Re: [Xen-devel] Design session report: Live-Updating Xen

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jul 18, 2019 at 11:00:23AM +0200, Juergen Gross wrote:
> On 17.07.19 00:27, Andrew Cooper wrote:
> > On 16/07/2019 05:20, Sarah Newman wrote:
> > > On 7/15/19 8:48 PM, Juergen Gross wrote:
> > > > On 15.07.19 21:31, Sarah Newman wrote:
> > > > > On 7/15/19 11:57 AM, Foerster, Leonard wrote:
> > > > > ...
> > > > > > A key cornerstone for Live-update is guest transparent live 
> > > > > > migration
> > > > > ...
> > > > > >      -> for live migration: domid is a problem in this case
> > > > > >          -> randomize and pray does not work on smaller fleets
> > > > > >          -> this is not a problem for live-update
> > > > > >          -> BUT: as a community we shoudl make this restriction go 
> > > > > > away
> > > > > 
> > > > > Andrew Cooper pointed out to me that manually assigning domain IDs
> > > > > is supported in much of the code already. If guest transparent live
> > > > > migration gets merged, we'll look at passing in a domain ID to xl,
> > > > > which would be good enough for us. I don't know about the other
> > > > > toolstacks.
> > > > 
> > > > The main problem is the case where on the target host the domid of the
> > > > migrated domain is already in use by another domain. So you either need
> > > > a domid allocator spanning all hosts or the change of domid during
> > > > migration must be hidden from the guest for guest transparent migration.
> > > 
> > > Yes. There are some cluster management systems which use xl rather
> > > than xapi.
> > > They could be extended to manage domain IDs if it's too difficult to
> > > allow
> > > the domain ID to change during migration.
> > 
> > For a v1 feature, having a restriction of "you must manage domids across
> > the cluster" is a fine.  Guest-transparent migration is a very important
> > feature, and one where we are lacking in relation to other hypervisors.
> > 
> > Longer term, we as the Xen community need to figure out a way to remove
> > the dependency on domids, at which point the cluster-wide management
> > restriction can be dropped.  This isn't going to be a trivial task, but
> > it will be a worthwhile one.
> 
> Another problem are Xenstore watches. With guest transparent LM they are
> lost today as there is currently no way to migrate them to the target
> Xenstore.

Hm, I guess I'm missing something, but xenstored running either in
dom0 or in a stubdomain should be completely unaware of the hypervisor
being updated under it's feet. The hypervisor itself don't have any
knowledge itself of xenstore state.

Roger.

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