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

[Daniel P. Berrange]

On Thu, Sep 29, 2016 at 11:17:52AM +0200, Kevin Wolf wrote:
> Am 29.09.2016 um 11:09 hat Fam Zheng geschrieben:
> > On Thu, 09/29 09:51, Daniel P. Berrange wrote:
> > > On Thu, Sep 29, 2016 at 04:43:25PM +0800, Fam Zheng wrote:
> > > > On Thu, 09/29 09:34, Daniel P. Berrange wrote:
> > > > > So my suggestion is that we deprecate "drive-mirror" and define a 
> > > > > fixed
> > > > > command "drive-mirror-blockdev" (or "blockdev-mirror" ?) that accepts
> > > > > the proper BlockdevOptions QAPI type for the target as above.
> > > > 
> > > > Are you aware that there is already a blockdev-mirror command? 
> > > > Supposedly it
> > > > can do what you need, together with blockdev-add once the latter is 
> > > > deemed
> > > > ready.
> > > 
> > > Clearly I'm not aware of that :-)  It seems libvirt does not yet use
> > > blockdev-mirror either, which is where I got the original bug report
> > > about drive-mirror from.
> > 
> > Libvirt doesn't support blockdev-add yet, because the command is still being
> > actively worked on at QEMU side, and is therefore thought to be not "stable"
> > yet. Though, I think blockdev-add + blockdev-{mirror,backup} are already 
> > useful
> > for common tasks (like your use case with LUKS).
> 
> 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.

Regards,
Daniel
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