[Ovirt-devel] Jumbo frame support on nodes

[Daniel P. Berrange]

On Mon, Sep 20, 2010 at 10:43:11PM +1000, Justin Clacherty wrote:
>  On 20/09/2010 10:30 PM, Arthur Clément wrote:
> >If you have time, you can take a look into libvirt doc or contact libvirt
> >developpers to see how jumbo frames work. It's half the work of this patch.
> 
> Is it a libvirt issue?  I'm talking about jumbo frame support on the 
> node which would be mounting iscsi volumes to export to the VMs running 
> on it.  Wouldn't that be part of the node configuration when it boots?  
> I'm not familiar with the node boot sequence so maybe that is delivered 
> via libvirt.  Of course once that's done the next step would be to make 
> jumbo frames available to VMs running on the node which would definitely 
> be a libvirt issue.

When setting up jumbo frames using bridging you need to several things

 - Set MTU on the physical NIC
 - Set MTU on the bridge
 - Set MTU on the TAP devices

you must do the first two before the bridge is brought online. when libvirt
creates a TAP device for a guest, it will automatically copy the MTU of
the bridge to the TAP device. So if ovirt is configuring the physical NIC
and bridge with a suitably large MTU, the libvirt bit will 'just work'.

Daniel
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