Re: [Qemu-devel] [PATCH v5 7/9] block: don't make snapshots for filters

[Kevin Wolf <kwolf@redhat.com>]

Am 28.09.2016 um 11:32 hat Pavel Dovgalyuk geschrieben:
> > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > Am 27.09.2016 um 16:06 hat Pavel Dovgalyuk geschrieben:
> > > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > > Am 26.09.2016 um 11:51 hat Pavel Dovgalyuk geschrieben:
> > > > > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > > > > Am 26.09.2016 um 10:08 hat Pavel Dovgalyuk geschrieben:
> > > > Originally, we only called bdrv_goto_snapshot() for all _top level_
> > > > BDSes, and this is still what you normally get. However, if you
> > > > explicitly create a BDS (e.g. with its own -drive option), it is
> > > > considered a top level BDS without actually being top level for the
> > > > guest, and therefore the snapshotting function is called for it.
> > > >
> > > > Of course, this is highly inefficient because the goto_snapshot request
> > > > is passed by the filter driver and then called another time for the
> > > > lower node, effectively loading the snapshot a second time.
> 
> Maybe double-saving/loading does the smallest damage then?
> And we should just document how to use blkreplay effectively?
> 
> > > >
> > > > On the other hand if you use a single -drive option to create both the
> > > > qcow2 BDS and the blkreplay filter, we do need to pass down the
> > > > goto_snapshot request because it won't be called for the qcow2 layer
> > > > otherwise.
> > >
> > > How this can be specified in command line?
> > > I believed that separate -drive option is required.
> > 
> > Something like this:
> > 
> >     -drive driver=blkreplay,image.driver=file,image.filename=test.img
> > 
> 
> I tried the following command line, but VM does not detect the hard drive
> and cannot boot.
> 
> -drive driver=blkreplay,if=none,image.driver=file,image.filename=testdisk.qcow,id=img-blkreplay 
> -device ide-hd,drive=img-blkreplay

My command line was assuming a raw image. It looks like you're using a
qcow (hopefully qcow2?) image. If so, then you need to include the qcow2
driver:

-drive driver=blkreplay,if=none,image.driver=qcow2,\
image.file.driver=file,image.file.filename=testdisk.qcow,id=img-blkreplay

Kevin