Recently on this blog, I wrote about in-memory filesystems in Rust, and concluded that I wasn’t able to detect a difference between any form of in-memory filesystem and using a regular SSD on macOS. I also asked anyone who found a counterexample to please let me know. Last week, David Barsky of ERSC sent me an extremely compelling counter-example, and I spent several days running benchmarks to understand it better. The top level summary is that the test suite for the jj VCS exhibits an absolutely huge difference between running on an SSD and running against a ramdisk. In my first reproduction attempt, I found the SSD took 239 seconds, while the ramdisk took just 37 seconds. That’s bananas! How was that even possible?