[libvirt] Question about save and restore OVS per-port data in live migration

[wangxin (U)]

> -----Original Message-----
> From: Michal Privoznik [mailto:mprivozn at redhat.com]
> Sent: Friday, May 31, 2019 3:25 PM
> To: wangxin (U) <wangxinxin.wang at huawei.com>; libvir-list at redhat.com
> Cc: Lichunhe (Cloud Networking) <lichunhe at huawei.com>
> Subject: Re: Question about save and restore OVS per-port data in live
> migration
> 
> On 5/30/19 3:50 AM, wangxin (U) wrote:
> >> -----Original Message-----
> >> From: Michal Privoznik [mailto:mprivozn at redhat.com]
> >> Sent: Wednesday, May 29, 2019 3:44 PM
> >> To: wangxin (U) <wangxinxin.wang at huawei.com>; libvir-list at redhat.com
> >> Cc: Lichunhe (Cloud Networking) <lichunhe at huawei.com>
> >> Subject: Re: Question about save and restore OVS per-port data in live
> >> migration
> >>
> >> On 5/29/19 6:23 AM, wangxin (U) wrote:
> >>> Hi
> >>>
> >>> When we try to migrate a guest with 12 NICs, we find libvirt will try
> >>> to get per-port data in finish phase, it makes the guest paused for
> >>> several hundred milliseconds.
> >>>
> >>>     qemuMigrationCookieAddNetwork
> >>>       -> virNetDevOpenvswitchGetMigrateData for per-nic
> >>>         -> run 'ovs-vsctl --if-exists get Interface vnetx external_ids:Port'
> >> (about 25ms)
> >>>
> >>> In our case, there's no portData available and it seems there is no need to
> >> run the cmd.
> >>>
> >>> Questions:
> >>> 1. Is it necessary to get/set per-port data in migration finish phase? It will
> >> increase the downtime.
> >>> 2. Is there any way (i.e. a switch) to not run the OVS cmd if there's no
> >> portData available?
> >>>
> >>
> >> IIUC migrating port data is required for openvswitch functionality on
> >> the destination. Although looking at the commit that introduced this
> >> does not say that explicitly (v1.0.0-rc1~8). But I vaguely recall that
> >> being the case. We should revisit that decission and ask ovd guys if
> >> that's still the case.
> >
> > It will be helpful to do this.
> >
> > And, is there a special reason to migrating port data in finish phase?
> > The finish phase should be as simple as possible.
> 
> Well, finish phase is the phase where we want to do this. Because until
> finish phase the domain is running on the source and if we'd transfer
> OVS private state before then there would be two different states - one
> on the source the other at the destination and we can't tell which one
> is valid. BTW this is exactly what's done with spice migration too.
> Well, qemu does it for us. But libvirt still has to wait for qemu to
> transfer spice internal state.

I see, it's always safe to migrate device data after VM stopped.

> 
> Anyway, let me ask OVS guys.
> 
> Done here:
> 
> https://mail.openvswitch.org/pipermail/ovs-discuss/2019-May/048739.html
> 
> Michal

Thanks,
Wangxin