cloud-hypervisor/docs/snapshot_restore.md

Snapshot and Restore

The goal for the snapshot/restore feature is to provide the user with the ability to take a snapshot of a previously paused virtual machine. This snapshot can be used as the base for creating new identical virtual machines, without the need to boot them from scratch. The restore codepath takes the snapshot and creates the exact same virtual machine, restoring the previously saved states. The new virtual machine is restored in a paused state, as it was before the snapshot was performed.

Snapshot a Cloud Hypervisor VM

First thing, we must run a Cloud Hypervisor VM:

./cloud-hypervisor \
    --api-socket /tmp/cloud-hypervisor.sock \
    --cpus boot=4 \
    --memory size=4G \
    --kernel vmlinux \
    --cmdline "root=/dev/vda1 console=hvc0 rw" \
    --disk path=focal-server-cloudimg-amd64.raw

At any point in time when the VM is running, one might choose to pause it:

./ch-remote --api-socket=/tmp/cloud-hypervisor.sock pause

Once paused, the VM can be safely snapshot into the specified directory and using the following command:

./ch-remote --api-socket=/tmp/cloud-hypervisor.sock snapshot file:///home/foo/snapshot

Given the directory was present on the system, the snapshot will succeed and it should contain the following files:

ll /home/foo/snapshot/
total 4194536
drwxrwxr-x  2 foo bar       4096 Jul 22 11:50 ./
drwxr-xr-x 47 foo bar       4096 Jul 22 11:47 ../
-rw-------  1 foo bar       1084 Jul 22 11:19 config.json
-rw-------  1 foo bar 4294967296 Jul 22 11:19 memory-ranges
-rw-------  1 foo bar     217853 Jul 22 11:19 state.json

config.json contains the virtual machine configuration. It is used to create a similar virtual machine with the correct amount of CPUs, RAM, and other expected devices. It is stored in a human readable format so that it could be modified between the snapshot and restore phases to achieve some very special use cases. But for most cases, manually modifying the configuration should not be needed.

memory-ranges stores the content of the guest RAM.

state.json contains the virtual machine state. It is used to restore each component in the state it was left before the snapshot occurred.

Restore a Cloud Hypervisor VM

Given that one has access to an existing snapshot in /home/foo/snapshot, it is possible to create a new VM based on this snapshot with the following command:

./cloud-hypervisor \
    --api-socket /tmp/cloud-hypervisor.sock \
    --restore source_url=file:///home/foo/snapshot

Or using two different commands from two terminals:

# First terminal
./cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock

# Second terminal
./ch-remote --api-socket=/tmp/cloud-hypervisor.sock restore source_url=file:///home/foo/snapshot

Remember the VM is restored in a paused state, which was the VM's state when it was snapshot. For this reason, one must explicitly resume the VM before to start using it.

./ch-remote --api-socket=/tmp/cloud-hypervisor.sock resume

Alternatively, the resume option can be used to automatically resume the VM after restore completes:

./cloud-hypervisor \
    --api-socket /tmp/cloud-hypervisor.sock \
    --restore source_url=file:///home/foo/snapshot,resume=true

At this point, the VM is fully restored and is identical to the VM which was snapshot earlier.

Restore also supports selecting how guest memory is populated:

./cloud-hypervisor \
    --api-socket /tmp/cloud-hypervisor.sock \
    --restore source_url=file:///home/foo/snapshot,memory_restore_mode=ondemand

If memory_restore_mode is omitted, Cloud Hypervisor uses the eager-copy restore path (copy).

With memory_restore_mode=ondemand, restore uses userfaultfd to fault snapshot pages in on first access instead of copying the full memory-ranges file into guest RAM before restore completes. This mode is strict: if Cloud Hypervisor cannot enable the userfaultfd restore path, restore fails instead of falling back to copy.

Current constraints for memory_restore_mode=ondemand:

• prefault=on is not supported
• the snapshot memory ranges must be page-aligned

Restore a VM with new Net FDs

For a VM created with FDs explicitly passed to NetConfig, a set of valid FDs need to be provided along with the VM restore command in the following syntax:

# First terminal
./cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock

# Second terminal
./ch-remote --api-socket=/tmp/cloud-hypervisor.sock restore source_url=file:///home/foo/snapshot net_fds=[net1@[23,24],net2@[25,26]]

In the example above, the net device with id net1 will be backed by FDs '23' and '24', and the net device with id net2 will be backed by FDs '25' and '26' from the restored VM.

Limitations

VFIO devices is out of scope.

Offload Snapshot and Restore

Cloud Hypervisor can hand the snapshot payload off to a user-provided offload daemon instead of writing files to a file:// directory. The daemon can transform the payload on the fly (encrypt, compress, stream to object storage, etc.) without ever touching local disk.

There is no dedicated API surface for offload: the daemon talks to CH over the existing local live-migration protocol, playing the migration peer role:

• On snapshot, CH acts as the migration sender and the daemon acts as the receiver. The source VM shuts down on success, exactly as it would for a local live migration. Memory is transferred via SCM_RIGHTS, CH handing off the daemon one memfd per guest-memory slot.
• On restore, CH acts as the migration receiver and the daemon acts as the sender. The daemon provides one memfd per slot, populated from its storage, and CH uses those memfds directly as guest RAM backing.

In practice, this means offload is driven through the existing vm.send-migration / vm.receive-migration endpoints (with local=on and a unix:<path> URL). The daemon is just another peer of these endpoints. This requires the VM to be configured with shared-memory backing, which is the same precondition that applies to local live migration today.

Snapshot offload usage

# 1. Run a VM with shared memory.
./cloud-hypervisor \
    --api-socket /tmp/cloud-hypervisor.sock \
    --cpus boot=2 \
    --memory size=1G,shared=on \
    --kernel vmlinux \
    --cmdline "root=/dev/vda1 console=hvc0 rw" \
    --disk path=focal-server-cloudimg-amd64.raw

# 2. Start your offload daemon. The reference implementation is shipped as
#    `offload_daemon` and persists snapshot data to a local directory.
./offload_daemon snapshot \
    --socket /tmp/offload.sock \
    --output-dir /var/snapshots/vm1

# 3. Issue a local live migration to the daemon's socket. CH connects to
#    /tmp/offload.sock, streams the snapshot, and exits on success.
./ch-remote --api-socket /tmp/cloud-hypervisor.sock pause
./ch-remote --api-socket /tmp/cloud-hypervisor.sock \
    send-migration destination_url=unix:/tmp/offload.sock,local=on

Restore offload usage

# 1. Start a CH process.
./cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock

# 2. Tell CH to listen for an inbound migration from the offload daemon.
./ch-remote --api-socket /tmp/cloud-hypervisor.sock \
    receive-migration receiver_url=unix:/tmp/restore.sock &

# 3. Start the daemon in restore mode pointing at the same saved snapshot.
#    With --resume, the restored VM starts running on completion;
#    without it, the VM is left paused (issue `resume` to start it).
./offload_daemon restore \
    --socket /tmp/restore.sock \
    --input-dir /var/snapshots/vm1 \
    --resume

The daemon protocol

The daemon implements the local live-migration wire protocol defined in vm-migration/src/protocol.rs. Two state machines are involved:

• Snapshot mode (migration receiver): walk Start → MemoryFd (×N) → Config → State → CompletePaused. For each MemoryFd command, receive a guest-memory fd via SCM_RIGHTS on the same UNIX socket.
• Restore mode (migration sender): walk the same sequence in reverse, emitting one MemoryFd per slot (with the memfd attached via SCM_RIGHTS) before sending Config and State. Finish with either CompletePaused (restored VM remains paused) or Complete (restored VM resumes).

Critical invariant on snapshot

On the snapshot path, the daemon must finish reading from every memory fd before it ACKs CompletePaused. Cloud Hypervisor blocks at the CompletePaused handshake until the daemon ACKs. Once it ACKs, the source VM shuts down and the daemon's fds are the only remaining record of guest RAM. The reference daemon dumps each slot to disk and fsyncs before ACKing.

Reference daemon

The in-tree offload_daemon binary is intentionally minimal: it just serialises the snapshot to a local directory and replays it back. Its purpose is to back the offload integration test and to serve as a working example for daemon authors. Use it as a template, not a production backend.

Limitations

• The VM must use shared-memory backing (shared=on or file-backed). Anonymous memory is rejected with the same error message that local live migration produces.
• Orchestrator-supplied network FDs (today carried by vm.restore's net_fds field) are not plumbed through vm.receive-migration, so VMs whose configuration relies on externally-provided net FDs cannot currently be restored via the offload path.
• Confidential VMs (CVMs) inherit the live-migration restriction: offload is not supported for CVMs.