Re: [Qemu-devel] How to introduce bs->node_name ?

["Benoît Canet" <benoit.canet@irqsave.net>]

Le Monday 04 Nov 2013 à 19:33:21 (+0800), Fam Zheng a écrit :
> 
> On 11/04/2013 07:06 PM, Kevin Wolf wrote:
> >Am 04.11.2013 um 10:48 hat Fam Zheng geschrieben:
> >>On 11/04/2013 05:31 PM, Stefan Hajnoczi wrote:
> >>>On Wed, Oct 30, 2013 at 02:49:32PM +0100, Markus Armbruster wrote:
> >>>>The first proposal is to add another parameter, say "id".  Users can
> >>>>then refer either to an arbitrary BDS by "id", or (for backward
> >>>>compatibility) to the root BDS by "device".  When the code sees
> >>>>"device", it'll look up the BB, then fetch its root BDS.
> >>>>
> >>>>CON: Existing parameter "device" becomes compatibility cruft.
> >>>>
> >>>>PRO: Clean and obvious semantics (in my opinion).
> >>>This proposal gets my vote.
> >>>
> >>>>The second proposal is to press the existing parameter "device" into
> >>>>service for referring to BDS node_name.
> >>>>
> >>>>To keep backward compatibility, we obviously need to ensure that
> >>>>whenever the old code accepts a value of "device", the new code accepts
> >>>>it as well, and both resolve it to the same BDS.
> >>>Different legacy commands given the same device name might need to
> >>>operate on different nodes.
> >>Could you give an example for this?
> >>
> >>
> >>>Dynamic renaming does not solve this
> >>>problem, so I'm not convinced we can always choose a device name
> >>>matching a node name.
> >>>
> >>>Device name commands are higher-level than graph node commands.  For
> >>>example, block_set_io_throttle makes sense on a device but less sense on
> >>>a graph node, unless we add the implicit assumption that the new
> >>>throttling node is created on top of the given node or updated in place
> >>>if the throttling node already exists (!!).
> >>Throttling a node could be useful too, for example if we want to
> >>throttle backing_hd which is on shared storage, but not to throttle
> >>on the local image.
> >>
> >>My ignorant question is: Why can't we just use one namespace, make
> >>sure no name collision between node_name and device_name, or even
> >>just drop device_name, so we treat the root node's node_name as
> >>device_name? For commands that only accept a device, this can be
> >>enforced in its implementation by checking against the whole graph
> >>to verify this.
> >Markus described it somewhere in this thread: Live snapshots.
> >Currently, the device_name moves to the new BDS on the top (and
> >compatibility requires us to keep it that way), whereas a node name
> >should, of course, stay at its node.
> >
> >When you consider this, the single namespace, as much as I would have
> >loved it, is pretty much dead.

Good everyone agree on the direction to take.
I'll write some code.

Best regards

Benoît

> 
> Thanks for explaining (again). I get the reason now.
> 
> Fam
>