Open Stack

Ian Wells <ijw.ubuntu at cack.org.uk> wrote:

>
> Right. Note that custom MTU works out of the box only starting from Mitaka.
>
> It's been in from at least Kilo (give or take a some bugfixes, it seems,  
> all of which deserve backporting).

It never worked as you would expect, though indeed a lot of code to  
calculate MTU was in place.

>
> - It won’t work in OVS hybrid setup, where intermediate devices (qbr)  
> will still have mtu = 1500, that will result in Jumbo frames dropped. We  
> have backports to fix it in Liberty at:  
> https://review.openstack.org/305782 and  
> https://review.openstack.org/#/c/285710/
>
> Indeed, you can actively request the MTU per virtual network as you  
> create them, subject to segment_mtu and path_mtu indicating they're  
> achievable.

No. MTU is not available for setting during network creation. It’s only  
calculated as per get_mtu() [relying on path_mtu and physical_network_mtus  
and segment_mtu; note the latter is renamed since Mitaka into  
global_physnet_mtu].

>
> In this instance, configure your switches with a 9000 MTU and set  
> segment_mtu = path_mtu = 9000.  The virtual network MTU will then default  
> to 8950 for a VXLAN network (the biggest possible packet inside VXLAN in  
> that circumstance) and you can choose to set it to anything else below  
> that number as you net-create.  The MTU should be correctly advertised by  
> DHCP when set.
>

As I said, no, you can’t set it on network creation.

Also, having the network using 8950 is not enough for Jumbo frames, because  
till Mitaka (and the next minor Liberty release) Nova was not using that  
value to bump MTU for intermediate devices participating in the hybrid  
bridge setup.

> I hope you don't find you have to do what Akihiro suggests.  That was  
> good advice about three releases back but nowadays it actually breaks the  
> code that's there to deal with MTUs properly.

Yes, indeed it’s not needed since Kilo to modify dnsmasq configuration to  
set the option. advertise_mtu is now True since Mitaka, and for earlier  
releases, you just set it to True explicitly.

Ihar