[libvirt] [RFC PATCH 02/30] qemu: domain: Format storage source node names into private data

[Kevin Wolf]

Am 20.04.2018 um 09:45 hat Peter Krempa geschrieben:
> On Fri, Apr 20, 2018 at 09:30:16 +0200, Kevin Wolf wrote:
> > Am 19.04.2018 um 17:24 hat Peter Krempa geschrieben:
> > > Save and restore node names if we know them in the status XML so that we
> > > don't need to recalculate them or don't lose them in some cases.
> > > 
> > > Signed-off-by: Peter Krempa <pkrempa at redhat.com>
> > > ---
> > >  src/qemu/qemu_domain.c | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > > 
> > > diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
> > > index e2a8450e2e..224b3b0478 100644
> > > --- a/src/qemu/qemu_domain.c
> > > +++ b/src/qemu/qemu_domain.c
> > > @@ -1934,6 +1934,9 @@ static int
> > >  qemuStorageSourcePrivateDataParse(xmlXPathContextPtr ctxt,
> > >                                    virStorageSourcePtr src)
> > >  {
> > > +    src->nodestorage = virXPathString("string(./nodename/@storage)", ctxt);
> > > +    src->nodeformat = virXPathString("string(./nodename/@format)", ctxt);
> > > +
> > >      if (virStorageSourcePrivateDataParseRelPath(ctxt, src) < 0)
> > >          return -1;
> > 
> > This format doesn't look very future-proof. I can see that using
> > x->nodestorage and x->nodeformat internally are probably easier for now,
> > but while the assumption of exactly two layers (except for backing
> > files) is probably correct for most cases, it will become less and less
> > common the more we use filter drivers.
> > 
> > Representing an arbitrary tree (or even non-tree graph) in the XML may
> > be unpractical now, but please be aware that you'll probably need this
> > change in the long run.
> 
> Yes. It will need to change e.g. if we allow IO throttling for other
> than the top layer. On the other hand it would be impractical to change
> to the topology qemu uses where a storage file is two layers minimum, so
> we will need to track everyting for one layer of XML.

The two layers minimum is not correct as far as QEMU is concerned. A
two-layer setup is what -drive creates by default if you don't specify a
protocol driver as driver=... (or format=... for older versions)
explicitly, but if you want, you can directly assign a 'file' node to a
guest device without adding a format layer on top.

> As the above uses attributes in the XML we can add an arbitrary amount
> of them if required.

Quorum takes an array of children, and VMDK needs a number of extents
that depends on the image file, so attributes probably won't do. If
we didn't have to consider compatibility, I think I'd choose something
like:

    <child-nodes>
        <child-node name="file" ref="my-file-node" />
        <child-node name="backing" ref="my-backing-node" />
    </child-nodes>

Kevin
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