----- Original Message -----
> From: "Igor Lvovsky" <ilvov...@redhat.com>
> To: "VDSM Project Development" <vdsm-devel@lists.fedorahosted.org>
> Cc: "Simon Grinberg" <si...@redhat.com>
> Sent: Wednesday, November 28, 2012 2:58:52 PM
> Subject: [vdsm] MTU setting according to ifcfg files.
> 
> Hi,
> 
> I am working on one of the vdsm bugs that we have and I found that
> initscripts (initscripts-9.03.34-1.el6.x86_64)
> behaviour doesn't fits our needs.
> So, I would like to raise this issue in the list.
> 
> The issue is MTU setting according to ifcfg files.
> I'll try to describe the flow below.
> 
> 1. I started with ifcfg file for the interface without MTU keyword at
> all
> and the proper interface (let say eth0) had the *default* MTU=1500
> (according to /sys/class/net/eth0/mtu).
> 2. I created a bridge with MTU=9000 on top of this interface.
> Everything went OK.
>    After I wrote MTU=9000 on ifcfg-eth0 and ifdown/ifup it, eth0 got
>    the proper MTU.
> 3. Now, I removed the bridge and deleted MTU keyword from the
> ifcfg-eth0.
>    But after ifup/ifdown the actual MTU of the eth0 stayed 9000.
>   
> The only way to change it back to 1500 (or something else) is
> explicitly set MTU in ifcfg file.
> According to Bill Nottingham it is intentional behaviour.
> If so, we have a problem in vdsm, because we never set MTU value
> until user ask it explicitly.

Actually you are, 

You where asked for MTU 9000 on the network, 
As implementation specif you had to do this all the way down the chain 
Now it's only reasonable that when you cancel the 9000 request then you'll do 
what is necessary to rollback the changes. 
It's pity that ifcfg-files don't have the option to set MTU='default', but as 
you can read this default before you change, then please keep it somewhere and 
revert to that.  


> It means that if we have interface with MTU=9000 on it just because
> once there was a bridge with such MTU
> attached to it and now we want to attach regular bridge with
> *default* MTU=1500 we have a problem.
> The only thing we can do to avoid this it's set explicitly MTU=1500
> in interface's ifcfg file.
> IMHO it's a bit ugly, but it looks like we have no choice.
> 
> As usual comments more than welcome...
> 
> Regards,
>    Igor Lvovsky
> _______________________________________________
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> vdsm-devel@lists.fedorahosted.org
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> 
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