We made our filesystem 47× faster by deleting it

Summary

A user in our Discord said microsandbox felt slow. Listing every file in the Python standard library took 5.3 seconds inside a sandbox; in Docker it took milliseconds. We went digging. We fixed it in v0.4: we replaced our user-space filesystem with a Linux disk image that the VM mounts directly. The geometric mean speedup across our mixed guest-visible filesystem suite is 47×, with the worst-case rows more than 1,000× faster, and the host filesystem code is about 5,300 lines shorter. My first try was monofs: a content-addressed filesystem with block-level dedup, compression, and distributed read replicas. It stored images at 1.3× their original size on disk, and microsandbox is local-first, so the long-tail dedup payoff wasn't worth the up-front cost. For v0.3 I switched to OCI plus a user-space overlay built on a libkrun hook; we got layer dedup and identical behavior on Linux and macOS, but everything still ran outside the kernel. Every file operation inside the VM had to bounce out to the host through FUSE, which is Linux's mechanism for letting an ordinary program act as a filesystem. To open a file, the VM hands the request to our host process, which walks every layer looking for the file and sends the answer back; the same trip happens for every stat, every readdir, and every cache miss. A single Python import triggers dozens of these round trips before your code even starts running, and a ten-layer image multiplies the cost of each one. We spent the next stretch of v0.3 trying to make that path faster: better caching, fewer syscalls, smaller responses. Each change shaved a few percent. None of them changed the order of magnitude. Docker doesn't have this problem because Docker uses the kernel's own layered-filesystem driver (overlayfs), so file operations never leave the kernel. We were trying to match a kernel filesystem from outside the kernel; no cache could close that gap. So we deleted the filesystem. The new plan was to stop bouncing every file operatio...