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

[Fam Zheng <famz@redhat.com>]

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.

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

Fam