Re: [Qemu-devel] [PATCH 1/2] commit: Add top-node/base-node options

[Peter Krempa <pkrempa@redhat.com>]

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On Tue, Sep 04, 2018 at 15:13:44 +0200, Alberto Garcia wrote:
> On Mon 03 Sep 2018 05:03:11 PM CEST, Kevin Wolf wrote:
> >> libvirt-3-format is read-write and all other node names are readonly in
> >> the above example.
> >> 
> >> The same also happens when using filenames:
> >> 
> >> {"execute":"block-commit",
> >>  "arguments" : {"device":"libvirt-3-format",
> >>                 "job-id":"libvirt-3-format",
> >>                 "top":"/var/lib/libvirt/images/rhel7.3.1483615252",
> >>                 "base":"/var/lib/libvirt/images/rhel7.3.1483605924",
> >>                 "auto-finalize":true,
> >>                 "auto-dismiss":false},
> >>  "id":"libvirt-13"}
> >> 
> >> {"id":"libvirt-13","error":{"class":"GenericError","desc":"Block node is read-only"}}
> >
> > I see what's happening here. So we have a graph like this:
> >
> >      guest device
> >           |
> >           v
> >     overlay-format -------> backing-format
> >     [read-only=off]         [read-only=on]
> >           |                        |
> >           v                        v
> >     overlay-proto           backing-proto
> >     [read-only=off]         [read-only=on]
> >
> > The difference between your -blockdev use and -drive is that you
> > explicitly specify the read-only option for backing-proto (and you use
> > a separate -blockdev option anyway), so it doesn't just inherit it
> > from backing-format.
> 
> Are these format and protocol block devices opened with four separate
> -blockdev parameters? Is that how libvirt does it?

Yes. This goes along with the fact that for 'blockdev-create' you need
to blockdev-add the file which you want to format, but the formatted
file is not automatically added.

If we'd use the approach where the protocol layer is opened as part of
the format layer it would complicate the snapshot code where we need to
add a file and then format it to qcow2. It would mean that we'd have to
blockdev-add a file, format it via blockdev-create, then blockdev-del it
and open it together with the format layer. Otherwise the disk
hot-unplug code would be plain crazy.

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