Migrating VMs from 8.4 to 9.0

[ljhardy]

I have an 8.4 cluster and I've added new nodes running 9.0. I'm trying to migrate a VM from an 8.4 node to a 9.0 node and am getting this error message when the VM tries to start on the 9.0 node:

2025-09-16 19:30:21 starting VM 111 on remote node 'proxmox11'
2025-09-16 19:30:23 [proxmox11] Unknown option: nets-host-mtu
2025-09-16 19:30:23 [proxmox11] 400 unable to parse option
2025-09-16 19:30:23 [proxmox11] qm start <vmid> [OPTIONS]
2025-09-16 19:30:23 ERROR: online migrate failure - target node proxmox11 is too old for preserving VirtIO-net MTU, please upgrade

Am I totally off here on how to migrate these VMs from 8.x to 9.x in this manner?

**SOLVED, specified MTU = 1500 on the existing VM on the Proxmox 8.4 node.

 

[fiona]

Hi,
it should also work if you upgrade the 9.x node to the latest version.

 

[masakrator]

This has not worked for me.
I had:
- MTU set explicitly on both bridges
- MTU set on VM
no matter what , it would not live migrate. I have migrated it cold then.

 

[fiona]

Hi @masakrator ,
what package versions did your source and target nodes have, i.e. pveversion -v? What was the exact error you got? Even better to share the full migration task log as well as the VM configuration qm config ID.

 

[masakrator]

Hello

it will be hard to say after I have cold-migrated vms, data, ISOs and backups to my newer node and decommissioned the old one (it is now removed from the cluster).
I will be provisioning another new one once my hardware arrives and it will be a greenfield build of pve 9 and I will test then and let you know.