Re: [Qemu-devel] Review of monitor commands identifying BDS / BB by name

[Kevin Wolf]

Am 04.12.2014 um 20:44 hat Eric Blake geschrieben:
> On 12/04/2014 08:56 AM, Markus Armbruster wrote:
> 
> > 
> > @device is a sub-optimal name for this single parameter.  Either we
> > accept that and move on, or we deprecate it in favor of a new parameter
> > with a better name.  I guess the better name isn't worth that much
> > trouble, in particular since the command name is already ugly.
> > 
> > When @node-name is given, @device must not be given.  So @device is
> > mandatory *except* in the @node-name usage we retain for compatibility.
> > Deprecate the usage.
> > 
> > Command documentation could then look like this:
> > 
> > ##
> > # @block-resize
> > #
> > # Resize a block image while a guest is running.
> > #
> > # @device: the name of the block backend or node to resize (node names
> > # supported since 2.3)
> > #
> > # @size: new image size in bytes
> > #
> > # Deprecated usage (since 2.3):
> > #
> > # @device: #optional the name of the block backend to resize
> > #
> > # @node-name: #optional name of the node to resize (since 2.0)
> > #
> > # Either @device or @node-name must be set but not both.
> 
> But this isn't discoverable.  You aren't changing whether any parameters
> are optional, only enhancing the semantics of existing parameters.  How
> is libvirt supposed to know if qemu is old (node names have to go
> through node-name) or new (node names are preferred through device)?
> Just blindly try the 'device' argument, and if it fails, fall back to an
> attempt with node-name?
> 
> On the other hand, if 'node-name' becomes the preferred interface, then
> libvirt has a solution: if node-name is present (it is easy during
> up-front QMP probing to determine whether 'node-name' is a recognized
> optional argument or an unknown argument), then always use node-name.
> As long as libvirt always names the nodes of each device (which it
> should be doing, but that's a patch series for another day and another
> list), then a device lookup is never needed.  If 'node-name' is not
> present, then only the 'device' form is supported, and libvirt can only
> manage the top image of a backend (but can make that point obvious to
> the end user that they should upgrade qemu for more functionality).

I thought libvirt didn't use any node names yet? Then it should probably
never try node-name, but only device.

There's probably little reason to resize a non-root node anyway, so if
you can't do that with qemu < 2.3, I don't think it would be a big
problem.

> > Let's get the easy case out of the way first: a query that reports only
> > backend properties and not node properties can return an array where
> > each element carries a backend name, like query-block does now.
> > 
> > For queries that report node properties, we have a couple of options:
> > 
> > * Flat array with node names
> > 
> >   Like current query-named-block-nodes.
> > 
> >   Can't report on nodes without names.  Jeff's project to give all nodes
> >   names would moot this issue.
> 
> I could live with this.
> 
> > 
> > * Array of trees mirroring the actual node forest,
> > 
> >   Similar to current query-blockstats.
> > 
> >   Tree roots correspond to backends, and backends have names.
> > 
> >   Unfortunately, the nodes don't actually form a forest: node trees may
> >   be shared.  To turn it into make a forest, we'd have to duplicate the
> >   shared trees.
> > 
> > * Hybrid: array of trees, but named sub-trees are represented by name
> > 
> >   Like the previous one, except the recursion stops at named nodes.
> >   Instead of including such a sub-tree, we reference it by name, then
> >   add it to the array if it's not already there.
> 
> This one seems like it might be easier for avoiding the reconstruction
> of relationships; but if management doesn't already know relationships,
> I'm not sure it is worth the complexity.
> 
> > 
> > "Flat array" seems simplest.
> 
> Simplest to implement.  Management can't easily reconstruct the tree,
> but for looking up information about a known node, iterating over the
> simpler structure will be faster than trying to find a known node in a
> complex structure.

I think what we need to add in the flat array is references (by name) to
the child nodes.

The approach is a bit complicated by the fact that we already include
random subtrees that appeared useful in some places.

Kevin

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