Re: [Qemu-devel] RFC: handling image options with drive-mirror/drive-backup

[Kashyap Chamarthy <kchamart@redhat.com>]

On Thu, Sep 29, 2016 at 10:43:37AM +0100, Daniel P. Berrange wrote:
> On Thu, Sep 29, 2016 at 11:17:52AM +0200, Kevin Wolf wrote:

[...]

> > Just to clarify what "not stable" means: Literally none of the
> > blockdev-add commands that used to work when it was originally merged
> > are still working. And we're considering another similar change
> > (removing the "options" indirection) that will change the command for
> > all users. So while I would encourage libvirt to write prototyp code for
> > supporting blockdev-add now, I would advise against enabling it in a
> > release yet.
> 
> Urgh, arbitrarily changing behaviour of existing commands is really
> very bad for libvirt, as it means we have to now write special case
> logic to detect whether we can use the command or not, instead of
> merely detecting whether it exists.
> 
> If commands are expected to change, they should have an 'x-' prefix
> and once that's removed they should never be changed in an incompatible
> manner again.

I too wondered about the "x-" prefix for 'blockdev-add' when I was
digging into the details a month ago.  I came to the conclusion that it
was just an inadvertant mistake that it wasn't marked as such --
because, you could see the experimental prefix for rest of them
commands: x-blockdev-change x-blockdev-del, x-blockdev-insert-medium,
x-blockdev-remove-medium

-- 
/kashyap