auto-fio — Automated Flexible I/O tester

Profile your storage device and get the optimal I/O block size — automatically.

Why

Block-sequential data pipelines (like CFM64) need one hardware-dependent number: the smallest block size that saturates your device's sequential-read throughput. Picking it by hand means running an fio sweep and eyeballing the knee of the curve. auto-fio automates exactly that — sweep, measure, detect the knee, return the number.

Install

Prerequisite: install fio. auto-fio drives the fio benchmark for its gold-standard measurements (it uses --direct=1 to bypass the OS page cache). Install it first so it's on your PATH:

# macOS
brew install fio
# Debian/Ubuntu
sudo apt install fio
# Fedora/RHEL
sudo dnf install fio

Then install auto-fio:

pip install auto-fio

Without fio, auto-fio falls back to a portable, zero-dependency python backend so a result is always available — but its numbers can be optimistic where the OS caches the file. Install fio for publication-grade accuracy. See Backends.

If fio is missing, auto-fio tells you at runtime (on stderr) with the exact install command for your OS, then continues on the python backend — it never installs anything or touches your system for you. --json stdout stays clean. Silence the notice with --backend python.

Use it — command line

auto-fio /path/on/the/device

auto-fio · backend=fio · threshold=95%

    512 KiB    1180.4 MB/s
      1 MiB    1910.2 MB/s
      2 MiB    2740.9 MB/s
      4 MiB    3120.5 MB/s
      7 MiB    3305.1 MB/s  <-- optimal
     16 MiB    3319.8 MB/s
     32 MiB    3322.0 MB/s

Optimal block size: 7340032 bytes (7.0 MiB) — reaches 95% of the 3322 MB/s peak.

Machine-readable output for scripts/CI: auto-fio /data --json.

Use it — Python

import auto_fio

# Just the number (bytes):
block_size = auto_fio.optimal_block_size("/data")

# Full sweep + metadata:
result = auto_fio.profile("/data", threshold=0.95)
print(result.optimal_block_size, result.peak_throughput_mbps, result.backend)

Backends

Force one with --backend fio / --backend python, or backend=... in Python. For publication-grade measurements, install fio and use the fio backend; the python backend exists so a result is always available and to cross-check fio.

How it decides — the "knee"

auto-fio measures throughput at each block size, finds the peak, and returns the smallest block size that reaches threshold (default 95%) of that peak. Smaller is better once saturated: it means fewer bytes buffered per read for the same bandwidth. This decision rule lives in a pure, unit-tested function (auto_fio.detect_knee) so it can be validated independently of any benchmark.

Companion to CFM64

import auto_fio
from cfm64 import CFM64Loader, TextBlockDataset

bs = auto_fio.optimal_block_size("/data/train")
dataset = TextBlockDataset("/data/train.csv", block_size_bytes=bs)
loader = CFM64Loader(dataset, batch_size=64, seed=42)

Run auto-fio once per machine, commit the number, and your pipeline is tuned to that hardware — no manual fio step.

License

MIT — see LICENSE.