Re: [Qemu-devel] [Qemu-block] [PATCH v7 00/11] Support streaming to an intermediate layer

[Alberto Garcia]

On Thu 18 Jun 2015 02:36:13 PM CEST, Eric Blake wrote:

   [Detecting support for intermediate block streaming]
>> One possibility is to try to stream to an intermediate node and see
>> if it fails.
>> 
>> Example: in a chain like [A] <- [B] <- [C], streaming to [B] using
>> [A] as the 'base' parameter is a no-op (there's even a test for that
>> in iotest 030).
>> 
>> If QEMU does support streaming to [B], the operation will succeed but
>> do nothing. Otherwise the operation will fail with a DeviceNotFound
>> error.

> In general, if a feature addition doesn't change API, but merely
> converts what was previously an error into something that works, then
> libvirt is probably okay with just trying the feature, and reporting
> the error message if it fails (assuming the qemu error message is
> sane).

With the example I gave above you should be able to detect easily if the
feature is present, but from what you say I understand that strictly
speaking you wouldn't even need to do that.

If you try to stream to a node and QEMU does not support that you will
get a 'device not found' error, which is I guess the kind of error that
you would expect. Since the only thing that changes with this feature is
the 'device' parameter I don't think there's any room for ambiguity.

Berto