[vdsm] MTU setting according to ifcfg files.

Igor Lvovsky ilvovsky at redhat.com
Wed Nov 28 12:58:52 UTC 2012


    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.
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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