----- Original Message -----
> From: "Saggi Mizrahi" <smizr...@redhat.com>
> To: "Simon Grinberg" <si...@redhat.com>
> Cc: "VDSM Project Development" <vdsm-devel@lists.fedorahosted.org>, "Igor 
> Lvovsky" <ilvov...@redhat.com>
> Sent: Wednesday, November 28, 2012 5:30:17 PM
> Subject: Re: [vdsm] MTU setting according to ifcfg files.
> 
> I suggest we don't have a default. If you don't specify an MTU it
> will use whatever is already configured.
> There is no way to "go back to the defaults" only to set a new value.
> The engine can assume 1500 (in case of ethernet devices) is the
> "recommended value".

I understand, 
This is why I've suggested to keep the old value and revert to that. 

Igor, alternately you may always calculate based on the hierarchy leafs, 
meaning  the 'trunk' interface always needs to be set to the maximal MTU 
required by any of the logical networks, and it needs to be recalculated every 
time you change something in the hierarchy 

The problem is what happens if all are removed and then another is configured 
with MTU set to not override, here you may need to use the saved one.  



> 
> ----- Original Message -----
> > From: "Simon Grinberg" <si...@redhat.com>
> > To: "Igor Lvovsky" <ilvov...@redhat.com>
> > Cc: "VDSM Project Development" <vdsm-devel@lists.fedorahosted.org>
> > Sent: Wednesday, November 28, 2012 9:53:48 AM
> > Subject: Re: [vdsm] MTU setting according to ifcfg files.
> > 
> > 
> > 
> > ----- 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
> > > _______________________________________________
> > > vdsm-devel mailing list
> > > vdsm-devel@lists.fedorahosted.org
> > > https://lists.fedorahosted.org/mailman/listinfo/vdsm-devel
> > > 
> > _______________________________________________
> > vdsm-devel mailing list
> > vdsm-devel@lists.fedorahosted.org
> > https://lists.fedorahosted.org/mailman/listinfo/vdsm-devel
> > 
> 
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