Skip to main content


The Flexible I/O Tester (fio) was originally written as a test tool for the kernel block I/O stack. Over the years, fio however gained many features and detailed performance statistics output that turned this tool into a standard benchmark application for storage devices.

fio source code is available on GitHub.

fio Zoned Block Device Support

Support for zoned block devices was added to fio with version 3.9. All previous versions do not provide guarantees for write command ordering with host managed zoned block devices. Executing workloads is still possible, but requires writing complex fio scripts.

Command Line Options

Changes to fio to support zoned block devices include several new options allowing a user to control zoned block device compliant workloads. fio already implemented the option --zonemode which allows defining workloads operating on disjoint ranges of blocks. This option was reused to define the new zbd zone mode.

When the zbd zone mode is used by an fio job, the --zonerange option is ignored and the --zonesize option is automatically set to the device zone size. Furthermore, the behavior of read and write commands is modified as follows.

  • Read and write commands are split when a zone boundary is crossed.

  • For sequential writes, the write stream is always started from a zone write pointer position. If the next zone to be written is not empty, the write stream "jumps" to that zone write pointer and resumes.

  • For random write workloads, write commands are always issued at the write pointer position of the target zone of the write command.

  • Any write command targeting a sequential zone that is full (entirely written) will trigger a reset of the zone write pointer before issuing the write I/O.

  • By default, all read commands always target written sectors, that is, sectors between the start sector and the write pointer position of sequential write zones. This behavior can be disabled, allowing read commands to be issued to any sector, using the new option read_beyond_wp.

Additionally, finer control over the workload operation can be added with the following new options.

  • --max_open_zones This option limits the number of zones that are being written by a workload. With this option, a random write workload cannot issue write commands targeting more zones than the limit set. Once a zone that is being written becomes full, another zone is chosen and writes are allowed to target the zone, resulting in a constant number of zones being written always at most equal to the max_open_zones limit.

  • --zone_reset_threshold and --zone_reset_frequency These two options allow a user to emulate the execution of zone reset commands being issued by an application.

In addition to these options, the zbd zone mode automatically enables job synchronization to ensure that a workload spanning multiple threads or processes can concurrently execute write I/Os targeting the same zone.

To indicate a zone unit for integer parameters, the z suffix can be used (e.g. --offset_increment=8z, --size=8z, ...).


As discussed in the kernel support document, using direct write I/O is mandatory for zoned block devices. The zbd zone mode, when enabled, enforces this requirement by checking that the option --direct=1 is specified for any job executing write I/Os.

The --offset and --size options must specify values that are aligned to the device zone size.

The --numjobs parameter should not exceed the --max_open_zones option for write workloads, otherwise jobs that do not obtain their required resources (e.g. opening a zone) immediately quit silently.