Modal explains ImageFS, the lazy-loaded filesystem behind sub-second cold starts
josh_wills · x · 2026-10-17
Modal engineer Amit Patel details the secret to Modal's speed: a custom lazily loaded container filesystem, ImageFS, built on a massive tiered distributed filesystem layering local NVMe disks, regional and p2p caches, and global blob stores.
- With this month's VM Sandboxes launch, the team wanted VMs to get the same ImageFS benefits as its gVisor containers without sacrificing sub-second cold starts.
- They built a new filesystem stack that tricks a VM's ext4 driver into thinking it's talking to a normal disk.
- Modal's blog post covers the engineering challenges and decisions behind the design.
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