To add to Bretts comment in #21, the proposed workaround is only
effective if you know about this situation beforehand. I.e. if you
enabled the override through vendor data before deploying any Focal or
newer instances.
Another possible workaround is to preform a "cold migration". When Nova
stops/starts an instance the domain XML on the hypervisor is re-created.
We can take advantage of this behavior, if an instance is stop/started
after a network MTU has been lowered the new domain XML will have the
new MTU which makes the libvirt driver enforce the MTU in the instance.
The instance configuration will still make systemd-networkd attempt to
set the hard coded MTU but it will not be allowed to:
ubuntu@u:~$ grep mtu /etc/netplan/50-cloud-init.yaml
mtu: 1442
ubuntu@u:~$ zgrep MTU /var/log/syslog.*
/var/log/syslog.2.gz:Dec 10 13:54:19 u systemd-networkd[287]:
/run/systemd/network/10-netplan-ens2.network: MTUBytes= in [Link] section and
UseMTU= in [DHCP] section are set. Disabling UseMTU=.
/var/log/syslog.2.gz:Dec 10 13:54:19 u systemd-networkd[287]: ens2: Could not
set MTU, ignoring: mtu greater than device maximum. Invalid argument
ubuntu@u:~$ ip link
2: ens2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1441 qdisc pfifo_fast state UP
mode DEFAULT group default qlen 1000
link/ether fa:16:3e:30:f3:82 brd ff:ff:ff:ff:ff:ff
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https://bugs.launchpad.net/bugs/1899487
Title:
cloud-init hard codes MTU configuration at initial deploy time
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