[Xen-users] booting a domU from a cd to run a linux distro installer

[Jeremy Blosser <jblosser-xen@xxxxxxxxxx>]

As the subject says, I wanted to boot a domU from a CD to run a linux
distribution installer (specifically, the 0.9.5-test2 installer ISO from
http://www.sourcemage.org/).  I looked for info on the web and in IRC on
doing this, and most things that discuss it seem to suggest doing the
install in a real system or using something line qemu first, then switching
it to xen.  I did get it to work using xen to initially boot the CD, so I
thought I would provide that info here for others.  Sorry if this is
repeated from somewhere, but I couldn't find it anywhere else.

Background: My distro's installer boots an initrd on the CD as its initial
/.  This initrd is larger than the default (important when getting the boot
parameters correct).  Once booted, the installer gives the user the chance
to insert modules, etc.  It's then supposed to switch to using the CD as /,
if it can find the CD.  If it can't find the CD you can tell it what
partition to use.  This style of install worked well for booting it in xen,
YMMV on other installers.

The real kernel boot line on the ISO is:

initrd=/boot/initrd.gz ramdisk_size=12288 devfs=mount root=/dev/ram0 rw

This is the xen config to get the same result:

kernel = "/boot/vmlinuz-2.6-xenU"
root = "/dev/ram0 rw"
extra = "ramdisk_size=12288"
ramdisk = "/mnt/cdrom/boot/initrd.gz"
memory = 64
name = "SMGL"
disk = ['phy:/dev/hdc,sdc,r',
        'file:/share/vm/smgl/smgl.install.disk,sda1,w']

/mnt/cdrom needs to be mounted so it can find the initial initrd for the
boot.  After that it boots and I am able to point it to /dev/sdc as the /
device to switch to and continue the install.

I was using the default xenU from 2.0.7.  There were warnings throughout
the install about the kernel modules directory missing, but this wasn't
actually enough to break anything.  I assume if I wanted to get rid of the
warnings I could build a kernel for the installer to use that didn't use
module support.

A couple of other things were flakey during the install as well, but
nothing that prevented me from booting to a working system when done.  Our
distribution install team will probably take a look at these warnings and
see if we can resolve them as well.

I hope this is useful to some people.  Thanks for this tool, we're a
low-budget distro and this should be useful in helping us debug things
without a lot of hardware available.

-- 
Jeremy Blosser
Assistant Project Lead, Source Mage GNU/Linux

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