Re: [Qemu-devel] More structured migration URIs?

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Jan 05, 2015 at 12:14:25PM +0000, Dr. David Alan Gilbert wrote:
> Hi,
>   I keep thinking of things where it might make sense to add
> options onto the migration URIs and wondered if it makes
> sense to restructure the migration URIs; my proposal would be:
> 
>   a) Restructure tcp:hhhh:pppp into   protocol=tcp,host=hhhh,port=pppp
>   b) Have a requirement that protocol= is the first entry in the list
>   c) If it doesn't start protocol= then it's the old format.
>   d) This would also change in the 'migrate' command to keep it symmetric
> 
> Eric/Daniel does this make sense for libvirt?
> 
> My current set of things I might want to add are:
>   1) A flag saying if a return channel is needed
>      For sockets qemu can open this afterwards when needed, but Dave Gibson's
>      review of my postcopy world pointed out that it might not be that
>      easy for all protocols to open the reverse later.
>   2) Flags for opening multiple sockets/FDs - e.g. to pass the pages down
>      a separate fd.
> 
> Thoughts?

The QEMU migration URI is exposed via the libvirt API to applications, so
they can control the host/port of the target explicitly (to override
libvirt's default guess of a suitable host/port).

These extra things that you suggest adding are not things we neccessarily
want to expose to applications. So if QEMU changed the format, libvirt
would probably not accept this new format from applications - we would
force applications to always use the URI syntax and only allow host+port
to be specified. Internally libvirt would translate that to the new
key/value pair format, and add the extra flags / options as needed. We
would possibly add some options to our public API to configure extra
features as desired, separately from the URI.

More generally though, what is the advantage of encoding new things in
the migration URI, as opposed to adding new parameters to the QMP
migration command. The use of URIs in this scenario is really a hang
over from days of HMP. If we were designing the migration command today
I don't think we'd use URIs at all - we'd just have a QMP command with
explicit parameters for the hostname, the port number and anything else
we might want to set.  So if there are new features I'd be inclined to
just add more optional parameters to the QMP migration command and not
touch the URI format at all.

Regards,
Daniel
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