[ovirt-users] change network MTU settings without taking all the VMs down?

[Lior Vernia]

Hi Darrell!

There's currently no clean way to do this - we'll be looking to fix this
in 3.6 (https://bugzilla.redhat.com/show_bug.cgi?id=1055454).

You haven't mentioned which version of oVirt you're running - if it's
3.4 or lower, I think it would suffice to change ifcfg files on your
hypervisors and restart the network service. The network will then
appear as out-of-sync in the GUI, but should be fully functional with
MTU 1500. You'd also want the network configuration to be saved in case
of future rollbacks - Dan, how would that be done?

If you're running 3.5, I think you need to run some vdsm shell commands
on the hypervisor as we've added an abstraction layer for configuration
persistence above ifcfg files - again I'll ask Dan to chime in.

As for getting the engine network configuration to MTU 1500 (for future
hypervisor configuration and for networks to not appear as out-of-sync
on existing ones) without taking down all the VMs (or hot-unplugging
NICs) at one point - I don't think there's currently a way other than
hacking the DB... Just leaving the network out-of-sync on the hosts
could result in inconveniences later on when configuring host networking.

Is it worth the trouble of getting MTU 1500 instead of 1448? I presume
the difference in performance would be negligible. Or are you
experiencing incoming frames being dropped due to having 1500 bytes
instead of 1448?... Either way, as you mentioned taking down the VMs can
be a last resort, or wait for 3.6 where it should be simpler :)

Yours, Lior.

On 27/01/15 19:43, Darrell Budic wrote:
> I finally got a couple of networks our from behind a wan based layer 2 bridge that required me to run at MTU 1448, and would like to get back up to MTU 1500. I see the GUI won’t let me do that while the network is in use. Any way around this, clean or otherwise? Restarting VMs to update them is ok, just trying to avoid having to take everything down at the same time.
> 
>  -Darrell
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