feature suggestion: migration network

[Dan Kenigsberg]

There's talk about this for ages, so it's time to have proper discussion
and a feature page about it: let us have a "migration" network role, and
use such networks to carry migration data

When Engine requests to migrate a VM from one node to another, the VM
state (Bios, IO devices, RAM) is transferred over a TCP/IP connection
that is opened from the source qemu process to the destination qemu.
Currently, destination qemu listens for the incoming connection on the
management IP address of the destination host. This has serious
downsides: a "migration storm" may choke the destination's management
interface; migration is plaintext and ovirtmgmt includes Engine which
sits may sit the node cluster.

With this feature, a cluster administrator may grant the "migration"
role to one of the cluster networks. Engine would use that network's IP
address on the destination host when it requests a migration of a VM.
With proper network setup, migration data would be separated to that
network.

=== Benefit to oVirt ===
* Users would be able to define and dedicate a separate network for
  migration. Users that need quick migration would use nics with high
  bandwidth. Users who want to cap the bandwidth consumed by migration
  could define a migration network over nics with bandwidth limitation.
* Migration data can be limited to a separate network, that has no
  layer-2 access from Engine

=== Vdsm ===
The "migrate" verb should be extended with an additional parameter,
specifying the address that the remote qemu process should listen on. A
new argument is to be added to the currently-defined migration
arguments:
* vmId: UUID
* dst: management address of destination host
* dstparams: hibernation volumes definition
* mode: migration/hibernation
* method: rotten legacy
* ''New'': migration uri, according to http://libvirt.org/html/libvirt-libvirt.html#virDomainMigrateToURI2 such as tcp://<ip of migration network on remote node>

=== Engine ===
As usual, complexity lies here, and several changes are required:

1. Network definition.
1.1 A new network role - not unlike "display network" should be
    added.Only one migration network should be defined on a cluster.
1.2 If none is defined, the legacy "use ovirtmgmt for migration"
    behavior would apply.
1.3 A migration network is more likely to be a ''required'' network, but
    a user may opt for non-required. He may face unpleasant surprises if he
    wants to migrate his machine, but no candidate host has the network
    available.
1.4 The "migration" role can be granted or taken on-the-fly, when hosts
    are active, as long as there are no currently-migrating VMs.

2. Scheduler
2.1 when deciding which host should be used for automatic
    migration, take into account the existence and availability of the
    migration network on the destination host.
2.2 For manual migration, let user migrate a VM to a host with no
    migration network - if the admin wants to keep jamming the
    management network with migration traffic, let her.

3. VdsBroker migration verb.
3.1 For the a modern cluster level, with migration network defined on
    the destination host, an additional ''miguri'' parameter should be added
    to the "migrate" command