[Engine-devel] Network Wiring

[Itamar Heim]

On 11/15/2012 02:29 PM, Dan Kenigsberg wrote:
> On Wed, Nov 14, 2012 at 02:12:07PM -0500, Simon Grinberg wrote:
>>>
>>> The intention is to use the new API VDSM.libvirtVm.updateVmInteface
>>> for
>>> performing the network rewire in a single command.
>>
>> What does it do? I could not find updateVmInteface in vdsm git.
>> Where is this defined?
>>
>> It's critical.
>>
>> - You can change the interface directly by moving the VM from one network to another
>> - You can do that but toggle the ling state so the VM will be aware.
>>
>> Which if these two?
>> If you do only the first then it's not the common use case. In most cases you must toggle the link status to the VM.
>> This will cause:
>> 1. Speed negotiation + arp request that also informs the switched about the change
>> 2. In case it's DHCP (which most likely the case for guests) it will trigger new DHCP request.
>>
>> If you don't baaad things will happen :)
>
> I think that baaaaad things are going to happen anyway. In "baaaaad
> things", I mean "stuff that require guest intervension".
>
> The guest may be moved from one subnet into another one, maybe on
> different VLAN or physical LAN. We can not expect that the applications
> running on it will be happy about these changes. A similar case appears
> if we rewire the network while the VM is down (or hibernated). When the
> VM is restarted, it is going to use mismatching IP addresses.
>
> You are right that it may make sense to request an new IP address after
> the rewiring, however, a little test I just did on my desktop showed that
> dhclient does not initiate a new request just because the carrier
> dropped for few seconds. So we should try harder if we want to refresh
> dhcp after rewiring: I think that it would be cool to have a guest agent verb
> that does it.

shouldn't this simulate a link disconnect/connect event to the OS?