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

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.

Checkpoint 4 can not be used without checkpoint 5 by design to it is not a problem that 3->4 is active. Nikolay