Support for mtu > 1500?

[larsks]

Is it possible to support network MTUs greater than 1500 bytes? I tried:

openstack network set --mtu 9000 mynetwork

And was told:

BadRequestException: 400: Client Error for url: https://esi.massopen.cloud:13696/v2.0/networks/d14b3d9d-c1b7-49ac-b7de-5111081db7c2, Invalid input for operation: Requested MTU is too big, maximum is 1500.

Why I care:

A suboptimal MTU size has been detected. Due to performance, we recommend setting an MTU of 9000 bytes or greater for the infra cluster that hosts the KubeVirt VirtualMachines. When smaller MTUs are used, the cluster will still operate, but network performance will be degraded due to fragmentation of the double encapsulation in OVN-Kubernetes.

[larsks]

Also, this means we're running our storage over a 1500 byte MTU network, which is going to impact storage performance.

[tzumainn]

Reading the documentation (and checking our Neutron configuration), it looks like the max mtu and default mtu setting is the same. I can update it to 9000, but all future Neutron networks will have their default mtu set to 9000, and the recommendation is to update all current Neutron networks to 9000.

Would there be possible consequences to this change? @hakasapl do you know if anything would have to change on the switch?

[hakasapl]

@tzumainn ESI would need to set the MTU on the switchport - does it currently do that? Other than that everything else upstream if 9216 MTU so it should be fine

[tzumainn]

The ansible playbooks currently have this line: https://github.com/CCI-MOC/network-runner/blob/devel/etc/ansible/roles/network-runner/providers/dellemc.os9.os9/conf_access_port.yaml#L8

Does this mean all our MTUs are already set to 9216 on the switch anyway?

[hakasapl]

@tzumainn yes we should be good in that case