[libvirt] [RFC] Design: Tunnelled NBD block disk migration

Pavel Boldin pboldin at mirantis.com
Fri Jul 3 14:24:54 UTC 2015

Dear All,

I continue my work on the tunnelled selective block disks migration and the
next step is to implement Tunnelled NBD block disk migration.

As far as I see libvirt uses the following algorithm for tunnelling the
QEMU migration stream that is unidirectional:
1. The qemuMigrationStartTunnel (src/qemu/qemu_migration.c) starts a thread
that reads the data from the local QEMU pipe and writes it to the virStream
using virStreamSend.
2. The stream used by virStreamSend is embedded in the remote protocol
messaging by the remoteDomainMigratePrepareTunnel3{,Params}
3. Remote links the daemonStream with the virFDStream connected  to the
write end of a pipe linked with the QEMU.
4. Overall stream path is then: QEMU -(pipe)> virStreamSend -> (message
passing to the remote) -> TCP -> (message received by deamon/*.c code) ->
virStreamSend -> virFDStreamWrite -(pipe)> QEMU.

Given that I wonder what should be the changes required to implement a
bidirectional QEMU tunnelled connection for the NBD device disk migration.

The way I see implementation details at the moment:
0. The `virStream` and corresponding `daemon/*.c` must be modified in a way
that multiple streams can be multiplexed through the only one remote
1. `Pipe`s used to interconnect to the QEMU on the both sides are obviously
to be replaced by the UNIX sockets since the pipes cannot support
bidirectional output due to the design. This is to be made *one for each*
block device, requiring the above change.
2. The `qemuMigrationIOFunc` must be rewritten in such a way it polls for
both read and write on the UNIX sockets *and* VM migration pipe and
tunnells these to the appropriate `virStream`.
3. The receiving code must be modified in such a way to tunnel each of the
opened UNIX socket to the according virFDStream.

Of the mentioned above the most intriguing for me is the zero-th item which
will require some changes to the binary remote messaging protocol.

Is my vision on the problem is correct? Are there any other difficulties
I'm going to face but not aware of due to the lack of the familiarity with
the code?

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