I don't want to keep the last configured MTU. It's problematic. Having a stack 
is even worse.
VDSM should try not to persist anything if possible.

Also, reverting to the last MTU is raceful and has weird corner cases. Best to 
just assume default it 1500 (Like all major OSs do).
But since it's not really a default I would call it a recommended setting.

----- Original Message -----
> From: "Igor Lvovsky" <ilvov...@redhat.com>
> To: "Saggi Mizrahi" <smizr...@redhat.com>
> Cc: "VDSM Project Development" <vdsm-devel@lists.fedorahosted.org>, "Simon 
> Grinberg" <si...@redhat.com>
> Sent: Wednesday, November 28, 2012 11:10:27 AM
> Subject: Re: [vdsm] MTU setting according to ifcfg files.
> 
> 
> 
> ----- 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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