[Engine-devel] Network Wiring

[Yaniv Kaul]

On 11/15/2012 02:33 PM, Itamar Heim wrote:
> 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.

Blame your OS if it doesn't do media sensing at all (or correctly).

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

I sincerely hope it does.
Y.

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