Firecracker is AWS’s open-source virtualization technology that powers Lambda, Fargate, and other serverless services. It boots microVMs in under 125ms with minimal memory overhead (~5MB), combining container-like density with VM-level security isolation.

The key insight: most serverless workloads don’t need full VMs with BIOS, bootloaders, and hundreds of device drivers. Strip that away, keep just KVM and a minimal device model, and you get Firecracker—fast enough to spin up per-request, secure enough for multi-tenant platforms.

I first experimented with Firecracker building a code execution sandbox. Docker containers weren’t isolated enough (shared kernel), and traditional VMs were too slow (10+ second boot times). Firecracker hit the sweet spot: VM security with near-container speed.

Architecture

Firecracker is built on KVM (Kernel-based Virtual Machine) and uses Rust for memory safety. Each microVM runs its own kernel and minimal device model:

  • virtio devices: Network, block storage (fast paravirtualized I/O)
  • Rate limiters: CPU and I/O rate limiting per VM
  • Jailer: Additional security layer (seccomp, cgroups, namespaces)
  • API-driven: HTTP API for VM lifecycle management

No BIOS, no PCI bus, no legacy devices. Just what’s needed for running workloads.

Read the Firecracker design document for architectural details.

Getting Started

Install Firecracker (requires Linux with KVM):

# Download latest release
ARCH=$(uname -m)
VERSION="v1.7.0"
curl -LOJ https://github.com/firecracker-microvm/firecracker/releases/download/${VERSION}/firecracker-${VERSION}-${ARCH}.tgz

tar -xzf firecracker-${VERSION}-${ARCH}.tgz
sudo mv release-${VERSION}-${ARCH}/firecracker-${VERSION}-${ARCH} /usr/local/bin/firecracker
sudo mv release-${VERSION}-${ARCH}/jailer-${VERSION}-${ARCH} /usr/local/bin/jailer

# Verify
firecracker --version

Create a MicroVM

Firecracker uses an HTTP API. Start the API server:

# Remove old socket if exists
rm -f /tmp/firecracker.socket

# Start Firecracker
firecracker --api-sock /tmp/firecracker.socket

In another terminal, configure and start the VM:

# Set boot source (kernel)
curl --unix-socket /tmp/firecracker.socket \
    -X PUT 'http://localhost/boot-source' \
    -H 'Content-Type: application/json' \
    -d '{
        "kernel_image_path": "/path/to/vmlinux",
        "boot_args": "console=ttyS0 reboot=k panic=1 pci=off"
    }'

# Set root filesystem
curl --unix-socket /tmp/firecracker.socket \
    -X PUT 'http://localhost/drives/rootfs' \
    -H 'Content-Type: application/json' \
    -d '{
        "drive_id": "rootfs",
        "path_on_host": "/path/to/rootfs.ext4",
        "is_root_device": true,
        "is_read_only": false
    }'

# Configure resources
curl --unix-socket /tmp/firecracker.socket \
    -X PUT 'http://localhost/machine-config' \
    -H 'Content-Type: application/json' \
    -d '{
        "vcpu_count": 2,
        "mem_size_mib": 1024
    }'

# Start the VM
curl --unix-socket /tmp/firecracker.socket \
    -X PUT 'http://localhost/actions' \
    -H 'Content-Type: application/json' \
    -d '{"action_type": "InstanceStart"}'

The VM boots in ~125ms. You’re now running a fully isolated microVM.

Pre-built Images

Use Firecracker’s CI scripts or pre-built images:

# Download Alpine Linux rootfs and kernel
curl -o vmlinux https://s3.amazonaws.com/spec.ccfc.min/img/quickstart_guide/x86_64/kernels/vmlinux.bin
curl -o rootfs.ext4 https://s3.amazonaws.com/spec.ccfc.min/img/quickstart_guide/x86_64/rootfs/bionic.rootfs.ext4

For production, build custom images with your application baked in. Tools like Packer work well.

Production Best Practices

After running Firecracker workloads at scale:

  1. Use the Jailer - Adds seccomp, cgroups, namespaces for defense-in-depth. Required for multi-tenant environments:
jailer --id microvm-1 \
    --exec-file /usr/local/bin/firecracker \
    --uid 123 --gid 456 \
    -- --api-sock /run/firecracker.socket

  1. Optimize boot time - Use minimal kernels and small rootfs. AWS’s production Lambda images boot in ~85ms.

  2. Resource limits - Set CPU and memory limits per VM:

{
  "vcpu_count": 2,
  "mem_size_mib": 1024,
  "cpu_template": "T2",  // Intel-specific optimizations
  "track_dirty_pages": true  // For live migration/snapshots
}
  1. Snapshots for faster starts - Create snapshots of fully booted VMs and restore them (< 5ms):
# Create snapshot
curl --unix-socket /tmp/firecracker.socket \
    -X PUT 'http://localhost/snapshot/create' \
    -d '{"snapshot_path": "/path/to/snapshot", "mem_file_path": "/path/to/mem"}'

# Restore from snapshot (much faster than boot)
firecracker --api-sock /tmp/firecracker.socket \
    --config-file /path/to/snapshot-config.json

  1. Monitor and observe - Track VM lifecycle, resource usage, and errors. Use Firecracker’s metrics.

  2. Network setup - Use TAP devices for networking:

# Create TAP device
sudo ip tuntap add tap0 mode tap
sudo ip addr add 172.16.0.1/24 dev tap0
sudo ip link set tap0 up

# Configure in Firecracker
curl --unix-socket /tmp/firecracker.socket \
    -X PUT 'http://localhost/network-interfaces/eth0' \
    -d '{
        "iface_id": "eth0",
        "guest_mac": "AA:FC:00:00:00:01",
        "host_dev_name": "tap0"
    }'
  1. Rate limiting - Prevent noisy neighbors:
{
  "bandwidth": {
    "size": 125000,  // Bytes
    "refill_time": 1000  // Milliseconds
  },
  "ops": {
    "size": 1000,
    "refill_time": 1000
  }
}
  1. Scale horizontally - Run many microVMs per host. AWS runs thousands per bare metal server.

When to Use Firecracker

Good fit:

  • Serverless platforms (function-as-a-service)
  • CI/CD runners (isolated build environments)
  • Code execution sandboxes (online IDEs, notebook kernels)
  • Multi-tenant SaaS (untrusted user code)

Not ideal:

  • Desktop applications (requires Linux KVM)
  • Long-running services (containers are simpler)
  • Windows guests (KVM is Linux-only)
  • Legacy apps needing full hardware emulation

Tools and Ecosystem

Conclusion

Firecracker proves you can have both security and speed. By rethinking virtualization for cloud-native workloads, AWS created something genuinely novel—microVMs that boot as fast as containers with VM-level isolation.

If you’re building multi-tenant platforms, serverless runtimes, or secure execution environments, Firecracker is worth serious evaluation. The operational complexity is real (you’re managing VMs), but the security and isolation guarantees are unmatched.

Further Reading:

Firecracker microVMs from April 2024, covering secure virtualization and fast boot times.