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

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [PATCH] block/snapshot: dirty all dirty bitmaps on snapshot-switch
Date: Fri, 17 Nov 2017 15:58:45 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

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.
> Kevin
The problem is that snapshotting of bitmaps will just provide wrong result.

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 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.


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