[libvirt] [RFC v2] external (pull) backup API

[Nikolay Shirokovskiy]

On 13.04.2018 14:41, Vladimir Sementsov-Ogievskiy wrote:
> 13.04.2018 11:51, Nikolay Shirokovskiy wrote:
>>
>> On 13.04.2018 03:04, John Snow wrote:
>>>
>>> On 04/12/2018 10:08 AM, Vladimir Sementsov-Ogievskiy wrote:
>>>> I propose, not to say that bitmap represents a checkpoint. It is simpler
>>>> to say (and it reflects the reality) that bitmap is a difference between
>>>> two consecutive checkpoints. And we can say, that active state is some
>>>> kind of a checkpoint, current point in time.
>>>>
>>>> So, we have checkpoints (5* is an active state) which are points in time:
>>>>
>>>> 1 2 3 4 5*
>>>>
>>> Oh -- the most recent checkpoint there doesn't belong to a ***specific
>>> time*** yet. It's a floating checkpoint -- it always represents the most
>>> current version. It's not really a checkpoint at all.
>>>
>>> 1, 2, 3, and 4 however are associated with a specific timestamp though.
>>>
>>>> And bitmaps, first three are disabled, last is enabled:
>>>>
>>>> "1->2", "2->3", "3->4", "4->5*"
>>>>
>>> OK; so 1->2, 2->3 and 3->4 define deltas between two ***defined***
>>> points in time.
>>>
>>> 4->5* however is only anchored by one specific point in time, and is
>>> floating just like the most recent checkpoint is floating.
>>>
>>>> So, remove first checkpoint: just remove bitmap "A->B".
>>> I assume you mean "1->2" here.
>>>
>>> And... yes, I agree -- if you don't care about your very first
>>> checkpoint anymore, you can just delete the first bitmap, too.
>>>
>>>> Remove any other checkpoint N: create new bitmap "(N-1)->(N+1)" =
>>>> merge("(N-1)->N", "N->(N+1)"), drop bitmaps "(N-1)->N" and "N->(N+1)".
>>> err, okay, so let's say we want to drop checkpoint 3:
>>>
>>> create: "2->4"
>>> merge: "2->3", "3->4" [and presumably store in "2->4"]
>>> drop: 2->3, 3->4
>>>
>>> OK, that makes more sense to me. In essence;
>>>
>>> (1) We could consider this 2->3 absorbing 3->4, or
>>> (2) 3->4 absorbing 2->3
>>>
>>> and in either case it's the same, really.
>>>
>>>> If the latter was active, the new one becomes active. And we cant remove
>>>> 5* checkpoint, as it is an active state, not an actual checkpoint.
>>> OK, crystal.
>>>
>>> --js
>>>
>> I prefer not talking of active checkpoint. It is a kind of controversial.
>> Better just keep in mind that last bitmap is active one. So for checkpoints 1 2 3 4
>> we have bitmaps:
>>
>> 1 1->2 2->3 3->4
>>
>> Note the first bitmap name. When it was created name 2 was unknown so we'd better
>> have this name for the first bitmap.
> 
> so here, 1->2 is a difference between checkpoints 2 and 3? I think it's unnatural.. Ofcource, when we create new active bitmap, we don't know the name of next checkpoint, but, why not rename it when we create next checkpoint?
> 
> So,
> 
> 1. have no checkpoints and bitmaps
> 2. create new checkpoint 1, and bitmap 1->?
> 3. create new checkpoint 2 and bitmap 2->?, disable bitmap 1->? and rename it to 1->2
> and so on.
> 
> this reflects the essence of things

Makes sense and I don't see any issue from implementation POV. I would only use > only or >> (or whatever times >)
instead of ->. This makes possible to restrict names to prohibit > only. - is typical for UUIDs.