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.
2. Remote Migration (TCP Migration): migrating a VM between two TCP/IP hosts.

⚠️ 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):

$ 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):

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

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

$ 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):

$ 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:

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:

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

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

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

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

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

Starting the Sender VM

Let's start the VM on the source machine:

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:

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:

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:

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

In a different terminal, prepare to receive the migration:

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:

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:

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

With migration parameters:

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.