On 11/17/2017 04:42 PM, Kevin Wolf wrote: > Am 17.11.2017 um 13:58 hat Denis V. Lunev geschrieben: >> On 11/17/2017 03:30 PM, Kevin Wolf wrote: >>> Am 23.10.2017 um 11:29 hat Vladimir Sementsov-Ogievskiy geschrieben: >>>> Snapshot-switch actually changes active state of disk so it should >>>> reflect on dirty bitmaps. Otherwise next incremental backup using >>>> these bitmaps will be invalid. >>>> >>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> >>> We discussed this quite a while ago, and I'm still not convinced that >>> this approach makes sense. >>> >>> Can you give just one example of a use case where dirtying the whole >>> bitmap while loading a snapshot is the desired behaviour? >>> >>> I think the most useful behaviour would be something where the bitmaps >>> themselves are snapshotted, too. But for the time being, the easiest and >>> safest solution might just be to error out in any snapshot operations >>> if any bitmaps are in use. >>> >> The problem is that snapshotting of bitmaps will just provide wrong >> result. > Only if the user does the wrong thing. :-) > >> Let us assume that we have bitmap named A. >> >> The user has started it and made full backup B. >> The user made snapshot S. At this moment bitmap A is saved as A' to bitmap. >> The user has made incremental backup B1. A is reset to 0. >> The user has made incremental backup B2. A is reset to 0 again. >> >> At this moment the user has reverted to snapshot S. >> What we need to make incremental backup at the moment? > The important point here is that the backup that you should make is not > incremental compared to B2, but to B (i.e. the last backup at the point > when the snapshot was made). > > If your snapshot chain branches, your backups have to branch, too. > >> The difference in between states B2 and S. This is __for sure__ >> not A'. Thus saving of the bitmap at the moment is quite >> useless and we need to reset bitmap to full. > A' still contains the differences between B and the point when S was > created. So if you take a new incremental backup B'1 based on B, the > correct bitmap to use would be A'. > > But I agree that this is complicated and probably easy to misuse, so > just erroring out is a serious option. And if we do this, at least we > don't set a bad solution in stone and can always add a better solution > if we can later think of one. > > I just don't think that marking everything dirty is a good solution. It > forces the user to make a full backup, so we could just as well ask the > user to delete the bitmap before they load a snapshot. > > Kevin Agree. This is also an option.
Den
