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

[Pavel Dovgalyuk]

> From: Kevin Wolf [mailto:address@hidden]
> > >
> > > However, global -snapshot is just a convenient shortcut for specifying
> > > snapshot=on for all -drive arguments. So if -snapshot is incompatible
> > > with replay, shouldn't manually marking all drives as snapshot=on be
> > > incompatible as well?
> > >
> > > Maybe you're really interested in some specific drive not having
> > > snapshot=on? But then it might be better to check that specific drive
> > > instad of forbidding just the shortcut for setting it.
> >
> > -snapshot adds the flag for top-level drive, making driver operations
> > dependent on temporary file structure.
> >
> > Moving this overlay beneath blkreplay driver makes drive operations
> > deterministic for the top-level device.
> 
> So the real requirement is that blkreplay is the top-level node of any
> guest device, right? And only because of this, you can't use -snapshot
> (or snapshot=on on the blkreplay driver).
> 
> If we instead check e.g. in blk_insert_bs() or blk_attach_dev() that in
> record/replay mode, the root node of the BlockBackend is blkreplay,
> wouldn't we catch many more incorrect setups?

That sounds interesting.
Will it help to check that every backend is connected to blkreplay?
How then this check has to be done?

Pavel Dovgalyuk