Migration has two pieces, a "source", that is, the host that already has the
instance, and a "sink", the host that's getting the instance. Currently,
pull mode, the source sets up an operation, and the sink connects
to the source and pulls the instance.
There are three websockets (channels) used in migration:
- the control stream
- the criu images stream
- the filesystem stream
When a migration is initiated, information about the instance, its configuration, etc. are sent over the control channel (a full description of this process is below), the criu images and instance filesystem are synced over their respective channels, and the result of the restore operation is sent from the sink to the source over the control channel.
In particular, the protocol that is spoken over the criu channel and filesystem channel can vary, depending on what is negotiated over the control socket. For example, both the source and the sink's LXD directory is on btrfs, the filesystem socket can speak btrfs-send/receive. Additionally, although we do a "stop the world" type migration right now, support for criu's p.haul protocol will happen over the criu socket at some later time.
Once all three websockets are connected between the two endpoints, the
source sends a MigrationHeader (protobuf description found in
/lxd/migration/migrate.proto). This header contains the instance
configuration which will be added to the new instance.
There are also two fields indicating the filesystem and criu protocol to speak.
For example, if a server is hosted on a btrfs filesystem, it can indicate that it
wants to do a
btrfs send instead of a simple rsync (similarly, it could
indicate that it wants to speak the p.haul protocol, instead of just rsyncing
the images over slowly).
The sink then examines this message and responds with whatever it supports. Continuing our example, if the sink is not on a btrfs filesystem, it responds with the lowest common denominator (rsync, in this case), and the source is to send the root filesystem using rsync. Similarly with the criu connection; if the sink doesn't have support for the p.haul protocol (or whatever), we fall back to rsync.