RE: [for-4.2 PATCH 3/6] replay: update docs for record/replay with block devices

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Kevin Wolf [mailto:kwolf@redhat.com]
> Am 11.10.2019 um 08:10 hat Pavel Dovgalyuk geschrieben:
> > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > Am 25.09.2019 um 11:02 hat Pavel Dovgalyuk geschrieben:
> > > > I started playing with -blockdev: added new blockdev for blkreplay and
> > > > constructed the following command line:
> > > >
> > > > -blockdev driver=file,filename=disk.img,node-name=hd0
> > > > -blockdev driver=blkreplay,file=hd0,node-name=hd0-rr
> > > > -device virtio-blk-device,drive=hd0-rr
> > > >
> > > > However, I get an error: "Could not open 'disk.img': Permission denied"
> > > > Everything works when I use this file in '-drive' parameter.
> > > > What am I doing wrong?
> > >
> > > The reason why I didn't reply immediately is because I don't see
> > > anything wrong in the options you used.
> > >
> > > Just to confirm, do you still get the same error when you use only the
> > > first -blockdev option and no other options at all?
> >
> > Ok, I tried again and got different error, which was caused by incorrect
> > QAPI schema for blkreplay.
> > Now it seems ok, but I still can't boot.
> 
> Hm... Are you actually using a raw image? If not, you need the format
> driver, too, and would end up with something like:
> 
>  -blockdev driver=file,filename=disk.qcow2,node-name=hd0
>  -blockdev driver=qcow2,file=hd0,node-name=hd0-qcow2
>  -blockdev driver=blkreplay,file=hd0-qcow2,node-name=hd0-rr
>  -device virtio-blk-device,drive=hd0-rr
> 
> (The first two can be combined into a single option by using a syntax
> like file.driver=file,file.filename=disk.qcow2, but defining each node
> separately is a bit cleaner.)

Ok, this works.
Now I'm trying to check root of the nodes in blk_insert_bs.
This command line leads to 2 invocations of this function:
1. bs->drv is file
2. bs->drv is blkreplay

How then can we check "snapshot" node attachment?

Pavel Dovgalyuk