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Re: [Qemu-devel] BdrvDirtyBitmap and bdrv_snapshot_goto

From: Max Reitz
Subject: Re: [Qemu-devel] BdrvDirtyBitmap and bdrv_snapshot_goto
Date: Mon, 7 Nov 2016 17:10:54 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 07.11.2016 16:24, Vladimir Sementsov-Ogievskiy wrote:
> Hi all!
> As I can see, in bdrv_snapshot_goto, existing dirty bitmaps are not
> handled.. Is it ok? Should not they be filled with ones or something
> like this?

Filling them with ones makes sense to me. I guess nobody noticed because
nobody was crazy enough to use block jobs alongside loadvm...

> Also, when we will have persistent bitmaps in qcow2, haw they should be
> handled on snapshot switching?

Good question. Since persistent bitmaps are not bound to snapshots, I'd
fill them with ones for now, too.

It would probably make sense to bind bitmaps to snapshots, though. This
could be achieved by adding a bitmap directory pointer to each snapshot
table entry. When switching snapshots, software (i.e. qemu) could then

(1) Fill the bitmaps with ones, thus treating them as "global" bitmaps.

(2) Save the current bitmap directory in the old snapshot and put the
one from the snapshot that is being switched to into the image header,
thus treating them as bound to the snapshot.

Of course, this could be a bitmap-specific property.


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