tun/tap and Vlans (was: Re: Network I/O performance)

[Lukas Kolbe <l-lists@einfachkaffee.de>]

Hi all,

On a sidenote:

> > I have also realized that when using the tun/tap configuration with
> > a bridge, packets are replicated on all tap devices when QEMU writes
> > packets to the tun interface. I guess this is a limitation of
> > tun/tap as it does not know to which tap device the packet has to go
> > to. The tap device then eventually drops packets when the
> > destination MAC is not its own, but it still receives the packet 
> > which causes more overhead in the system overall.
> 
> Right, I guess you'd see this with a real switch as well?  Maybe have 
> your guest send a packet out once in a while so the bridge can learn its 
> MAC address (we do this after migration, for example).

Does this mean that it is not possible for having each tun device in a
seperate bridge that serves a seperate Vlan? We have experienced a
strange problem that we couldn't yet explain. Given this setup:

Guest            Host          
kvm1 --- eth0 -+- bridge0 --- vlan1 \
               |                     +-- eth0
kvm2 -+- eth0 -/                     /
      \- eth1 --- bridge1 --- vlan2 +

When sending packets through kvm2/eth0, they appear on both bridges and
also vlans, also when sending packets through kvm2/eth1. When the guest
has only one interface, the packets only appear on one bridge and one
vlan as it's supposed to be.

Can this be worked around?

-- 
Lukas