Re: [Xen-users] how can you boot an existing linux domU off the cdrom on sles11 dom0?

["Nick Couchman" <Nick.Couchman@xxxxxxxxx>]

>>> On 2010/04/23 at 05:06, James Pifer <jep@xxxxxxxxxxxxxxxx> wrote: 
> On Thu, 2010-04-22 at 15:51 -0400, James Pifer wrote:
>> I have a sles10 domU running on a sles11 dom0. xen is version 3.4. We're
>> trying to pass through a scsi card to the domU. We assign the domU the
>> scsi card through virt manager. When we try to boot the domU it won't
>> boot. We need to update fstab and menu.lst, but we need to boot with the
>> hardware attached to see how it's being seen by the domU. 
>> 
>> On a physical machine we usually boot off the sles boot cd in rescue
>> mode and modify the file accordingly. In this case, I can't get the domU
>> to boot from the cd. 
>> 
>> Any suggestions on how to do this? I've done it with Citrix XenServer,
>> but have never been able to do it on a sles dom0. 
>> 
>> I've tried adding a cdrom and using xm commands to save config, delete
>> the domU, modify the config enabling the cdrom as bootable, then
>> restoring the domU with xm new -F command, but then it won't boot at
>> all. 
> 
> Can anyone else provide some assistance here? Really need to be able to
> boot off a CD. 
> 
> Thanks,
> James

PV domUs don't really boot off of a device, per se.  PV domUs must first load a 
kernel (and possibly initrd), then the software inside the domU deals with 
finishing off the boot process.  In SLES, this happens using either domUloader 
or pygrub to find and grab the kernel out of the domU disk and pass the correct 
argument.  vm-install searches for a kernel file or kernel package on the ISO 
or CD, and unpacks and/or loads that.

All that said, here's what I'd suggest:
- Use virt-manager and/or vm-install to kick off an installation.
- Specify your existing domU disk, and the SLES11 CD-ROM
- Configure hardware as needed
- On the final screen, where you choose the installation media, make sure to 
specify the CD-ROM as the install source (it usually defaults to hard disk if 
it finds something on it)
- On that same screen is a place for extra arguments.  Add "rescue=1" as an 
extra argument and boot.  This should take you into the rescue system for the 
CD-ROM, and you can modify the files as needed.

Another alternative is to do the following:
- Edit your VM config file and add the following for the extra= line: 
"init=/bin/bash"
- Reload the config file and boot.
- The system should boot and take you directly to a bash prompt, where you can 
remount the root device as R/W (mount -o remount,rw /) and then edit the files.
- Once you're done, remount as R/O, shut down (or destroy) the domain, and undo 
the change to the config file.

-Nick



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