cloud-hypervisor/docs/live_migration.md

# Live Migration

This document gives examples of how to use the live migration support
in Cloud Hypervisor:

1. **Local Migration**: Migrating a VM from one Cloud Hypervisor instance to another on the same machine; also called
  UNIX socket migration.
1. **Remote Migration** (TCP Migration): migrating a VM between two TCP/IP hosts.

> :warning: These examples place sockets in /tmp. This is done for
> simplicity and should not be done in production.

## Local Migration (Suitable for Live Upgrade of VMM)

Launch the source VM (on the host machine):

```console
$ target/release/cloud-hypervisor
    --kernel ~/workloads/vmlinux \
    --disk path=~/workloads/focal.raw \
    --cpus boot=1 --memory size=1G,shared=on \
    --cmdline "root=/dev/vda1 console=ttyS0"  \
    --serial tty --console off --api-socket=/tmp/api1
```

Launch the destination VM from the same directory (on the host machine):

```console
$ target/release/cloud-hypervisor --api-socket=/tmp/api2
```

Get ready for receiving migration for the destination VM (on the host
machine):

```console
$ target/release/ch-remote --api-socket=/tmp/api2 receive-migration unix:/tmp/sock
```

Start to send migration for the source VM (on the host machine):

```console
$ target/release/ch-remote --api-socket=/tmp/api1 send-migration destination_url=unix:/tmp/sock,local=true
```

When the above commands completed, the source VM should be successfully
migrated to the destination VM. Now the destination VM is running while
the source VM is terminated gracefully.

## Remote Migration (TCP Migration)

_Hint: For developing purposes, same-host TCP migrations are also supported._

In this example, we will migrate a VM from one machine (`src`) to
another (`dst`) across the network. To keep it simple, we will use a
minimal VM setup without storage.

### Preparation

Make sure that `src` and `dst` can reach each other via the
network. You should be able to ping each machine. Also each machine
should have an open TCP port.

You will need a kernel and initramfs for a minimal Linux system. For
this example, we will use the Debian netboot image.

Place the kernel and initramfs into the _same directory_ on both
machines. This is important for the migration to succeed. We will use
`/var/images`:

```console
src $ export DEBIAN=https://ftp.debian.org/debian/dists/stable/main/installer-amd64/current/images/netboot/debian-installer/amd64
src $ mkdir -p /var/images
src $ curl $DEBIAN/linux > /var/images/linux
src $ curl $DEBIAN/initrd.gz > /var/images/initrd
```

Repeat the above steps on the destination host.

### Unix Socket Migration

If Unix socket is selected for migration, we can tunnel traffic through "socat".

#### Starting the Receiver VM

On the receiver side, we prepare an empty VM:

```console
dst $ cloud-hypervisor --api-socket /tmp/api
```

In a different terminal, configure the VM as a migration target:

```console
dst $ ch-remote --api-socket=/tmp/api receive-migration unix:/tmp/sock
```

In yet another terminal, forward TCP connections to the Unix domain socket:

```console
dst $ socat TCP-LISTEN:{port},reuseaddr UNIX-CLIENT:/tmp/sock
```

#### Starting the Sender VM

Let's start the VM on the source machine:

```console
src $ cloud-hypervisor \
        --serial tty --console off \
        --cpus boot=2 --memory size=4G \
        --kernel /var/images/linux \
        --initramfs /var/images/initrd \
        --cmdline "console=ttyS0" \
        --api-socket /tmp/api
```

After a few seconds the VM should be up and you can interact with it.

#### Performing the Migration

First, we start `socat`:

```console
src $ socat UNIX-LISTEN:/tmp/sock,reuseaddr TCP:{dst}:{port}
```

> Replace {dst}:{port} with the actual IP address and port of your destination host.

Then we kick-off the migration itself:

```console
src $ ch-remote --api-socket=/tmp/api send-migration unix:/tmp/sock
```

When the above commands completed, the VM should be successfully
migrated to the destination machine without interrupting the workload.

### TCP Socket Migration

If TCP socket is selected for migration, we need to consider migrating
in a trusted network.

#### Starting the Receiver VM

On the receiver side, we prepare an empty VM:

```console
dst $ cloud-hypervisor --api-socket /tmp/api
```

In a different terminal, prepare to receive the migration:

```console
dst $ ch-remote --api-socket=/tmp/api receive-migration tcp:0.0.0.0:{port}
```

#### Starting the Sender VM

Let's start the VM on the source machine:

```console
src $ cloud-hypervisor \
        --serial tty --console off \
        --cpus boot=2 --memory size=4G \
        --kernel /var/images/linux \
        --initramfs /var/images/initrd \
        --cmdline "console=ttyS0" \
        --api-socket /tmp/api
```

After a few seconds the VM should be up and you can interact with it.

#### Performing the Migration

Initiate the Migration over TCP:

```console
src $ ch-remote --api-socket=/tmp/api send-migration destination_url=tcp:{dst}:{port}
```

With migration parameters:

```console
src $ ch-remote --api-socket=/tmp/api send-migration destination_url=tcp:{dst}:{port},downtime_ms=200,timeout_s=3600,timeout_strategy=cancel
```

> Replace {dst}:{port} with the actual IP address and port of your destination host.

After completing the above commands, the source VM will be migrated to
the destination host and continue running there. The source VM instance
will terminate normally. All ongoing processes and connections within
the VM should remain intact after the migration.

#### Migration Parameters

Cloud Hypervisor supports additional parameters to control the
migration process. Via the API or `ch-remote`, you may specify:

- `downtime_ms <milliseconds>`: \
  The maximum downtime the migration aims for, in milliseconds.
  Defaults to `300ms`.
- `timeout_s <seconds>`: \
  The timeout for the migration (maximum total duration), in seconds.
  Defaults to `3600s` (one hour).
- `timeout_strategy <strategy>` (`[cancel, ignore]`): \
  The strategy to apply when the migration timeout is reached.
  Cancel will abort the migration and keep the VM running on the source.
  Ignore will proceed with the migration regardless of the downtime requirement.
  Defaults to `cancel`.
- `connections <amount>`: \
  The number of parallel TCP connections to use for migration.
  Must be between `1` and `128`. Defaults to `1`.
  Multiple connections are not supported with local UNIX-socket migration.

## Version Compatibility

Cloud Hypervisor live migration compatibility has two dimensions: the
Cloud Hypervisor version and the migration protocol version.

Cloud Hypervisor uses a versioned migration protocol for the messages exchanged
between source and destination. Each Cloud Hypervisor release sends its current
migration protocol version and accepts that version plus the immediately
previous protocol version.

This means migration is supported from an older protocol version to the same or
next protocol version. If the source and destination are more than one migration
protocol version apart, the VM must be migrated through an intermediate
Cloud Hypervisor version first.

### Technical Details

The source sends its migration protocol version in the initial `Start` request.
The destination accepts the migration only if that protocol version is supported.
A zeroed `Start` command header is handled as protocol `v0`, so older
deployments that do not explicitly send a protocol version remain compatible.

The migration protocol version covers the protocol spoken after a migration
connection has been established. Examples include adding a mandatory migration
command, changing the order of migration protocol messages, or changing the
framing or encoding of protocol command payloads.

The migration protocol version does not cover migration transport setup. For
example, choosing TCP vs. UNIX sockets or opening the initial connection needs
separate compatibility handling.

Migration protocol versioning is separate from snapshot state compatibility.
Device and VM state changes still need to be handled by the respective snapshot
serialization/deserialization code. That compatibility is Cloud Hypervisor
version dependent, but it is not the responsibility of migration protocol
versioning.