Re: [Qemu-devel] [PATCH 1/2] qapi: support external bitmaps in block-dirty-bitmap-merge

[John Snow <jsnow@redhat.com>]

On 5/17/19 10:09 AM, Vladimir Sementsov-Ogievskiy wrote:
> 17.05.2019 16:50, Eric Blake wrote:
>> On 5/16/19 7:32 PM, John Snow wrote:
>>>
>>>
>>> On 5/16/19 8:27 AM, Vladimir Sementsov-Ogievskiy wrote:
>>>> Add new optional parameter making possible to merge bitmaps from
>>>> different nodes. It is needed to maintain external snapshots during
>>>> incremental backup chain history.
>>>>
>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>>> ---
>>>>   qapi/block-core.json | 13 ++++++++++---
>>>>   block/dirty-bitmap.c |  9 ++++++---
>>>>   blockdev.c           | 46 ++++++++++++++++++++++++++++++--------------
>>>>   3 files changed, 48 insertions(+), 20 deletions(-)
>>
>>>> -# @bitmaps: name(s) of the source dirty bitmap(s)
>>>> +# @bitmaps: name(s) of the source dirty bitmap(s). The field is optional
>>>> +#           since 4.1.
>>>> +#
>>>> +# @external-bitmaps: additional list of source dirty bitmaps with specified
>>>> +#                    nodes, which allows merging bitmaps between different
>>>> +#                    nodes. (Since: 4.1)
>>>>   #
>>>>   # Since: 4.0
>>>>   ##
>>>>   { 'struct': 'BlockDirtyBitmapMerge',
>>>> -  'data': { 'node': 'str', 'target': 'str', 'bitmaps': ['str'] } }
>>>> +  'data': { 'node': 'str', 'target': 'str', '*bitmaps': ['str'],
>>>> +            '*external-bitmaps': ['BlockDirtyBitmap'] } }
>>>>   
>>>
>>> I guess you can specify one, or both, or maybe neither! Seems fine.
>>
>>
>>>
>>> I don't think I like the name "external-bitmaps" but I guess I don't
>>> really have a better suggestion.
>>
>> I do - we could use an alternate type instead:
>>
>> { 'alternate': 'BitmapSource',
>>    'data': { 'local': 'str',
>>               'external': 'BlockDirtyBitmap' } }
>>
>> then use 'bitmaps': ['BitmapSource']
>>
>> so that the caller can pass:
>>
>> "bitmaps": [ "bitmap1", { "node": "other", "name", "bitmap2" } ]
>>
>> and we only have to deal with one array at all times, and not have the
>> name 'external-bitmaps' to worry about.
>>
> 
> Oh, I wanted to do something like this, but looked at union type,  which also needs some
> discriminator field, and decided that it's impossible to make it backward-compatible.
> 
> Will resend.
> 

Excellent! Thanks Eric!

--js