libvirt users - Oct 2019 - Re: [libvirt] Some questions about live migration

Hi libvirt experts,

I have some questions about live migration.

* If a live migration failed during migrating, will the domain exist on the
destination host?

* Is the flag VIR_MIGRATE_PAUSED make sense to live migration? It's a little
confusing for me. Does that indicate if I set this flag, then the domain on
the destination will not disappear even if the migration is failed, and it will
in 'paused' status? If not setting this flag, what will it be? It may be
similar
to the question above.
I filed a bug https://bugzilla.redhat.com/show_bug.cgi?id=1762703 about this
flag,
since its description is not very clear.

Thanks in advance, looking forward to your reply.

Regards,
Luyao


-- 
2.7.4

On Fri, Oct 18, 2019 at 15:00:19 +0800, Luyao Zhong
wrote:
> Hi libvirt experts,
> 
> I have some questions about live migration.
I'm assuming you are not asking about post-copy migration, since it is a
bit more complicated as the current state if split between the source
and destination hosts and none of them can keep running without the
other until migration finishes.
> * If a live migration failed during migrating, will the domain exist on the
> destination host?
No, the domain will keep running on the source host and disappear from
the destination host.
> * Is the flag VIR_MIGRATE_PAUSED make sense to live migration?
You can use this combination, but it most likely doesn't make sense.
Usually you would do live migration to minimize downtime. But
VIR_MIGRATE_PAUSED would cause the domain not to be automatically
resumed after migration so the downtime will likely be even longer than
with non-live migration (until the user manually resumes the domain on
destination). That said, the live and non-live migration when
VIR_MIGRATE_PAUSED is used (for both of them) differ only in the moment
the domain is paused. With non-live migration the domain is paused at
the beginning of migration, while with live migration the domain keeps
running during migration and it is paused once migration is almost done.
> It's a little confusing for me. Does that indicate if I set this flag,
> then the domain on the destination will not disappear even if the
> migration is failed, and it will in 'paused' status?
No. It means that if migration succeeds, the domain will remain paused
on the destination (while normally it would be running at that point).
The flag has no effect if the migration fails.

Jirka