FiO

fio, short for Flexible I/O tester, is an open-source storage I/O benchmarking tool. It is designed to simulate a variety of I/O workloads on block devices and files, across multiple operating systems, to measure performance characteristics such as IOPS, bandwidth, and latency.

fio uses pluggable I/O engines, including libaio (Linux native asynchronous I/O), posixaio (POSIX AIO), sync, and mmap, among others. It can run with multiple simultaneous jobs, use different I/O sizes, and support different patterns such as sequential and random reads/writes (read, write, randread, randwrite, randrw, readwrite). It supports time-based workloads, per-job and per-queue depth configurations (iodepth), and options to constrain or vary CPU usage, caching, and file system effects.

Workloads are described in a job file or on the command line. A typical test specifies parameters such as size, block size (bs), I/O depth (iodepth), number of jobs (numjobs), the I/O pattern (rw), and runtime or total data to transfer. fio reports metrics including IOPS, bandwidth, latency percentiles, and per-job statistics. It can direct I/O to real devices or to files, and can use direct I/O to bypass cache (direct=1).

Developed by Jens Axboe and others, fio is cross-platform with primary support on Linux. It is maintained as open-source software under the GNU General Public License, with releases available on multiple platforms, including Linux, BSD, and macOS.

Because results depend on hardware, kernel version, caching, and workload mix, fio tests should be designed to reflect real-world usage and repeated under controlled conditions. It is widely used in storage testing, capacity planning, and performance tuning of disks, SSDs, NVMe devices, and filesystems.