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Re: [Xen-users] Creating initial file system for a new Xen domain

To: David Stone <unclestoner@xxxxxxxxx>
Subject: Re: [Xen-users] Creating initial file system for a new Xen domain
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Fri, 19 Oct 2007 16:48:21 +0100
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> I've moderately familiar with using XenSource distributions and also
> administering Xen with tools like virt-manager on Fedora.  However now
> I'm building xen (3.0.3) from source and trying to use what I built
> myself.
> My first question is why is it that tools like virt-manager don't work
> when I boot to the Xen hypervisor and Dom0 I built?  The virt-manager
> I'm trying to use is from FC6.  Is this known to be generally
> impossible for some reason (missing APIs), or is it supposed to work
> at least sometimes?  What are my other options for tools that will
> create a Xen domain configuration file...I know this can be done by
> hand but there must be more friendly tools out there somewhere...

I'm not entirely sure, to be honest.  I would guess that maybe virt-manager 
isn't up to date enough to use your hypervisor version, but I'm not really 
familiar enough with it to say for sure.  Does xm, etc work correctly for 

What distro are you on?  You could perhaps try updating to a newer 
virt-manager (e.g. the one from FC6?  Or from source, or from SVN?).

> My second question is very basic, forgive me: using just the tools
> that are built with the xen source distribution, how does one install
> Linux on a DomU?  I know you just point to a kernel/ramdisk in the
> domain configuration file, but what about the root filesystem?  Using
> XenExpress and virt-manager, I always run through a Linux install, so
> that populates the root filesystem with all the stuff essential for
> the system.  I know that you can point a domain to a partition in the
> domain's configuration file, but how do you populate that partition?

If you want an HVM domU, you create a configuration file that points to the 
virtual disk of the domain, and gives it a virtual cdrom drive pointing to 
the .iso of the install CD.  Then proceed basically as for a normal machine.

If you want a PV domU, the process varies.  To install a Redhat-type guest, 
you ferret around on the install CD until you find the installation kernel 
and initrd for Xen, then create a config file that boots these, passing it 
the virtual disks for it's install destination and the .iso file (don't pass 
this as a cdrom, it won't work).  For other guests, there are other ways - 
you can also install RH-like (and maybe other) RPM distros using "rpmstrap".  
And you can install Debian-likes using "debootstrap".  Both of these tools 
can be used to drop a distro install into a filesystem tree (e.g. a mounted 
virtual disk partition).

Various distros (Redhat, SuSE, etc) also provide their own tools for 
installing a guest.  The libvirt folks were looking at supporting SuSE guests 
from virt-manager, which would be cool.

> My third and final question is: when using virt-install a file-backed
> VBD is created for the DomU.  The domain's configuration file has:
> disk = [ 'tap:aio:/vmdisks/Fedora6Guest3,xvda,w', ]
> I'd like to mount this in Dom0.  I execute
> xm block-attach 0 tap:aio:/vmdisks/Fedora6Guest3 /dev/xvda1 w 0
> which seems to work.  But how do mount the device /dev/xvda1 in Dom0?
> I think the problem is that it isn't really an ext3 filesystem,
> because it contiains multiple partitions (one for /boot and one for
> /), so I can't just say
> mount -t ext3 /dev/xvda1 /tmp/mountpoint

Easiest way is to use the lomount command; it's included in the Xen 
distribution and originally came from the Qemu distribution.  It allows you 
to select a partition from a virtual disk image and specify a directory to 
mount its filesystem on.  This uses the Linux loopback device.  Make sure you 
unmount before booting the guest, or bad things may happen ;-)

Hope that helps!

Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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