Re: [Qemu-devel] [PATCH 1/3] migration: Create block capabilities for shared and enable

[Juan Quintela <quintela@redhat.com>]

Eric Blake <eblake@redhat.com> wrote:
> On 05/15/2017 04:46 AM, Dr. David Alan Gilbert wrote:
>> * Juan Quintela (quintela@redhat.com) wrote:
>>> Eric Blake <eblake@redhat.com> wrote:
>>>> On 05/11/2017 11:32 AM, Juan Quintela wrote:
>>>>> Those two capabilities were added through the command line.  Notice that
>>>>> we just created them.  This is just the boilerplate.
>>>>>
>>>>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>>>>> Reviewed-by: Eric Blake <eblake@redhat.com>
>>>>>
>>>>> --
>>>>>
>>>>> Make migrate_set_block_* take a boolean argument.
>>>>
>>>> Question - do we support the orthogonal selection of all 4 combinations
>>>> under HMP 'migrate' (no argument, -b alone, -i alone, -b and -i
>>>> together), or are there only 3 actual states? If the latter, should we
>>>> represent this as a single enum-valued property, rather than as two
>>>> independent boolean properties?
>>>
>>> { 'enum': 'MigrationCapability',
>>>   'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
>>>            'compress', 'events', 'postcopy-ram', 'x-colo', 'release-ram'] }
>>>
>>>
>>> My understanding is that we can only have boolean capabilities here.
>>> Or, how could we put a non-boolean capability?
>
> If we want a non-boolean, then we make it a migration parameter rather
> than a migration capability.  There may be other advantages to using
> MigrationParameter instead of MigrationCapability (such as making it
> easier to figure out whether the parameter settings are persistent or
> apply per-migration).

Block migration is a capability as far as I can see/understand.  The
block shared bit could be a parameter, though.  Not sure if that would
be better/worse.

>
>> 
>> Lets keep this simple and stick with the booleans.
>> 
>> Dave
>> 
>>> There are three states as far as I can see.
>
> I'll leave it up to you as maintainers which way you prefer, I'm just
> offering the potential design tradeoffs for simplicity of booleans (but
> complexity in an unused state) vs. simplicity of design (but complexity
> in code).

As I expect to deprecate the old interface, I think that the best thing
to do is to use two capabilities or a capability(block enabled) and a
parameter (block shared).

What do you think?

Later, Juan.