Snapshots Guide

Save and restore VM state — crash recovery, fast boot, debugging.

Overview

SwarmCracker can snapshot a running Firecracker microVM and restore it later. A snapshot captures the full VM state: guest memory plus CPU/device state (Firecracker snapshot_type: Full). This requires Firecracker v1.14.0+ — the version installed by swarmcracker setup install.

Calling create pauses the VM, writes the snapshot, and the VM can then resume.

Use Cases

Use Case	Benefit
Crash recovery	Restore a VM to a known-good state
Fast boot	Resume from a snapshot faster than a cold boot
Debugging	Capture exact VM state at a point in time
Pre-update safety	Roll back a workload after a bad update

CLI Commands

Snapshots live under swarmcracker vm snapshot:

# Create a snapshot of a running VM (task)
swarmcracker vm snapshot create <task-id>

# With metadata (all optional)
swarmcracker vm snapshot create <task-id> \
  --service <service-id> \
  --node <node-id> \
  --rootfs /var/lib/firecracker/rootfs/<image>.ext4 \
  --vcpus 2 \
  --memory 512

# List snapshots (optionally filtered)
swarmcracker vm snapshot list
swarmcracker vm snapshot list --task <task-id>
swarmcracker vm snapshot list --service <service-id>
swarmcracker vm snapshot list --node <node-id>

# Restore a VM from a snapshot
swarmcracker vm snapshot restore <snapshot-id>

# Delete a snapshot
swarmcracker vm snapshot delete <snapshot-id>

# Remove snapshots older than a duration
swarmcracker vm snapshot cleanup --max-age 168h

create determines the Firecracker API socket from --socket (default: <socket-dir>/<task-id>.sock). restore can set a new socket with --socket.

Configuration

snapshot:
  enabled: true
  snapshot_dir: "/var/lib/firecracker/snapshots"
  max_snapshots: 3        # per service (0 = unlimited)
  max_age: 168h           # cleanup threshold (0 = unlimited)
  auto_snapshot: false    # snapshot automatically on start
  compress: false

Option	Default	Description
`enabled`	`true`	Enable the snapshot feature
`snapshot_dir`	`/var/lib/firecracker/snapshots`	Snapshot storage directory
`max_snapshots`	`3`	Max snapshots per service
`max_age`	`168h` (7 days)	Age threshold used by `cleanup`
`auto_snapshot`	`false`	Snapshot automatically on VM start
`compress`	`false`	Compress snapshot files

Snapshot Storage

Each snapshot gets its own ID and directory:

/var/lib/firecracker/snapshots/
└── snap-a1b2c3d4e5f67890/
    ├── vm.state      # VM state (~15 KB)
    ├── vm.mem        # Memory image (≈ VM RAM size)
    └── …             # metadata (JSON)

The metadata records the snapshot ID, task/service/node IDs, creation time, vCPU count, memory size, rootfs path, and a SHA-256 checksum of the state file.

Workflow Examples

Pre-Update Snapshot

# Find the task behind the service
swarmcracker service ps <service>

# Snapshot before updating
swarmcracker vm snapshot create <task-id>

# Update the service
swarmcracker service update <service> --image nginx:1.25-alpine

# If something breaks, restore
swarmcracker vm snapshot restore <snapshot-id>

Crash Recovery

# Snapshot before a risky operation
swarmcracker vm snapshot create <task-id>

# If the VM dies, restore it
swarmcracker vm snapshot restore <snapshot-id>

swarmctl Alternative

The lightweight swarmctl debug client (manager node only) can also manage snapshots. Note the name is a positional argument:

swarmctl snapshot create <task-id> <snapshot-name>
swarmctl snapshot list
swarmctl snapshot restore <snapshot-name>
swarmctl snapshot rm <snapshot-name>

Limitations

VM must be paused before snapshot (handled automatically by create).
Snapshots are node-local — they are not replicated across the cluster.
Size — the memory file is roughly the VM’s RAM size.
Rootfs path — the rootfs must be accessible at the same path on restore.
Firecracker version — requires v1.14.0+ for the current snapshot API.
Network state — active network connections may not survive a restore.

Troubleshooting

Snapshot Fails

# Check the VM/task is running
swarmcracker task ls
swarmcracker vm list

# Check the snapshot directory is writable and has space
ls -la /var/lib/firecracker/snapshots
df -h /var/lib/firecracker/snapshots

Restore Fails

# Verify the snapshot exists
swarmcracker vm snapshot list

# Confirm the files are present
ls /var/lib/firecracker/snapshots/<snapshot-id>/

Snapshots Too Large

# Create VMs with less memory
swarmcracker vm create --memory 256 alpine:latest

# Or reclaim space
swarmcracker vm snapshot cleanup --max-age 24h

See Also: Configuration | CLI Reference | Operations