[libvirt] networking with two hosts

[Radek Hladik]

Andreas Rittershofer napsal(a):
> 
> Am 25.02.2009 um 00:11 schrieb David Lutterkort:
> 
>> On Tue, 2009-02-24 at 14:25 +0100, Andreas Rittershofer wrote:
>>> Task 1.) Each VM must have network access to each other VM and to the
>>> outside.
>>> Task 2.) When one host fails, all VMs are running on the remainig host
>>> and 1.) must still be fulfilled.
>>>
>>> Does somebody has any hints about realising this scenario with libvirt
>>> and KVM?
>>
>> What you want is a 'shared network interface'[1]
>>
>> David
>>
>> [1]
>> http://wiki.libvirt.org/page/Networking#Bridged_networking_.28aka_.22shared_physical_device.22.29 
>>
>>
>>
> 
> Thank you for this answer - but it is too short.
> 
> I'm working since two days on this problem but I cannot solve it until now.
> 
> Some configurations are working when the VMs are on different nodes but 
> are failing when all machines are running on one node and vice versa.
> 
> Can you please go in more details?
> 
> mfg ar
> 

Basically the idea is like this. You have your hosts on one ethernet 
segment. You create bridge on each hosts and put hosts physical adapter 
into it (lets call the bridges BrA and BrB). The bridge behaves like a 
virtual ethernet switch. You should configure IP on the bridge not on 
the physical interface itself. And now if you add any other adapter into 
the bridge, it will act as if it is directly on the ethernet. This has 
been used for bridging two or more ethernet segments into one for a long 
time.
If you setup your VMs correctly (see the wiki page for details), 
libivirt will create hosts virtual adapter and add it to the bridge. For 
example for VM1 running on A it will create i.e. vnet1 and add it to the 
bridge BrA. The virtualized interface in the VM1 will have direct access 
to the ethernet segment. It can have IP address from the same range as 
the hosts, etc...
If the host A fails, then VM1 and VM2 will be down but the VM3 and VM4 
wont be affected (besides they can not communicate with VM1 or VM2 of 
course). However if you (live) migrate i.e. VM1 from A to B, its network 
interface will be removed from BrA, destroyed, brought up on B and 
inserted into BrB with the same MAC address. It will also send ARP 
packet for all switches to update new location of that MAC address.
This can be done with very shot downtime and network operations 
continues on new host without interruption.
Do not forget to setup firewall on hosts as it is described in wiki page.

Radek