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".
----- Original Message -----
> From: "Simon Grinberg" <si...@redhat.com>
> To: "Igor Lvovsky" <ilvov...@redhat.com>
> Cc: "VDSM Project Development" <firstname.lastname@example.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" <email@example.com>
> > 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
> 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
> > firstname.lastname@example.org
> > https://lists.fedorahosted.org/mailman/listinfo/vdsm-devel
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