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

[Kevin Wolf <kwolf@redhat.com>]

Am 18.10.2019 um 14:06 hat Pavel Dovgalyuk geschrieben:
> > 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?

Ah, I see what's going on there... :-/

bdrv_open_inherit() creates an internal temporary BlockBackend for image
format probing. And it creates that BlockBackend even when we don't need
to do image probing because we already explicitly selected a driver. It
would be better if it didn't do that.

Kevin