Daniel P. Berrangé <berra...@redhat.com> wrote:
> On Tue, Nov 14, 2023 at 11:28:28AM +0100, Juan Quintela wrote:
>> Daniel P. Berrangé <berra...@redhat.com> wrote:
>> > On Thu, Nov 02, 2023 at 03:25:25PM +0100, Markus Armbruster wrote:
>> >> Now let's try to apply this to migration.
>> >>
>> >> As long as we can have just one migration, we need just one QAPI object
>> >> to configure it.
>> >>
>> >> We could create the object with -object / object_add. For convenience,
>> >> we'd probably want to create one with default configuration
>> >> automatically on demand.
>> >>
>> >> We could use qom-set to change configuration. If we're not comfortable
>> >> with using qom-set for production, we could do something like
>> >> blockdev-reopen instead.
>> >
>> > Do we even need to do this via a QAPI object ?
>> >
>> > Why are we not just making the obvious design change of passing everything
>> > with the 'migrate' / 'migrate-incoming' commands that kick it off:
>> >
>> > ie:
>> >
>> > { 'command': 'migrate',
>> > 'data': {'uri': 'str',
>> > '*channels': [ 'MigrationChannel' ],
>> > '*capabilities': [ 'MigrateCapability' ],
>> > '*parameters': [ 'MigrateParameters' ],
>> > '*detach': 'bool', '*resume': 'bool' } }
>>
>> Once that we are doing incompatible changes:
>
> This is not incompatible - it is fully backcompatible with existing
> usage initially, which should make it pretty trivial to introduce
> to the code. Mgmt apps can carry on using migrate-set-capabilities
> and migrate-set-parameters, and ignore these new 'capabilities'
> and 'parameters' fields if desired.
>
> Only once we decide to deprecate migrate-set-capabilities, would
> it become incompatible.
Oh, I mean that the interface is incompatible. Not that we can't do the
current one on top of this one.
>> - resume can be another parameter
>
> Potentially yes, but 'resume' is conceptually different to all
> the other capabilities and parameters, so I could see it remaining
> as a distinct field as it is now
It is conceptually different. But it is the _only_ one needed that
capability. And putting that on the parameters and just checking it
first will achieve the same result. I think that being special here
don't help, for instance, to check for incompatible things, we need to
also pass resume (it is only valid for postcopy).
>> - detach is not needed. QMP don't use it, and HMP don't need to pass it
>> to qmp_migrate() to make the non-detached implemntation.
>
> We could deprecate that today then.
Yeap. Will do it.
>> > (deprecated bits trimmed for clarity)
>> >
>> > and the counterpart:
>> >
>> > { 'command': 'migrate-incoming',
>> > 'data': {'*uri': 'str',
>> > '*channels': [ 'MigrationChannel' ],
>> > '*capabilities': [ 'MigrateCapability' ],
>> > '*parameters': [ 'MigrateParameters' ] } }
>> >
>> > such that the design is just like 99% of other commands which take
>> > all their parameters directly. We already have 'migrate-set-parameters'
>> > remaining for the runtime tunables, and can deprecate the usage of this
>> > when migration is not already running, and similarly deprecate
>> > migrate-set-capabilities.
>>
>> This makes sense to me, but once that we change, we could try to merge
>> capabilities and parameters. See my other email on this topic.
>> Basically the distition is arbitrary, so just have one of them.
>>
>> Or better, as I said in the other email, we have two types of
>> parameters:
>> - the ones that need to be set before migration starts
>> - the ones that can be changed at any time
>>
>> So to be simpler, I think that 1st set should be passed to the commands
>> themselves and the others should only be set with
>> migrate_set_parameters.
>
> As a mgmt app dev I don't want there to be an arbitrary distinction
> between what I can pass with 'migrate' and what I have to use a
> separate command for.
If it ever wants to set the parameter that it "can" change after
migration starts, it needs to know that they are different.
Once told that, I don't write management apps and you do so I am not
discussing further O:-)
> If I'm starting a migration, I just want to
> pass all the settings with the 'migrate' command. I should not have
> to care about separate 'migrate-set-parameters' command at all, unless
> I actually need to change something on the fly (many migrates will
> never need this).
What OpenStack/CNV do?
If my memory is right, at least one of them used progressive downtimes
every couple of iterations or something like that.
Later, Juan.
> With regards,
> Daniel
