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Engineering
9 min read
Booting a Linux VM in 400 milliseconds
Firecracker, a trimmed kernel and a lot of pre‑warming. A tour of what happens between the API call and the first shell prompt.
Where the time goes
A default cloud VM spends most of its boot in firmware, device enumeration and a full init system. We removed all three. Firecracker skips the BIOS, the guest kernel is compiled with only the drivers we ship, and init is a static binary that mounts the workspace and starts the agent supervisor.
The remaining time is dominated by pulling the root filesystem, which is why images are cached per region and snapshotted after first boot. Subsequent boots restore from the snapshot instead of running init at all.
