Re: [Qemu-devel] [RFC V3 3/7] qapi: Add skeletton of command to query a drive bs graph.

[Benoît Canet]

Le Thursday 05 Dec 2013 à 07:38:37 (-0700), Eric Blake a écrit :
> On 12/05/2013 07:24 AM, Benoît Canet wrote:
> 
> >>
> >> Am I correct that it will be possible to have named nodes that are not
> >> currently associated with any device?  If so, how do we learn about
> >> those nodes?  Would it be better to have a command that returns an array
> >> of structs for all known node roots, with an optional member describing
> >> which device owns that node root?  Something like:
> > 
> > The code have a list of all named nodes but not a list of named nodes roots.
> > Also it's difficult to get the device name for a named node because the 
> > bses don't
> > have any backward pointers to their parents.
> > It could be done by recursing into all the blockbackend bs but it's twisted.
> 
> Still worth thinking about how to structure things so we could add it in
> the future if it turns out to be useful to management, but I can
> understand why you aren't providing it right away.
> 
> > 
> > In fact I am wondering if we really need something to spit out the named 
> > nodes
> > topology in QMP for the simple reason that the names of the nodes are given 
> > by the
> > management so the management should already know the topology.
> 
> There's one case where management might not know - if libvirtd gets
> restarted while in the middle of an operation that was attempting to
> create a named node, then on restart and reconnection to the monitor,
> libvirt would want to query to see if the node actually got created or
> if the command needs to be attempted again.  I'm not a fan of write-only
> interfaces - and making management responsible to track all named nodes
> with no way to query if qemu actually agrees with the topology that
> management thinks it has commanded feels like a write-only interface.

Would a command returning info about a specific named node be sufficient for
libvirt checks ?
It's far less complex to implement than exposing the whole graph.
We could also provide a simple command to list the names of the named nodes.

Best regards

Benoît

> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>