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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 19:25:26 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

Am 17.11.2017 um 19:15 hat John Snow geschrieben:
> On 11/17/2017 10:01 AM, Max Reitz wrote:
> > On 2017-11-17 13:30, 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.
> > 
> > I think it at least makes more sense than not handling this case at all.
> > 
> >> 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.
> > 
> > Agreed.
> > 
> >>                                  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.
> > 
> > Sounds OK, too.  I personally don't have an opinion either way.
> > 
> > But in any case, what we did before this patch was definitely wrong so I
> > consider it an improvement.
> > 
> This is how I feel about it too. Erroring out entirely is an option, but
> code-wise just dirtying everything is at least verifiably not-wrong and
> pretty simple to implement.

But that's exactly the problem: If something is just plain wrong, you
can always replace it with something that makes sense. If it errors out,
you can still remove that error later. But if you have something that
doesn't make a whole lot of sense, but kinda sorta works (like after
this patch), it's ABI and you can't implement something more useful
later any more.

> It's an improvement... Don't do it, but at least you won't get
> something wrong after, just something heinously unoptimal.

It's a short-term improvement that may become a long-term burden.


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