On 07/29/2011 09:03 AM, Kevin Wolf wrote:
Am 26.07.2011 14:37, schrieb Anthony Liguori:
Hrm, I'm not sure I agree with these conclusions.
Management tools should do their best job to create two compatible
device models.
Given two compatible device models, there *may* be differences in the
structure of the device models since we evolve things over time. We may
rename a field, change the type, etc. To support this, we can use
filters both on the destination and receive end to do our best to
massage the device model into something compatible.
But creating two creating compatible device models is not the job of the
migration protocol. It's the job of management tools.
I'm not sure if I agree with this.
Let's forget about management tools for a moment, and just think of a
qemu instance with a given set of command line option describing its
devices. Then you start another instance with different options and
-incoming and start a migration. The result will be something, but
definitely not a successfully migrated VM (even though it might look
like one at first).
This is why I believe that the information about which devices to use
actually belongs into the migration data. There's no way to make use of
it with different options.
I agree with you actually.
Right now, it's the management tools job. The complexity is daunting.
Recreating the same object model, particularly after hotplug, is
difficult and in many cases impossible.
I think the primary problem is that there are so many ways to create
things. -M pc creates a bunch of stuff that there's no way to create
individually. The stuff it creates can sort of be manipulated by using
-global but not on a per device basis. Much of it isn't even addressable.
Creating backends is a totally different mechanism and each backend has
different mechanisms to enumerate and create.
The result is that introspecting what's there and recreating it is
insanely complex today.
That's the motivation behind QOM. plug_list lists *everything*. All
objects, whether they are created as part of the PIIX3 or whether it's a
backing file, can be directly addressed and manipulated.
If you look at qsh, there's an import command. The export command is
trivial and I don't remember if I've already added it. But the point is
that you should be able to 'qsh export' everything and then 'qsh import'
everything to create the exact same device model in another QEMU instance.
And yeah, this should end up becoming part of the migration protocol.
5) Once we're here, we can implement the next 5-year format. That
could be ASN.1 and be bidirectional or whatever makes the most sense.
We could support 50 formats if we wanted to. As long as the transport
is distinct from the serialization and compat routines, it really
doesn't matter.
This means finishing the VMState support, once there, only thing needs
to change is "copy" the savevm, and change the "visitors" to whatever
else that we need/want.
There's no need to "finish" VMState to convert to visitors. It's just
sed -e 's:qemu_put_be32:visit_type_int32:g'
Actually I think the real question is whether we want to have VMState or
not.
VMState doesn't give me what I want by itself.
I want to be able to marshal the device tree to an in-memory
representation that can be manipulated. One approach to doing that is
first completing VMState, and then writing something that can walk the
VMState descriptions. The VMState descriptions are fairly complicated
but it's doable.
Another approach, which I'm arguing is much simpler, the imperative
nature of our current serialization and use visitors.
There may be other advantages of a declarative description of VMState
that would justify completing the conversions. But I don't think we
need it to start improving the migration protocol.
Regards,
Anthony Liguori
