Filesystem fragmentation is a first-order performance problem that has been the target of many heuristic and algorithmic approaches. Real-world application benchmarks show that common filesystem operations cause many filesystems to fragment over time, a phenomenon known as filesystem aging.

This paper examines the common assumption that space pressure will exacerbate fragmentation. Our microbenchmarks show that space pressure can cause a substantial amount of inter-file and intra-file fragmentation. However, on a “real-world” application benchmark, space pressure causes fragmentation that slows subsequent reads by only 20% on ext4, relative to the amount of fragmentation that would occur on a file system with abundant space. The other file systems show negligible additional degradation under space pressure.

Our results suggest that the effect of free-space fragmentation on read performance is best described as accelerating the filesystem aging process. The effect on write performance is non-existent in some cases, and, in most cases, an order of magnitude smaller than the read degradation from fragmentation cause by normal usage.


July, 2019


  • Alex Conway
  • Eric Knorr
  • Yizheng Jiao
  • Michael A. Bender
  • William Jannen
  • Rob Johnson
  • Don Porter
  • Martin Farach-Colton

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Research Areas

  • File systems