Re: [Xen-users] Need mega basic Xen help

[Michael Jinks <mjinks@xxxxxxxxxxxx>]

Hi, Rob.

I'm a confused newbie too but I think I can help with this one...

> Here's my error message when watching the console:
> 
> > VFS: Cannot open root device "sda1" or unknown-block(0,0)
> > Please append a correct "root=" boot option
> > Kernel panic - not syncing: VFS: Unable to mount root fs on
> > unknown-block(0,0)

Okay, kernel don't know no sda1, probably because it doesn't have a Xen
block device driver loaded...

> And my config:
> 
> > kernel = "/boot/vmlinuz-2.6.18-53.1.14.el5xen"
> > memory = "256"
> > name = "ubuntu"
> > vif = ['']
> > dhcp = "dhcp"
> > disk = ['file:/var/lib/xen/images/ubuntu.img,sda1,w',
> > 'file:/var/lib/xen/images/ubuntu.swap,sda2,w']
> > root = "/dev/sda1 ro"

This all looks fine to me, but there's no 'ramdisk' line, and I bet the
el5 kernel stores its block drivers in its ramdisk (or not; see below).
Alongside wherever you got your kernel, there should be a file named
something like "initrd-[same exact version string].img".  Copy it into
the same area where you're storing your Xen kernel, and add

  ramdisk = "/boot/initrd-2.6.18-53.1.14.el5xen.img"

...(tweaking the details as appropriate) to your xm config file.

That might not get you out of the woods though.  I found that the RHEL
kernels tended to be missing bits that I needed, like the ability to
recognize PCI cards passed through from the dom0; they're also notorious
for leaving the xenblk driver out of the ramdisk, meaning you have to
generate your own using mkinitrd.  If you still have trouble after
copying over the initrd file for your kernel, here's a page that talks
about that, including an example of a fix:

  http://wiki.xensource.com/xenwiki/Fedora5Xen0DomU

I've also found that mixing distro X with a kernel from distro Y is
asking for mysterious hangups at the moment when init should be taking
over from the kernel boot process; your mileage may vary, try it and see.

> I didn't know which kernel to grab, so I just used the CentOS.  However, if
> that's not right, I don't know what to use!

I have yet to get anybody's install CD to work properly.  The closest I
got was OpenSolaris, which booted and happily installed the OS, but the
system then kernel panics on boot.  I could get the RHEL4 install CD to
boot, but even with a monolithic kernel it wouldn't recognize any block
devices, including the CD it had just booted from (so I know the problem
wasn't within Xen), so I couldn't use it to actually set up the system.
The only reliable method I've found so far is to build the domU inside a
chroot on the dom0.

HTH,
-j

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