Scaling Firecracker: Using OverlayFS to Save Disk Space

OverlayFS is a Linux filesystem that efficiently manages multiple Firecracker instances by layering one filesystem over another, creating a merged view without duplicating data. This technique uses a read-only lower directory containing the root filesystem and a writable upper directory for modifications, employing copy-on-write (COW) to maintain the integrity of the base system. By leveraging OverlayFS, users can significantly reduce storage requirements and avoid copying hundreds of megabytes for each instance, which is especially beneficial when running thousands of instances. The blog post explains how to set up OverlayFS with Firecracker, detailing the creation and configuration of Squashfs images for read-only filesystems and ext4 images for writable overlays. Additionally, it demonstrates configuring Firecracker to utilize OverlayFS, allowing for scalable and efficient instance management without the need for persistent storage, making it ideal for scenarios requiring numerous small instances.