I made some test with queens ans rocky:

on queens the vm migrated makes an arp request and when you ping it , receives arp replay from the physical router. Ol rocky it makes an arp request but when you ping it, it does not receive any arp replay. It starts to responds only when it send traffic for example polling ntp server

Il giorno mar 28 apr 2020 alle ore 07:52 Ignazio Cassano <ignaziocassano@gmail.com> ha scritto:
Hello, I googled for my issue and I found the following:



Il giorno lun 27 apr 2020 alle ore 19:50 Sean Mooney <smooney@redhat.com> ha scritto:
On Mon, 2020-04-27 at 18:19 +0200, Ignazio Cassano wrote:
> Hello, I have this problem with rocky or newer with iptables_hybrid
> firewall.
> So, can I solve using post copy live migration ???
so this behavior has always been how nova worked but rocky the
spec intoduced teh ablity to shorten the outage by pre biding the port and activating it when
the vm is resumed on the destiation host before we get to pos live migrate.

this reduces the outage time although i cant be fully elimiated as some level of packet loss is
always expected when you live migrate.

so yes enabliy post copy live migration should help but be aware that if a network partion happens
during a post copy live migration the vm will crash and need to be restarted.
it is generally safe to use and will imporve the migration performace but unlike pre copy migration if
the guess resumes on the dest and the mempry page has not been copied yet then it must wait for it to be copied
and retrive it form the souce host. if the connection too the souce host is intrupted then the vm cant
do that and the migration will fail and the instance will crash. if you are using precopy migration
if there is a network partaion during the migration the migration will fail but the instance will continue
to run on the source host.

so while i would still recommend using it, i it just good to be aware of that behavior change.

> Thanks
> Ignazio
> Il Lun 27 Apr 2020, 17:57 Sean Mooney <smooney@redhat.com> ha scritto:
> > On Mon, 2020-04-27 at 17:06 +0200, Ignazio Cassano wrote:
> > > Hello, I have a problem on stein neutron. When a vm migrate from one node
> > > to another I cannot ping it for several minutes. If in the vm I put a
> > > script that ping the gateway continously, the live migration works fine
> >
> > and
> > > I can ping it. Why this happens ? I read something about gratuitous arp.
> >
> > qemu does not use gratuitous arp but instead uses an older protocal called
> > RARP
> > to do mac address learning.
> >
> > what release of openstack are you using. and are you using iptables
> > firewall of openvswitch firewall.
> >
> > if you are using openvswtich there is is nothing we can do until we
> > finally delegate vif pluging to os-vif.
> > currently libvirt handels interface plugging for kernel ovs when using the
> > openvswitch firewall driver
> > https://review.opendev.org/#/c/602432/ would adress that but it and the
> > neutron patch are
> > https://review.opendev.org/#/c/640258 rather out dated. while libvirt is
> > pluging the vif there will always be
> > a race condition where the RARP packets sent by qemu and then mac learning
> > packets will be lost.
> >
> > if you are using the iptables firewall and you have opnestack rock or
> > later then if you enable post copy live migration
> > it should reduce the downtime. in this conficution we do not have the race
> > betwen neutron and libvirt so the rarp
> > packets should not be lost.
> >
> >
> > > Please, help me ?
> > > Any workaround , please ?
> > >
> > > Best Regards
> > > Ignazio
> >
> >