[Ubuntu-US-CA] qemu-kvm/libvirt/virsh & MTU ...

[Elizabeth K. Joseph]

On Thu, Dec 10, 2015 at 1:16 AM, Michael Paoli
<Michael.Paoli at cal.berkeley.edu> wrote:
> qemu-kvm/libvirt/virsh & MTU ...
> Question came up at Ubuntu Hour San Francisco (not sure if the OS in
> question was Ubuntu, or Debian (or ???), but in any case ...)
>
> Roughly paraphrasing scenario from memory - a guest Virtual Machine (VM)
> under qemu-kvm/libvirt/virsh, and I'm presuming (and from what I seem to
> recall) a relatively default network configuration (VM NATed on default
> bridge behind the physical hosting host's interface(s) to upstream
> network(s) (e.g. Internet or other larger network
> environment/infrastructure).  The issue was described, at least very
> roughly, as highly poor throughput from(/to?) Internet - very slow
> performance and/or drops/stalls, etc.  It was also noted the MTU on
> interface of VM, and MTU of the physical host's upstream network
> connectivity were different - if I recall correctly, lower for the
> upstream on physical host (perhaps through some VPN or tunnel or PPPoE
> or whatever).  Anyway, there was slight bit of discussion regarding the
> MTUs, and I was guestimating (it turns out incorrectly), that one could
> probably tweak that for the default network/bridge device under
> qemu-kvm/libvirt/virsh, or if not there, perhaps on the, again default,
> built-in DHCP server on that subnet that qemu-kvm/libvirt/virsh sets up
> there by default.  Seems I was incorrect on my guesses.  Anyway,
> picking up from that point ...

Thanks for this thorough email.

As an initial test this morning we manually adjusted the mtu with the
ip command and confirmed that we were on the right track with mtu, her
networking is much better now :) Now to play around with the bridge
itself so we don't need to run this command on every VM...

> Oh, also, for apt-get, if client and server are dual stack (IPv4 &
> IPv6), but IPv6 is broken or semi-broken or randomly broken between
> client and server, one can force apt-get to use IPv4 only
> "
> Add -o Acquire::ForceIPv4=true when running apt-get.

This wasn't it, but I forgot to try it so I appreciate you including
it, just in case.

-- 
Elizabeth Krumbach Joseph || Lyz || pleia2