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Re: [Xen-devel] [PATCH] [Xend] Move some backend configuration

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] [Xend] Move some backend configuration
From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
Date: Tue, 30 Sep 2008 16:30:54 +0100
Cc: Pascal Bouchareine <pascal@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, John Levon <levon@xxxxxxxxxxxxxxxxx>
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On Tue, Sep 30, 2008 at 04:09:49PM +0100, Keir Fraser wrote:
> On 30/9/08 15:58, "John Levon" <levon@xxxxxxxxxxxxxxxxx> wrote:
> >> Which guaranteed API would that be? These paths are private to the 
> >> toolstack
> >> implementation. Perhaps the only exception is the
> > 
> > Precisely the problem, there's absolutely no idea or indication what is
> > and isn't private. Thus you get libvirt looking in places it maybe
> > shouldn't, but how are they supposed to know?
> > 
> > I'm pretty sure this patch breaks libvirt again.
> I don't really see why it would. Beyond the console-client-to-daemon
> interface change (which we can make not depend on the /vm_path stuff) the
> rest is changing things which I believe are internal to xend.
> If libvirt is really going in at this private state, perhaps we could patch
> xend so that it writes to the new locations *in addition to* the old
> locations. It then has its own guaranteed-private and -consistent state,
> while libvirt can still read from old locations.

I've done a quick check of what libvirt accesses in xenstore:

General VM metadata


Console data


For fetching disk/net statistics


The first group is not being changed, so that's not a problem. Likewise
the latter is not impacted. The only problem I see is with the console
TTY paths and VNC port.

While I would prefer it not to change, or have backwards compatability
data written in existing fields, we have the ability to check multiple
different data sources. So if the TTY info and VNC port is available
via the 'xm list --long' output we could make libvirt check there if it
fails to find it in xenstore. We only historically used xenstored for
the few occassions when the 'xm list --long' output was incomplete.

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