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Re: [Xen-devel] A dumb question

On Fri, 2003-10-10 at 14:04, Nicolas Bougues wrote: 
> On Thu, Oct 09, 2003 at 04:38:17PM +0100, Rolf Neugebauer wrote:
> > the "hosting kernel" is the virtual machine monitor (or hypervisor) Xen.
> > You can build it from source by typing 'make' in the xen directory of
> > the source distro.
> > 
> > The readme should contain information on how to set up grub to boot xen
> > and the first virtual machine (domain 0) running xenolinux.
> > 
> Thanks Rolf, it works very fine.
> I'm now struggling with another stupid bit : I don't seem to be able
> to mount a vbd device in domain 0. I was able to create the vd, but
> the java source doesn't seem to allow to mount a vbd in domain 0. For
> a good reason, probably.

I attached an email by Ian Pratt to sven kretzschmar from yesterday
providing some details. I haven't actually tried this myself

> What I'm trying to do is basically :
> - create a virtual drive
> - populate it with a live linux fs and distro (from domain 0)
> - booting a xenolinux on this virtual drive
this should be possible

> I have another, more general question : why are the tools based on a
> java front end ? As far as I understand, the java stuff just calls the
> low level C stuff. The java build and runtime environnement looks even
> uglier, when in presence of the nice Xen "do it light and fast" stuff.

there is a lower level interface with tools called xi_*. you could use
them (I normally do). xenctl is a wrapper around these but also keeps
some state and default values around. The idea is that the xi_* tools
might change if the domain0 control interface changes but that the
xenctl interface will be more stable. It's really up to you what you


---- start of attached email

Ian Pratt
keir.fraser@xxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxxx, 
Re: [Xen-devel]
Re:Trying to get
HyperSCSI and Xen
to work... ;-)
Thu, 09 Oct 2003
22:50:39 +0100

> 1.) Is it possible to access the created vd (virtual disk) or the
> vbd (virtual block device)  _without_ booting and logging in to domain
1 ??
> I have found no way to access either the created vd or vbd in domain 0
> in order to create a filesystem on it and to fill it with something

You should be able to use "xenctl vdb create -k<key> -v0 -w -n0"
to make the virtual block device accessible from domain0 as
/dev/xvda (major 125, minor 0 -- use tools/misc/xen-mkdevnodes if
the device doesn't exist)

I haven't tried this in a while, but it should work unless it got
accidentally broken when the "xenctl physical grant" stuff got
> 2.) Is it possible to boot domains > 1 from a vbd (virtual block
>      which is defined as the root device in the kernel command line ?

Yes, e.g. root=/dev/xvdb3  (4th attached vbd, 3rd partition).

See xenolinux-2.4.22/init/do_mounts.c for the grim way that Linux
selects boot devices.

> 3.) Does the number of the created vbd coresspond to the name of
>      the block device during boot time of domains > 0,  such that, if
>      the vbd number is 1 then the device name is /dev/xvda1 (according
>      to the README.CD file) ?
>      At least in domain 0  I could not find such a device entry before
>      or after creating the vbd in domain 0 ?

The numbering is per domain, i.e. it is controlled by the -vX
parameter to "xenctl vbd create". You can connect a virtual disk
(vd) as the first vbd in one domain (/dev/xvda) and
simultaneously as e.g. the 3rd vbd in another domain
(/dev/xvdc). The optional number suffixed to a disk is a PC
partition number if present.  If you don't put a partition table
on a virtual disk, Linux is pretty good at guessing that you mean
the whole device.

Hope this helps.


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