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Re: [Qemu-devel] BdrvDirtyBitmap and bdrv_snapshot_goto

From: Kevin Wolf
Subject: Re: [Qemu-devel] BdrvDirtyBitmap and bdrv_snapshot_goto
Date: Tue, 8 Nov 2016 13:18:59 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 08.11.2016 um 12:08 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 08.11.2016 14:05, Kevin Wolf wrote:
> >Am 07.11.2016 um 17:10 hat Max Reitz geschrieben:
> >>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...
> >What's the use case in which ones make sense?
> >
> >It rather seems to me that an active dirty bitmap and snapshot switching
> >should exclude each other because the bitmap becomes meaningless by the
> >switch. And chances are that after switching a snapshot you don't want
> >to "incrementally" backup everything, but that you should access a
> >different backup.
> In other words, dirty bitmaps should be deleted on snapshot switch?
> All? Or only named?

As Max said, we should probably integrate bitmaps with snapshots. After
reloading the old state, the bitmap becomes valid again, so throwing it
away in the active state seems only right if we included it in the
snapshot and can bring it back.


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