How to Fragment Your File System

File systems attempt to avoid aging, or fragmentation over time, by strategically allocating space for files. System implementers and users alike treat aging as a solved problem. Here, we present a realistic workload, based on Git, that can cause these best-guess file-block placement heuristics to fail, inducing large performance declines due to aging. This performance decline cannot be prevented with more caching or larger disks, and SSDs reduce but do not eliminate the aging effects. Our Git-based aging scheme can simulate a year of aging in under an hour. To make it easy for practitioners to incorporate aging into benchmarks, we have open-sourced our aging scripts at betrfs.org.