A small change to the Linux FAT driver, included in Linux 7.4, substantially speeds up a specific file-allocation operation on FAT32. When software pre-allocates disk space using FALLOC_FL_KEEP_SIZE, it reserves storage without increasing the file’s reported length. Previously, the driver handled this by repeatedly allocating one cluster at a time as the file’s reserved space grew. Developer Matheus Alves de Almeida changed the allocation path so it can reserve multiple clusters in a single operation.
Benchmarks shared by Almeida show improvements of up to 265 times, with gains across the tested cases. The result comes from reducing the repeated work involved in cluster-by-cluster allocation, rather than changing FAT32’s on-disk format or the behavior of the KEEP_SIZE flag. The patch is only a few lines long and was merged into Linux 7.4 through a non-memory-management pull request submitted by Andrew Morton. The reported speed-up applies to this particular pre-allocation workload; it does not indicate that all FAT32 operations or general file access are hundreds of times faster.
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