Re: [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API

[John Snow <jsnow@redhat.com>]

On 12/19/2017 11:07 AM, Vladimir Sementsov-Ogievskiy wrote:
> 13.12.2017 07:12, Fam Zheng wrote:
>> On Mon, 11/13 19:20, Vladimir Sementsov-Ogievskiy wrote:
>>> Hi all.
>>>
>>> There are three qmp commands, needed to implement external backup API.
>>>
>>> Using these three commands, client may do all needed bitmap
>>> management by
>>> hand:
>>>
>>> on backup start we need to do a transaction:
>>>   {disable old bitmap, create new bitmap}
>>>
>>> on backup success:
>>>   drop old bitmap
>>>
>>> on backup fail:
>>>   enable old bitmap
>>>   merge new bitmap to old bitmap
>>>   drop new bitmap
>>>
>>> Question: it may be better to make one command instead of two:
>>> block-dirty-bitmap-set-enabled(bool enabled)
>>>
>>> Vladimir Sementsov-Ogievskiy (4):
>>>    block/dirty-bitmap: add lock to bdrv_enable/disable_dirty_bitmap
>>>    qapi: add block-dirty-bitmap-enable/disable
>>>    qmp: transaction support for block-dirty-bitmap-enable/disable
>>>    qapi: add block-dirty-bitmap-merge
>>>
>>>   qapi/block-core.json         |  80 +++++++++++++++++++++++
>>>   qapi/transaction.json        |   4 ++
>>>   include/block/dirty-bitmap.h |   2 +
>>>   block/dirty-bitmap.c         |  21 ++++++
>>>   blockdev.c                   | 151
>>> +++++++++++++++++++++++++++++++++++++++++++
>>>   5 files changed, 258 insertions(+)
>>>
>> I think tests are required to merge new features/commands.  Can we
>> include tests
>> on these new code please?  We should cover error handling, and also
>> write tests
>> that demonstrate the intended real world use cases.
>>
>> Also should we add new sections to docs/interop/bitmaps.rst?
>>
>> Meta: John started a long discussion about the API design but I think
>> after all
>> it turns out exposing dirty bitmap objects and the primitives is a
>> reasonable
>> approach to implement incremental backup functionalities. The comment
>> I have is
>> that we should ensure we have also reviewed it from a higher level
>> (e.g. all the
>> potential user requirements) to make sure this low level API is both
>> sound and
>> flexible. We shouldn't introduce a minimal set of low level commands
>> just to
>> support one particular use case, but look a bit further and broader
>> and come up
>> with a more complete design? Writing docs and tests might force us to
>> think in
>> this direction, which I think is a good thing to have for this series,
>> too.
>>
>> Fam
> 
> Nikolay, please describe what do you plan in libvirt over qmp bitmap API.
> 
> Kirill, what do you think about this all?
> 
> (brief history:
> we are considering 3 new qmp commands for bitmap management, needed for
> external incremental backup support
>  - enable (bitmap will track disk changes)
>  - disable (bitmap will stop tracking changes)
>  - merge (merge bitmap A to bitmap B)
> )
> 

Yeah, it would be helpful to know what the full workflow for the API
will be ... before I get ahead of myself again (sorry) ...

but I'd like to see a quick writeup of your vision for the pull-mode
backups (which I assume this is for, right?) from start-to-finish, like
a mockup of annotated QMP output or something.

Nothing fancy, just something that lets me orient where we're headed,
since you're doing most of the work and I just want to get out of your
way, but having a roadmap helps me do that.

--js