom0 that enables 
> tools in dom0 to also access the disk. For a HVM guest this is used by qemu 
> for accessing the guest's disk through emulation. It is this same device that 
> pygrub operates against.
> 
> I'm not sure why pygrub isn't working for you. You should be able to run it 
> manually against the block device and see why its failing.

You can use the 'xe-edit-bootloader' script from dom0 to ensure that the 
dom0-block-attach is working correctly (valid VHD file, etc).

It drops you into a shell inside the guest filesystem to fix grub.conf issues 
which would otherwise leave a PV VM unbootable.  If the VHD file is block 
attached successfully, then the failing pygrub output should be in 
/var/xensource/xapi.log.

If you're not in a XenServer dom0 for some reason, then the script is in the 
repo at 
http://xenbits.xen.org/xapi/xen-api.hg?file/25e9038fce00/scripts/xe-edit-bootloader
 

-anil
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