Re: [Xen-devel] question about migration

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Andrew Cooper writes ("Re: [Xen-devel] question about migration"):
> On 04/01/16 15:31, Ian Jackson wrote:
> > * It is not possible to resume the domain in the source after it has
> >   suspended.
> 
> This functionality exists and is already used in several circumstances,
> both by libxl, and other toolstacks.

Oh!

> xl has an added split-brain problem here that plain demonic toolstacks
> don't have; specifically that there are two completely independent
> processes playing with the domain state at the same time.
> 
> The daemonic xl needs to ignore DOMAIN_SHUTDOWN and tidy up only after
> DOMAIN_DEATH.  Under these circumstances, a failed migrate which resumes
> the domain won't result in qemu being cleaned up.

I think there is some kind of further underlying problem here.

Suppose a domain goes into SHUTDOWN with reason code SUSPEND.  Then
later it is resumed again (perhaps the migration failed).  And later
it shuts down cleanly.

Will this generate a second @releaseDomain ?  How is a toolstack
supposed to discover this situation ?

Should libxl simply treat domains in state SHUTDOWN/SUSPEND as if they
were running, and not issue a notification to anyone ?


> > * @releaseDomain does not have a specific domain which is the "subject
> >   of @releaseDomain".  Arguably this is unhelpful, but it is not
> >   libxl's fault.  It arises from the VIRQ generated by Xen.  Note that
> >   xenstored needs to search its own list of active domains to see what
> >   has happened; it generates the @releaseDomain event and throws away
> >   the domid.
> 
> The semantics of @releaseDomain are quite mad, but this is have it has
> always been.

Yes.

> The current semantics are a scalability limitation which someone in
> XenServer will likely get around to in due course (we support 1000 VMs
> per host).

Jolly good :-).


Ian.

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