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Re: [Qemu-devel] [PATCH] block/snapshot: dirty all dirty bitmaps on snap

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] block/snapshot: dirty all dirty bitmaps on snapshot-switch
Date: Fri, 17 Nov 2017 14:42:47 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

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 <address@hidden>
> > 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.


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