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Firecracker Backend

The --fc backend: trade features for boot speed and density.

Overview

Cocoon supports Firecracker as an alternative hypervisor for workloads that prioritize boot speed and resource density.

# Run with Firecracker (--fc only needed for create/run/debug)
cocoon vm run --fc --name fast-vm ghcr.io/cocoonstack/cocoon/ubuntu:24.04

# Other commands auto-detect the backend — no --fc needed
cocoon vm list              # shows both CH and FC VMs
cocoon vm console fast-vm
cocoon vm stop fast-vm

# Clone infers backend from the snapshot
cocoon snapshot save fast-vm --name my-snap
cocoon vm clone my-snap --name clone-vm

Feature Comparison

Feature Cloud Hypervisor Firecracker
OCI images (direct boot) Y Y
Cloud images (UEFI boot) Y N
Windows guests Y N
Snapshot / Clone / Restore Y Y
Multi-queue networking Y N
Memory balloon Y Y
qcow2 storage Y N
Interactive console Y Y
HugePages Y Y
Boot time ~200-500ms ~125ms
Memory overhead ~10-20 MiB/VM <5 MiB/VM

Limitations

  • OCI images only: --fc is mutually exclusive with --windows and rejects cloudimg (UEFI boot) images
  • Raw disks only: Firecracker uses raw virtio-blk without serial support; disks are referenced by device path (/dev/vdX)
  • Single-queue networking: NetworkConfig.NumQueues is ignored
  • Snapshot portability requires same directory layout: FC snapshots store absolute paths in the vmstate binary (not patchable); cross-host export/import requires the target host to use the same root_dir/run_dir and have the same OCI image pulled
  • Console via PTY relay: a background relay process bridges FC's serial (stdin/stdout) to console.sock

OCI Image Compatibility

OCI images must include a resolve_disk() init script that supports device paths (e.g., /dev/vda) in addition to virtio serial names. Images built from os-image/ubuntu/overlay.sh (v0.3+) support both formats automatically.