Re: [PATCH v3 1/1] vhost-user-fs: add migration type property

[Michael S. Tsirkin]

On Wed, Mar 01, 2023 at 05:03:03PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 01.03.23 00:24, Michael S. Tsirkin wrote:
> > > Said that checking on destination would need another flag and the safe
> > > way of using this feature would require managing two flags instead of one
> > > making it even more fragile. So I'd prefer not to make it more complex.
> > > 
> > > In my opinion the best way to use this property by orchestrator is to
> > > leave default unmigratable behavior at start and just before migration 
> > > when
> > > destination is known enumerate all vhost-user-fs devices and set 
> > > properties
> > > according to their backends capability with QMP like you mentioned. This
> > > gives us single point of making the decision for each device and avoids
> > > guessing future at VM start.
> > this means that you need to remember what the values were and then
> > any failure on destination requires you to go back and set them
> > to original values.
> 
> Why do we need to restore old values?

To get back to where you were before you were starting migration.

> For me, this new property is a kind of per-device migration
> capability. Do we care to restore migration capabilities to the values
> that they had before setting them for failed migration? We don't need
> it, as we just always set capabilities as we want before each
> migration. Same thing for this new property: just set it properly
> before migration and you don't need to care about restoring it after
> failed migration attempt.

If you really trust your management then we can just remove the
migration blocker and be done with it. All this song and dance
with changing properties is to catch errors. If one has to
carefully play with QOM to achieve the desired result then
IMHO we failed in this.


> > With possibility of crashes on the orchestrator
> > you also need to recall the temporary values in some file ...
> > This is huge complexity much worse than two flags.
> > 
> > Assuming we need two let's see whether just reload on source is good
> > enough.
> > 
> 
> -- 
> Best regards,
> Vladimir