[Users] oVirt 3.4 - MTU Override not honoured by virtual NICs

[Moti Asayag]

----- Original Message -----
> From: "Jonas Israelsson" <jonas at israelsson.com>
> To: users at ovirt.org
> Sent: Saturday, March 1, 2014 3:28:15 PM
> Subject: [Users] oVirt 3.4 - MTU Override not honoured by virtual NICs
> 
> I have a SAN Network defined in oVirt where I have enabled Jumbo frames
> (by setting the MTU to 9000).
> Still I see now VM Nic;s attached to this network get MTU 1500 and are
> therefore basically not working.
> 
> I have created several VMs before all (still) have Jumbo Frames on the
> their san-interface, but newly created VMs get 1500.
> Currently on 3.4 Beta2 (upgraded from alpha --> beta1 --->) and this
> behaviour have started somewhere along the way..
> 
> The host physical interface
> 
> em2       Link encap:Ethernet  HWaddr D0:67:E5:F9:2E:1C
>            inet6 addr: fe80::d267:e5ff:fef9:2e1c/64 Scope:Link
>            UP BROADCAST RUNNING MULTICAST  MTU:9000  Metric:1
>            RX packets:503209263 errors:0 dropped:0 overruns:0 frame:0
>            TX packets:483040537 errors:0 dropped:0 overruns:0 carrier:0
>            collisions:0 txqueuelen:1000
>            RX bytes:2395173035170 (2.1 TiB)  TX bytes:408352341106
> (380.3 GiB)
> 
> 
> And on a VM
> 
> vnet25    Link encap:Ethernet  HWaddr FE:1A:4A:2F:D2:A3
>            inet6 addr: fe80::fc1a:4aff:fe2f:d2a8/64 Scope:Link
>            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>            RX packets:167 errors:0 dropped:0 overruns:0 frame:0
>            TX packets:137 errors:0 dropped:0 overruns:0 carrier:0
>            collisions:0 txqueuelen:500
>            RX bytes:17662 (17.2 KiB)  TX bytes:26684 (26.0 KiB)
> 
> If manully setting the MTU everything work as expected.. (Until you
> reboot the machine)
> 
> Anyone else seen this ?
> 

This sounds like a reasonable behaviour since there is a clear separation
between the host networking configuration to the vms networking configuration.
What you're attempting to achieve is guest-os level network configuration which
isn't controlled by the ovirt-engine.

The current method to control guest network internals is by using the cloud-init-integration
however MTU is not part of its api [1]. 

You can add it to the 3.5 planning thread as a requested item.

[1] http://www.ovirt.org/Features/Cloud-Init_Integration

> Rgds Jonas
> 
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