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Re: [Xen-users] Assumption is the mother...

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Subject: Re: [Xen-users] Assumption is the mother...
From: Sascha Retzki <sretzki@xxxxxx>
Date: Wed, 29 Jun 2005 15:10:29 +0200
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On Wed, Jun 29, 2005 at 02:57:34PM +0200, John Smith wrote:
> Hi all,
>       you know how to finish that line. Recently started trying xen
> (awsome!) and since the documentation is in some aspects still a bit 
> vague, I would like to get a few assumptions confirmed/denied and
> some questions answered.
>       Domain0 is the top level kernel and manages a series of domainu
> kernels which can be of several different flavours at the moment, name-
> ly at least Linux, Free-, Open and NetBSD. Windows is in the works and

Don't know about OpenBSD, I assume they don't want it. NetBSD works
even as dom0, but xentools are a bit linux-centric so its a bit harder
to achieve goals, imho. FreeBSD xen support is on the way.

> expected to be supported with release 3.0. All domainu kernels run as
> child processes of the domain0 proces.
>       All kernels still have to be separately compiled with xen spe-
> cific options (so no really native kernels now).

Right. Xen3 is said to change that, tho.

>       All kernels and their direct dependencies (/lib/modules for 
> Linux, -how about the *BSD's?-) are stored on domain0's filesystem, the 
> domains are described in domain0:/etc/xen/auto and started by 
> domain0:/etc/init.d/xendomains.

That is not a must. To be particular, I think it is even to depreciate.
You can make a file, a LVM, a NFS-exported fs or a real partition 
visible to unprev. domains as harddisks (imho).

>       A kernel on disc can be shared by an unlimited number of domains.

I don't see a "why not"-reason, but never did that.
>       It is recommended that each of the domains (or virtual machines)
> including domain0, have their own filesystem(s), although it may be wise
> to share read-only filesystem like /usr. 

Yes - two considerations from on top of my head:

- security: If the security of one domain is broken, the security of all
other domains is, too.

- well a bit stupid, but NetBSD cannot share the userspace with FreeBSD
and so on ;-)

>       Can vm's share local filesystems and if so how do they look at 
> them, NFS, local ..., and how are conflicts -filelocking etc.- handled?

A, good one, add that to the above list ;-)

>       All network communication with domain0 on a single nic machine
> (the default) is handled through a virtual bridge interface on the 
> single nic which allows access to the localhost ( address of
> domain0.

I think so, yes. Did not get that far yet ;)

Networking in Xen seems a bit strange (to me?), so I'd like to comment
and explain on that a bit further, hoping that more advanced users
may correct me here:

Domain0 sees all the real hardware and assigns it drivers to the NICs.
Basing on the configuration (nics = directive), DomainUs may see 0-N
NICs, but in a virtual way. The network traffic  can then be NATed by
Domain0 so that DomainUs can communicate with other 'puters on the 

So you have a PF in between them, and can control which NICs a domU 
actually "sees".

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