Re: [pve-devel] [PATCH qemu-server v2 stable-bookworm 4/4] migration: preserve host_mtu for virtio-net devices

[Fiona Ebner <f.ebner@proxmox.com>]

Am 04.09.25 um 11:11 AM schrieb Fabian Grünbichler:
> On September 3, 2025 4:22 pm, Fiona Ebner wrote:
>> The virtual hardware is generated differently (at least for i440fx
>> machines) when host_mtu is set or not set on the netdev command line
>> [0]. When the MTU is the same value as the default 1500, Proxmox VE
>> did not add a host_mtu parameter. This is problematic for migration
>> where host_mtu is present on one end of the migration, but not on the
>> other [1]. Moreover, the effective setting in the guest (state) will
>> still be the host_mtu from the source side, even if a different value
>> is used for host_mtu on the target instance's commandline. This will
>> not lead to an error loading the migration stream in QEMU, but having
>> a larger host_mtu than the bridge MTU is still problematic for certain
>> network traffic like
>>> iperf3 -c 10.10.10.11 -u -l 2k
>> when host_mtu=9000 and bridge MTU=1500.
> 
> since the effective actual MTU value is determined by the migration
> state, this means 8->8 migrations potentially also break traffic:
> 
> VM config: mtu=1 (use bridge MTU)
> source node: bridge MTU 9000
> host_mtu for starting source VM: 9000
> 
> target node: bridge MTU 1500
> host_mtu for starting target VM: 1500
> effective MTU of the vNIC: 9000, because that's what the source VM's
> migration stream tells the target OS?
> 
> right? so it might make sense to still detect and handle this also on
> 8.x? can be done as a follow-up of course..

Yes, we can also backport "api: vm start: introduce nets-host-mtu
parameter for migration compat" too :)


_______________________________________________
pve-devel mailing list
pve-devel@lists.proxmox.com
https://lists.proxmox.com/cgi-bin/mailman/listinfo/pve-devel