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

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.