Re: [Xen-users] MTU limits?

["Christoph Dwertmann" <lists.cd@xxxxxxxxx>]

> On Friday 02 September 2005 17:58, Patrick O'Rourke wrote:
> > The network device on our hardware uses a rather large MTU (~16k), but I
> > noticed that when setting up bridging, the MTU for the bridge and veth
> > device is set to 1500.  Trying to set the MTU on the bridge, or eth
> > device to match that of the peth is unsuccessful:
> >
> > [root@por-hv-proto3-p4 ~]# ip link set eth0 mtu 16896
> > SIOCSIFMTU: Invalid argument
> > [root@por-hv-proto3-p4 ~]#  ip link set xen-br0 mtu 16896
> > SIOCSIFMTU: Invalid argument
> >
> > We're using xen-unstable, but a very similar issue was discussed
> > regarding 2.0.6 on the list a while back, see:
> >
> > http://lists.xensource.com/archives/html/xen-users/2005-07/msg00187.html
> >
> > IIRC, the poster was suggesting that the 1500 MTU limit is in the bridge
> > or the vif code and I was wondering if this is the case.
> >
> Afaik the vif code of 2.0.x uses a 4k max ringbuffer for transfering packets,
> and does not handle overflows of that buffer, meaning that a MTU of >4k will
> truncate or drop all larger packets anyways.
>
> But the bridge code seems to limit the mtu to 1500, even with no real
> interfaces attached:
>
> # brctl addbr test
> # ip link set test mtu 1600
> SIOCSIFMTU: Invalid argument
>
> So, with bridging you're stuck at 1500, with routing it might be possible to
> raise the MTU, but that will most likely reduce overall network performance,
> as the number of truncated packets with invalid checksum will raise...
>
> See one of the recent threads about network hangs, ping packet loss, ssh
> session hanging etc for details...
>
>
> /Ernst
Has someone found a way to do this yet? My Xen network interfaces are
not bridged, and I can neither set the MTU in domU nor in dom0 to
> 1500.
I've tried to patch the kernel, but I can't find the piece of code
that prevents me from setting MTU values >1500. Can anyone please
point me in the right direction?

--
Christoph Dwertmann
cdwertmann at gmx dot de

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