Samej Spenser

FAT32, the file system your USB sticks and SD cards have been quietly putting up with since the 90s, is getting up to a 265x speedup in Linux 7.4. And the fix is basically "stop doing it one at a time." Matheus Alves de Almeida from UFRGS in Brazil noticed that when you preallocate space on FAT (fallocate with FALLOC_FL_KEEP_SIZE), the kernel grabbed clusters one by one in a while loop. On a sync mount, that means writing the FAT table back to disk for every single cluster. Tens of thousands of tiny trips to the disk just to reserve a few megabytes. His patch swaps the loop for a single fat_add_clusters() call. Net change in fs/fat/file.c: +4 lines, -6 lines. The numbers: sixteen 16 MiB files with 4 KiB clusters on a sync mount went from 92.489 seconds to 0.348 seconds. Even without sync, it's 5x to 19x faster. In his words: "The improvement is especially large on synchronous mounts, where the new code can batch FAT updates instead of synchronously handling each cluster allocation separately." It won't make ordinary file copies faster. It's preallocation only, so think embedded devices, cameras and anything that reserves space up front. Andrew Morton sent it to Linus in his non-MM pull, and it's now in Linux 7.4.