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

[Fam Zheng]

On Thu, 09/29 10:15, Daniel P. Berrange wrote:
> On Thu, Sep 29, 2016 at 05:09:20PM +0800, Fam Zheng wrote:
> > 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).
> 
> In what way is it "unstable", aside from the obvious limitation that some
> of the blockdev backends are not yet represented in BlockdevOptions QAPI
> schema ?

I think another risk is that it is so flexible in terms of building up the
block node graph (especially with BdrvChild referencing) that things may go
loose unexpectedly if the user is doing things we haven't really expected.

I don't have the full list in mind. Maybe Kevin can explain more.

Fam