Re: [pve-devel] [PATCH docs/manager/qemu-server v2 0/3] Make VirtIO network devices always inherit MTU from bridge

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

Am 22.04.25 um 13:33 schrieb Stefan Hanreich:
> If the MTU of the bridge was larger than 1500, but the NIC was set to
> 1500, then the VM was just sending packets that are too small, but the
> setup would work, assuming the bridge MTU is the correct one for the
> network.
> 
> A possible regression I can think of is: If the bridge was set to the
> wrong MTU (e.g. 9000) at some point, but external devices in the same
> LAN are still set to use a lower MTU (e.g. 1500). If users never
> configured the larger MTU anywhere else besides the bridge, then this
> would break.

Above is basically what I meant.

>> FWIW, we could also tie this behavior to a machine version to avoid changing
>> the behavior for any existing VM. But I would be fine with applying this only
>> for PVE 9 then and add a notice to the pve8to9 checker script that lists all
>> VMs that will change their MTU including the respective value.
> 
> I think it would be a good idea to include this in pve8to9 with warnings
> at least and mention it in the release notes. It might make for some
> noise and unsettle some users though. Since we cannot really tell what
> MTU is set inside the VM, we'd have to show warnings for basically every
>  network device on a bridge with MTU != 1500.

Well, yes, but I used "notice" over "warning" for a reason, as we have
that level in the checker script, and it fits this change well IMO.

> Would also be open to tie this to a new machine version if we want to be
> really careful and avoid the unnecessary warnings.

After rethinking this it's IMO not worth it.


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