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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
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Date: Tue, 20 Sep 2016 16:37:13 +0300
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Subject: Re: [Qemu-devel] [PATCH v3 4/8] replay: save/load initial state
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To: 'Paolo Bonzini' <pbonzini@redhat.com>, 'Pavel Dovgalyuk' <pavel.dovgaluk@ispras.ru>, qemu-devel@nongnu.org
Cc: peter.maydell@linaro.org, jasowang@redhat.com, quintela@redhat.com, mst@redhat.com, 'Kevin Wolf' <kwolf@redhat.com>

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> On 20/09/2016 14:39, Pavel Dovgalyuk wrote:
> > > > +    replay_snapshot = g_strdup(qemu_opt_get(opts, "rrsnapshot"));
> > > > +
> > > >      replay_enable(fname, mode);
> > > >
> > >
> > > Should you set snapshot = 1 here if there is no rrsnapshot option?
> >
> > No, because there is default snapshot name for the case when user
> > specifies overlay for the drives.
> 
> There are three possibilities:
> 
> a) these patches:
>    with implicit overlay:
>       -drive file=disk.raw,if=none,id=img-direct
>       -drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay
> 
>    with explicit overlay:
>      -drive file=disk.raw,if=none,id=img-direct
>      -drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay,overlay=foo.qcow2
> 
>    Advantages:
>    - does the right thing in the "implicit overlay" case.

     - automatically creates overlay

> 
>    Disadvantages:
>    - no need really to specify disk.raw in the "explicit overlay" case, since
>      it's already specified when you create the overlay with qemu-img.
> 
>    Vote for implicit overlay: excellent
>    Vote for explicit overlay: bad (need to track two file names)

Disadvantage is for replay only. Running QEMU in record mode automatically
creates overlay. Therefore two filenames are required.

> c) no rrsnapshot implies -snapshot:
>    without overlay:
>       -drive file=disk.raw,if=none,id=img-direct
>       -drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay
> 
>    with overlay:
>      -drive file=foo.qcow2,if=none,id=img-direct
>      -drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay
>      -icount ...,rrsnapshot=snapname

But how record will create this overlay?
This method requires creating overlay manually, because backing file is
not specified at all.


Pavel Dovgalyuk