[libvirt] virsh edit not saving interface target specification

[Dan Kenigsberg]

On Sun, Oct 25, 2009 at 01:13:01PM -0500, Neil Aggarwal wrote:
> Nikolai:
> 
> > As i said in the kvm list, did you do virsh define 
> > /path/to/xml before launching the vm again ?
> 
> According to what I am seeing on the Internet,
> virsh edit is equivalent to dumping the xml,
> editing it, and calling define to update
> it so it should be the same thing.
> 
> However, just to test it, I did the following:
> 1. Stopped the virtual machine
> 2. Added the target directive to the
>    /etc/libvirt/qemu/jamm12a.xml file
> 3. Ran virsh define /etc/libvirt/qemu/jamm12a.xml
> 
> If I look at the jamm12a.xml file after the define
> command, the target directive is no longer there.
> For some reason, libvirt is removing it.
> 
> If I start the virtual machine, it attaches
> to vnet0 instead of vnet1.

heh, surprisingly I stumbled on this today. target devices with the
'vnet' prefix are special: if you do not specify a target device, a
device with the vnet prefix is created automatically. conversely, if
libvirt sees a target device with that name, it assumes it's a remnant
of an automatically-created device name.

if you care about the name of the tap device, you must choose a more
"original" name. but beware: anything but automatically-created names
would cause collisions during migration in a multi-domain multi-host
environment.

Dan