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
