Xen: Installing guest VMs

Jeremy Katz katzj at redhat.com
Mon Feb 28 16:16:20 UTC 2005

On Sat, 2005-02-26 at 16:16 -0500, Robin Green wrote:
>So, is the plan to modify the Xeno-kernel so that it makes virtual block 
>devices into fake IDE or SCSI devices, or is the plan to modify anaconda 
>to install on Xen without kludges, or some combination? (Let's ignore 
>driver domains for the minute!)

Personally, I don't want them to be fake IDE or SCSI devices, but fake
_disk_ devices, yes.  Faking IDE or SCSI is hard (and only done poorly
at present).  But there's also some other stuff needed so that device
probing can be sane (see some of my posts to xen-devel from a few weeks
ago and Rusty's mail from over the weekend)

>I am currently working on the latter approach. I chose that because

This isn't the approach I really want :-)  Mostly because it ends up
making things drastically different from a user experience point of
view.  Adding partitions makes little to no sense, you're adding a
_device_.  And then you use it however makes sense.  Which could be
using the whole device, but most of our toolset assumes the existence of
some partition table on the device.

>An intriguing possibility is having a pre-install stage of Anaconda for 
>guest installs, which runs on the host as a normal program, e.g. for 
>things like hooking up LVM volumes to the new VM. 

There's likely to be a need for a "guest setup" utility that does a lot
of the device set up and then starts the installer and kicks things off
from there.  My initial thinking is that it basically asks all the
information needed for device  setup and where you want to do an install
from and then kicks things off with all of the information needed to get
through the first stage passed in.

> You could even rewrite
>to run the ENTIRE install process on the host, without even booting the 
>guest kernel, although my suspicion is that that route would involve an 
>unnecessary amount of effort.

Having the entire install run in the host is less desirable from a
security point of view.  It also means that the host has to have support
for any filesystem of any guest you want to install, etc.  Not to
mention the difficulty it adds of doing things like an FC5 guest install
on an FC4 host.


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