Ping: [PATCH] Qemu: migration: Not bind RAM info with active migration status

[Jiri Denemark]

On Tue, Jan 05, 2021 at 21:54:05 +0800, Keqian Zhu wrote:
> On 2021/1/5 21:34, Daniel P. Berrangé wrote:
> > On Tue, Jan 05, 2021 at 09:28:27PM +0800, Keqian Zhu wrote:
> >>>> The purpose of QEMU commit 65ace0604551 (migration: add postcopy total blocktime into query-migrate)
> >>>> is to query some postcopy related information on destination side.

The total post-copy block time is only computed when postcopy-blocktime
migration capability is enabled. Libvirt does not support this
capability so I believe this patch is fixing something that can never
happen.

> >>>>
> >>>> We can call query-migrate on destination side *after* migration complete, thanks.
> >>>
> >>> But nothing in libvirt ever tries to call query-migrate on the dest
> >>> side. 
> >> Yes, but the dest side does not always act as dest. After migration completion, the dest side enters
> >> to a normal status and libvirt does not forbid us to query migration status.
> >>
> >> Before QEMU commit 65ace0604551, we can successfully query the migration status, which is
> >> MIGRATION_STATUS_NONE. But this commit will return valid status (MIGRATION_STATUS_COMPLETED)
> >> without ram info, causing libvirt reports error (migration was active, but no RAM info was set).
> >>
> >>>
> >>> Do you have more patches that add such calls ? If so, then please send a
> >>> patch series that does the full job.
> >> I do not add new feature to libvirt, but just manually execute query-migrate on dest side *after
> >> migration completion* will trigger this problem.
> > 
> > How are you running query-migrate ? If you're doing that with monitor
> > command passthrough then we shouldn't need to change the libvirt migration
> > code.
> OK. I am not very familiar with libvirt code logic, and my work flow is:
> 
>      source side                                dest side
> 
>  virsh start domain_name                            |
>       |                                             |
>  virsh migrate --live domain_name       (wait for migrate complete)
>       |                                             |
>   (migrate complete)                                |
>                                         virsh domjobinfo domain_name
>                                                     |
>                                  (migration was active, but no RAM info was set)
> 
> Hope the above work flow helps.

You're trying to fetch statistics about a completed migration, for which
virsh domjobinfo --completed should be used.

Instead of hacking around it, we should properly implement support for
postcopy-blocktime and fetch the blocktime once migration completes
internally to make it available via the existing virsh domjobinfo
--completed command. Libvirt users should not need to know that
domjobinfo may magically be called even when migration is not running to
fetch some additional data.

Jirka