----- 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".
> 

This is not related to engine. You are right that the actually MTU will the 
last configured one,
but this is exactly a problem.
As I already mentioned, if you will add another bridge without custom MTU its 
users (VMs) 
can assume that the MTU is 1500

> ----- 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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