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[Kent Wu <wu@xxxxxxxxxxxxxxxxx>]

Public bug reported:

Here is the specific code path I was talking about:

https://github.com/openstack/nova/blob/stable/mitaka/nova/virt/libvirt/vif.py#L531

As you can see in the code,

1. self.plug_ovs_hybrid(instance, vif) -> self._plug_bridge_with_port()
which will take care of creation of Linux bridge and setting of MTU
properly.

2. however self.plug_ovs_bridge(instance, vif) itself does nothing so
MTU is not being set/honored at all. The end-result of this is that the
MTU of the VM's tap interface remains 1500 instead of honoring the MTU
of the corresponding neutron network.....Note that in this code path we
don't need to create the Linux bridge and veth pair at all however we
still need to set the MTU properly for the VM tap interface.

We believe this is an overlook from this MTU feature patch:
https://review.openstack.org/#/c/285710/

Let us know if you need more info then.

** Affects: nova
     Importance: Undecided
         Status: New


** Tags: mtu

** Description changed:

  Here is the specific code path I was talking about:
  
  https://github.com/openstack/nova/blob/stable/mitaka/nova/virt/libvirt/vif.py#L531
  
  As you can see in the code,
  
  1. self.plug_ovs_hybrid(instance, vif) -> self._plug_bridge_with_port()
  which will take care of creation of Linux bridge and setting of MTU
  properly.
  
  2. however self.plug_ovs_bridge(instance, vif) itself does nothing so
  MTU is not being set/honored at all. The end-result of this is that the
  MTU of the VM's tap interface remains 1500 instead of honoring the MTU
- of the corresponding neutron network.....
+ of the corresponding neutron network.....Note that in this code path we
+ don't need to create the Linux bridge and veth pair at all however we
+ still need to set the MTU properly for the VM tap interface.
  
  We believe this is an overlook from this MTU feature patch:
  https://review.openstack.org/#/c/285710/
  
  Let us know if you need more info then.

-- 
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https://bugs.launchpad.net/bugs/1665798

Title:
  nova-compute is not setting MTU at all when following the
  plug_ovs_bridge() call path

Status in OpenStack Compute (nova):
  New

Bug description:
  Here is the specific code path I was talking about:

  https://github.com/openstack/nova/blob/stable/mitaka/nova/virt/libvirt/vif.py#L531

  As you can see in the code,

  1. self.plug_ovs_hybrid(instance, vif) ->
  self._plug_bridge_with_port() which will take care of creation of
  Linux bridge and setting of MTU properly.

  2. however self.plug_ovs_bridge(instance, vif) itself does nothing so
  MTU is not being set/honored at all. The end-result of this is that
  the MTU of the VM's tap interface remains 1500 instead of honoring the
  MTU of the corresponding neutron network.....Note that in this code
  path we don't need to create the Linux bridge and veth pair at all
  however we still need to set the MTU properly for the VM tap
  interface.

  We believe this is an overlook from this MTU feature patch:
  https://review.openstack.org/#/c/285710/

  Let us know if you need more info then.

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