ext4 is the file system that powers most Linux servers. It organizes your hard drive into a superblock, inode tables, block bitmaps, and a write-ahead journal so your data survives crashes. localStorage is a tiny key-value store in your browser, designed to remember things like "dark mode: on." We reimplemented ext4's architecture on top of localStorage. Even a simple touch has to update the inode bitmap, write an inode, allocate a data block, update the parent directory, and sync the superblock. Hit "Crash" to simulate a crash mid-write. The journal detects the incomplete transaction and rolls back every block it touched.