fio man page

fio — flexible I/O tester


fio [options] [jobfile]...


fio is a tool that will spawn a number of threads or processes doing a particular type of I/O action as specified by the user. The typical use of fio is to write a job file matching the I/O load one wants to simulate.



Enable verbose tracing type of various fio actions. May be `all' for all types or individual types separated by a comma (e.g. `--debug=file,mem' will enable file and memory debugging). `help' will list all available tracing options.


Parse options only, don't start any I/O.


Write output to filename.


Set the reporting format to `normal', `terse', `json', or `json+'. Multiple formats can be selected, separate by a comma. `terse' is a CSV based format. `json+' is like `json', except it adds a full dump of the latency buckets.


Limit run time to runtime seconds.


Generate aggregate bandwidth logs.


Print statistics in a terse, semicolon-delimited format.


Print statistics in selected mode AND terse, semicolon-delimited format. Deprecated, use --output-format instead to select multiple formats.


Set terse version output format (default `3', or `2', `4', `5').


Print version information and exit.


Print a summary of the command line options and exit.


Perform test and validation of internal CPU clock.


Test the speed of the built-in checksumming functions. If no argument is given, all of them are tested. Alternatively, a comma separated list can be passed, in which case the given ones are tested.


Print help information for command. May be `all' for all commands.


List all commands defined by ioengine, or print help for command defined by ioengine. If no ioengine is given, list all available ioengines.


Convert jobfile to a set of command-line options.


Turn on safety read-only checks, preventing writes. The --readonly option is an extra safety guard to prevent users from accidentally starting a write workload when that is not desired. Fio will only write if `rw=write/randwrite/rw/randrw' is given. This extra safety net can be used as an extra precaution as --readonly will also enable a write check in the I/O engine core to prevent writes due to unknown user space bug(s).


Specifies when real-time ETA estimate should be printed. when may be `always', `never' or `auto'.


Force a new line for every time period passed. When the unit is omitted, the value is interpreted in seconds.


Force full status dump every time period passed. When the unit is omitted, the value is interpreted in seconds.


Only run specified section name in job file. Multiple sections can be specified. The --section option allows one to combine related jobs into one file. E.g. one job file could define light, moderate, and heavy sections. Tell fio to run only the "heavy" section by giving `--section=heavy' command line option. One can also specify the "write" operations in one section and "verify" operation in another section. The --section option only applies to job sections. The reserved *global* section is always parsed and used.


Set the internal smalloc pool size to kb in KiB. The --alloc-size switch allows one to use a larger pool size for smalloc. If running large jobs with randommap enabled, fio can run out of memory. Smalloc is an internal allocator for shared structures from a fixed size memory pool and can grow to 16 pools. The pool size defaults to 16MiB. NOTE: While running `.fio_smalloc.*' backing store files are visible in `/tmp'.


All fio parser warnings are fatal, causing fio to exit with an error.


Set the maximum number of threads/processes to support to nr.


Start a backend server, with args specifying what to listen to. See CLIENT/SERVER section.


Background a fio server, writing the pid to the given pidfile file.


Instead of running the jobs locally, send and run them on the given hostname or set of hostnames. See CLIENT/SERVER section.


Tell fio server to load this local file.


Report CPU idleness. option is one of the following:


Run unit work calibration only and exit.


Show aggregate system idleness and unit work.


As system but also show per CPU idleness.


Inflate and output compressed log.


Execute trigger command when file exists.


Execute trigger at this time.


Set this command as local trigger.


Set this command as remote trigger.


Use this path for fio state generated files.

Job File Format

Any parameters following the options will be assumed to be job files, unless they match a job file parameter. Multiple job files can be listed and each job file will be regarded as a separate group. Fio will stonewall execution between each group.

Fio accepts one or more job files describing what it is supposed to do. The job file format is the classic ini file, where the names enclosed in [] brackets define the job name. You are free to use any ASCII name you want, except *global* which has special meaning. Following the job name is a sequence of zero or more parameters, one per line, that define the behavior of the job. If the first character in a line is a ';' or a '#', the entire line is discarded as a comment.

A *global* section sets defaults for the jobs described in that file. A job may override a *global* section parameter, and a job file may even have several *global* sections if so desired. A job is only affected by a *global* section residing above it.

The --cmdhelp option also lists all options. If used with an command argument, --cmdhelp will detail the given command.

See the `examples/' directory for inspiration on how to write job files. Note the copyright and license requirements currently apply to `examples/' files.

Job File Parameters

Some parameters take an option of a given type, such as an integer or a string. Anywhere a numeric value is required, an arithmetic expression may be used, provided it is surrounded by parentheses. Supported operators are:

addition (+)

subtraction (-)

multiplication (*)

division (/)

modulus (%)

exponentiation (^)

For time values in expressions, units are microseconds by default. This is different than for time values not in expressions (not enclosed in parentheses).

Parameter Types

The following parameter types are used.


String. A sequence of alphanumeric characters.


Integer with possible time suffix. Without a unit value is interpreted as seconds unless otherwise specified. Accepts a suffix of 'd' for days, 'h' for hours, 'm' for minutes, 's' for seconds, 'ms' (or 'msec') for milliseconds and 'us' (or 'usec') for microseconds. For example, use 10m for 10 minutes.


Integer. A whole number value, which may contain an integer prefix and an integer suffix.

[*integer prefix*] **number** [*integer suffix*]

The optional *integer prefix* specifies the number's base. The default is decimal. *0x* specifies hexadecimal.

The optional *integer suffix* specifies the number's units, and includes an optional unit prefix and an optional unit. For quantities of data, the default unit is bytes. For quantities of time, the default unit is seconds unless otherwise specified.

With `kb_base=1000', fio follows international standards for unit prefixes. To specify power-of-10 decimal values defined in the International System of Units (SI):

K means kilo (K) or 1000

M means mega (M) or 1000**2

G means giga (G) or 1000**3

T means tera (T) or 1000**4

P means peta (P) or 1000**5

To specify power-of-2 binary values defined in IEC 80000-13:

Ki means kibi (Ki) or 1024

Mi means mebi (Mi) or 1024**2

Gi means gibi (Gi) or 1024**3

Ti means tebi (Ti) or 1024**4

Pi means pebi (Pi) or 1024**5

With `kb_base=1024' (the default), the unit prefixes are opposite from those specified in the SI and IEC 80000-13 standards to provide compatibility with old scripts. For example, 4k means 4096.

For quantities of data, an optional unit of 'B' may be included (e.g., 'kB' is the same as 'k').

The *integer suffix* is not case sensitive (e.g., m/mi mean mebi/mega, not milli). 'b' and 'B' both mean byte, not bit.

Examples with `kb_base=1000':

4 KiB: 4096, 4096b, 4096B, 4k, 4kb, 4kB, 4K, 4KB

1 MiB: 1048576, 1m, 1024k

1 MB: 1000000, 1mi, 1000ki

1 TiB: 1073741824, 1t, 1024m, 1048576k

1 TB: 1000000000, 1ti, 1000mi, 1000000ki

Examples with `kb_base=1024' (default):

4 KiB: 4096, 4096b, 4096B, 4k, 4kb, 4kB, 4K, 4KB

1 MiB: 1048576, 1m, 1024k

1 MB: 1000000, 1mi, 1000ki

1 TiB: 1073741824, 1t, 1024m, 1048576k

1 TB: 1000000000, 1ti, 1000mi, 1000000ki

To specify times (units are not case sensitive):

D means days

H means hours

M mean minutes

s or sec means seconds (default)

ms or msec means milliseconds

us or usec means microseconds

If the option accepts an upper and lower range, use a colon ':' or minus '-' to separate such values. See irange parameter type. If the lower value specified happens to be larger than the upper value the two values are swapped.


Boolean. Usually parsed as an integer, however only defined for true and false (1 and 0).


Integer range with suffix. Allows value range to be given, such as 1024-4096. A colon may also be used as the separator, e.g. 1k:4k. If the option allows two sets of ranges, they can be specified with a ',' or '/' delimiter: 1k-4k/8k-32k. Also see int parameter type.


A list of floating point numbers, separated by a ':' character.

Job Parameters

With the above in mind, here follows the complete list of fio job parameters.



Select the interpretation of unit prefixes in input parameters.


Inputs comply with IEC 80000-13 and the International System of Units (SI). Use:

  • power-of-2 values with IEC prefixes (e.g., KiB)
  • power-of-10 values with SI prefixes (e.g., kB)

Compatibility mode (default). To avoid breaking old scripts:

- power-of-2 values with SI prefixes

- power-of-10 values with IEC prefixes

See bs for more details on input parameters.

Outputs always use correct prefixes. Most outputs include both side-by-side, like:

bw=2383.3kB/s (2327.4KiB/s)

If only one value is reported, then kb_base selects the one to use:

1000 -- SI prefixes

1024 -- IEC prefixes


Base unit for reporting. Allowed values are:


Use auto-detection (default).


Byte based.


Bit based.

Job description


ASCII name of the job. This may be used to override the name printed by fio for this job. Otherwise the job name is used. On the command line this parameter has the special purpose of also signaling the start of a new job.


Text description of the job. Doesn't do anything except dump this text description when this job is run. It's not parsed.


Run the specified number of iterations of this job. Used to repeat the same workload a given number of times. Defaults to 1.


Create the specified number of clones of this job. Each clone of job is spawned as an independent thread or process. May be used to setup a larger number of threads/processes doing the same thing. Each thread is reported separately; to see statistics for all clones as a whole, use group_reporting in conjunction with new_group. See --max-jobs. Default: 1.

Target file/device


Prefix filenames with this directory. Used to place files in a different location than `./'. You can specify a number of directories by separating the names with a ':' character. These directories will be assigned equally distributed to job clones created by numjobs as long as they are using generated filenames. If specific filename(s) are set fio will use the first listed directory, and thereby matching the filename semantic which generates a file each clone if not specified, but let all clones use the same if set.

See the filename option for information on how to escape ':' and '´ characters within the directory path itself.


Fio normally makes up a filename based on the job name, thread number, and file number (see filename_format). If you want to share files between threads in a job or several jobs with fixed file paths, specify a filename for each of them to override the default. If the ioengine is file based, you can specify a number of files by separating the names with a ':' colon. So if you wanted a job to open `/dev/sda' and `/dev/sdb' as the two working files, you would use `filename=/dev/sda:/dev/sdb'. This also means that whenever this option is specified, nrfiles is ignored. The size of regular files specified by this option will be size divided by number of files unless an explicit size is specified by filesize.

Each colon and backslash in the wanted path must be escaped with a '´ character. For instance, if the path is `/dev/dsk/foo@3,0:c' then you would use `filename=/dev/dsk/foo@3,0\:c' and if the path is `F:\\filename' then you would use `filename=F\:\\filename'.

On Windows, disk devices are accessed as `\\\\.\\PhysicalDrive0' for the first device, `\\\\.\\PhysicalDrive1' for the second etc. Note: Windows and FreeBSD prevent write access to areas of the disk containing in-use data (e.g. filesystems).

The filename `-' is a reserved name, meaning *stdin* or *stdout*. Which of the two depends on the read/write direction set.


If sharing multiple files between jobs, it is usually necessary to have fio generate the exact names that you want. By default, fio will name a file based on the default file format specification of `jobname.jobnumber.filenumber'. With this option, that can be customized. Fio will recognize and replace the following keywords in this string:


The name of the worker thread or process.


The incremental number of the worker thread or process.


The incremental number of the file for that worker thread or process.

To have dependent jobs share a set of files, this option can be set to have fio generate filenames that are shared between the two. For instance, if `testfiles.$filenum' is specified, file number 4 for any job will be named `testfiles.4'. The default of `$jobname.$jobnum.$filenum' will be used if no other format specifier is given.


To avoid collisions between networked clients, fio defaults to prefixing any generated filenames (with a directory specified) with the source of the client connecting. To disable this behavior, set this option to 0.


Recursively open any files below directory str.


Fio defaults to not locking any files before it does I/O to them. If a file or file descriptor is shared, fio can serialize I/O to that file to make the end result consistent. This is usual for emulating real workloads that share files. The lock modes are:


No locking. The default.


Only one thread or process may do I/O at a time, excluding all others.


Read-write locking on the file. Many readers may access the file at the same time, but writes get exclusive access.


Number of files to use for this job. Defaults to 1. The size of files will be size divided by this unless explicit size is specified by filesize. Files are created for each thread separately, and each file will have a file number within its name by default, as explained in filename section.


Number of files to keep open at the same time. Defaults to the same as nrfiles, can be set smaller to limit the number simultaneous opens.


Defines how fio decides which file from a job to service next. The following types are defined:


Choose a file at random.


Round robin over opened files. This is the default.


Finish one file before moving on to the next. Multiple files can still be open depending on openfiles.


Use a Zipf distribution to decide what file to access.


Use a Pareto distribution to decide what file to access.


Use a Gaussian (normal) distribution to decide what file to access.


Alias for normal.

For random, roundrobin, and sequential, a postfix can be appended to tell fio how many I/Os to issue before switching to a new file. For example, specifying `file_service_type=random:8' would cause fio to issue 8 I/Os before selecting a new file at random. For the non-uniform distributions, a floating point postfix can be given to influence how the distribution is skewed. See random_distribution for a description of how that would work.


Attempt to switch the device hosting the file to the specified I/O scheduler before running.


If true, serialize the file creation for the jobs. This may be handy to avoid interleaving of data files, which may greatly depend on the filesystem used and even the number of processors in the system. Default: true.


fsync(2) the data file after creation. This is the default.


If true, don't pre-create files but allow the job's open() to create a file when it's time to do I/O. Default: false -- pre-create all necessary files when the job starts.


If true, fio will only run the setup phase of the job. If files need to be laid out or updated on disk, only that will be done -- the actual job contents are not executed. Default: false.


If true, fio is permitted to create files as part of its workload. If this option is false, then fio will error out if the files it needs to use don't already exist. Default: true.


If this isn't set, fio will abort jobs that are destructive (e.g. that write) to what appears to be a mounted device or partition. This should help catch creating inadvertently destructive tests, not realizing that the test will destroy data on the mounted file system. Note that some platforms don't allow writing against a mounted device regardless of this option. Default: false.


If this is given, files will be pre-read into memory before starting the given I/O operation. This will also clear the invalidate flag, since it is pointless to pre-read and then drop the cache. This will only work for I/O engines that are seek-able, since they allow you to read the same data multiple times. Thus it will not work on non-seekable I/O engines (e.g. network, splice). Default: false.


Unlink the job files when done. Not the default, as repeated runs of that job would then waste time recreating the file set again and again. Default: false.


Unlink job files after each iteration or loop. Default: false.


Divide a file into zones of the specified size. See zoneskip.


Give size of an I/O zone. See zoneskip.


Skip the specified number of bytes when zonesize data has been read. The two zone options can be used to only do I/O on zones of a file.

I/O type


If value is true, use non-buffered I/O. This is usually O_DIRECT. Note that ZFS on Solaris doesn't support direct I/O. On Windows the synchronous ioengines don't support direct I/O. Default: false.


If value is true, attempt to use atomic direct I/O. Atomic writes are guaranteed to be stable once acknowledged by the operating system. Only Linux supports O_ATOMIC right now.


If value is true, use buffered I/O. This is the opposite of the direct option. Defaults to true.

readwrite=str, rw=str

Type of I/O pattern. Accepted values are:


Sequential reads.


Sequential writes.


Sequential trims (Linux block devices only).


Random reads.


Random writes.


Random trims (Linux block devices only).


Sequential mixed reads and writes.


Random mixed reads and writes.


Sequential trim+write sequences. Blocks will be trimmed first, then the same blocks will be written to.

Fio defaults to read if the option is not specified. For the mixed I/O types, the default is to split them 50/50. For certain types of I/O the result may still be skewed a bit, since the speed may be different.

It is possible to specify the number of I/Os to do before getting a new offset by appending `:<nr>' to the end of the string given. For a random read, it would look like `rw=randread:8' for passing in an offset modifier with a value of 8. If the suffix is used with a sequential I/O pattern, then the `<nr>' value specified will be added to the generated offset for each I/O turning sequential I/O into sequential I/O with holes. For instance, using `rw=write:4k' will skip 4k for every write. Also see the rw_sequencer option.


If an offset modifier is given by appending a number to the `rw=str' line, then this option controls how that number modifies the I/O offset being generated. Accepted values are:


Generate sequential offset.


Generate the same offset.

sequential is only useful for random I/O, where fio would normally generate a new random offset for every I/O. If you append e.g. 8 to randread, you would get a new random offset for every 8 I/Os. The result would be a seek for only every 8 I/Os, instead of for every I/O. Use `rw=randread:8' to specify that. As sequential I/O is already sequential, setting sequential for that would not result in any differences. identical behaves in a similar fashion, except it sends the same offset 8 number of times before generating a new offset.


Fio normally reports statistics on a per data direction basis, meaning that reads, writes, and trims are accounted and reported separately. If this option is set fio sums the results and report them as "mixed" instead.


Seed the random number generator used for random I/O patterns in a predictable way so the pattern is repeatable across runs. Default: true.


Seed all random number generators in a predictable way so results are repeatable across runs. Default: false.


Seed the random number generators based on this seed value, to be able to control what sequence of output is being generated. If not set, the random sequence depends on the randrepeat setting.


Whether pre-allocation is performed when laying down files. Accepted values are:


Do not pre-allocate space.


Use a platform's native pre-allocation call but fall back to none behavior if it fails/is not implemented.


Pre-allocate via posix_fallocate(3).


Pre-allocate via fallocate(2) with FALLOC_FL_KEEP_SIZE set.


Backward-compatible alias for none.


Backward-compatible alias for posix.

May not be available on all supported platforms. keep is only available on Linux. If using ZFS on Solaris this cannot be set to posix because ZFS doesn't support pre-allocation. Default: native if any pre-allocation methods are available, none if not.


Use posix_fadvise(2) to advise the kernel what I/O patterns are likely to be issued. Accepted values are:


Backwards compatible hint for "no hint".


Backwards compatible hint for "advise with fio workload type". This uses FADV_RANDOM for a random workload, and FADV_SEQUENTIAL for a sequential workload.




Advise using FADV_RANDOM.


Use fcntl(2) to advise the kernel what life time to expect from a write. Only supported on Linux, as of version 4.13. Accepted values are:


No particular life time associated with this file.


Data written to this file has a short life time.


Data written to this file has a medium life time.


Data written to this file has a long life time.


Data written to this file has a very long life time.

The values are all relative to each other, and no absolute meaning should be associated with them.


Start I/O at the provided offset in the file, given as either a fixed size in bytes or a percentage. If a percentage is given, the next blockalign-ed offset will be used. Data before the given offset will not be touched. This effectively caps the file size at `real_size - offset'. Can be combined with size to constrain the start and end range of the I/O workload. A percentage can be specified by a number between 1 and 100 followed by '%', for example, `offset=20%' to specify 20%.


If this is provided, then the real offset becomes `offset + offset_increment * thread_number', where the thread number is a counter that starts at 0 and is incremented for each sub-job (i.e. when numjobs option is specified). This option is useful if there are several jobs which are intended to operate on a file in parallel disjoint segments, with even spacing between the starting points.


Fio will normally perform I/Os until it has exhausted the size of the region set by size, or if it exhaust the allocated time (or hits an error condition). With this setting, the range/size can be set independently of the number of I/Os to perform. When fio reaches this number, it will exit normally and report status. Note that this does not extend the amount of I/O that will be done, it will only stop fio if this condition is met before other end-of-job criteria.


If writing to a file, issue an fsync(2) (or its equivalent) of the dirty data for every number of blocks given. For example, if you give 32 as a parameter, fio will sync the file after every 32 writes issued. If fio is using non-buffered I/O, we may not sync the file. The exception is the sg I/O engine, which synchronizes the disk cache anyway. Defaults to 0, which means fio does not periodically issue and wait for a sync to complete. Also see end_fsync and fsync_on_close.


Like fsync but uses fdatasync(2) to only sync data and not metadata blocks. In Windows, FreeBSD, and DragonFlyBSD there is no fdatasync(2) so this falls back to using fsync(2). Defaults to 0, which means fio does not periodically issue and wait for a data-only sync to complete.


Make every N-th write a barrier write.


Use sync_file_range(2) for every int number of write operations. Fio will track range of writes that have happened since the last sync_file_range(2) call. str can currently be one or more of:







So if you do `sync_file_range=wait_before,write:8', fio would use `SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE' for every 8 writes. Also see the sync_file_range(2) man page. This option is Linux specific.


If true, writes to a file will always overwrite existing data. If the file doesn't already exist, it will be created before the write phase begins. If the file exists and is large enough for the specified write phase, nothing will be done. Default: false.


If true, fsync(2) file contents when a write stage has completed. Default: false.


If true, fio will fsync(2) a dirty file on close. This differs from end_fsync in that it will happen on every file close, not just at the end of the job. Default: false.


Percentage of a mixed workload that should be reads. Default: 50.


Percentage of a mixed workload that should be writes. If both rwmixread and rwmixwrite is given and the values do not add up to 100%, the latter of the two will be used to override the first. This may interfere with a given rate setting, if fio is asked to limit reads or writes to a certain rate. If that is the case, then the distribution may be skewed. Default: 50.


By default, fio will use a completely uniform random distribution when asked to perform random I/O. Sometimes it is useful to skew the distribution in specific ways, ensuring that some parts of the data is more hot than others. fio includes the following distribution models:


Uniform random distribution


Zipf distribution


Pareto distribution


Normal (Gaussian) distribution


Zoned random distribution

When using a zipf or pareto distribution, an input value is also needed to define the access pattern. For zipf, this is the `Zipf theta'. For pareto, it's the `Pareto power'. Fio includes a test program, fio-genzipf, that can be used visualize what the given input values will yield in terms of hit rates. If you wanted to use zipf with a `theta' of 1.2, you would use `random_distribution=zipf:1.2' as the option. If a non-uniform model is used, fio will disable use of the random map. For the normal distribution, a normal (Gaussian) deviation is supplied as a value between 0 and 100.

For a zoned distribution, fio supports specifying percentages of I/O access that should fall within what range of the file or device. For example, given a criteria of:

60% of accesses should be to the first 10%

30% of accesses should be to the next 20%

8% of accesses should be to the next 30%

2% of accesses should be to the next 40%

we can define that through zoning of the random accesses. For the above example, the user would do:


similarly to how bssplit works for setting ranges and percentages of block sizes. Like bssplit, it's possible to specify separate zones for reads, writes, and trims. If just one set is given, it'll apply to all of them.


For a random workload, set how big a percentage should be random. This defaults to 100%, in which case the workload is fully random. It can be set from anywhere from 0 to 100. Setting it to 0 would make the workload fully sequential. Any setting in between will result in a random mix of sequential and random I/O, at the given percentages. Comma-separated values may be specified for reads, writes, and trims as described in blocksize.


Normally fio will cover every block of the file when doing random I/O. If this option is given, fio will just get a new random offset without looking at past I/O history. This means that some blocks may not be read or written, and that some blocks may be read/written more than once. If this option is used with verify and multiple blocksizes (via bsrange), only intact blocks are verified, i.e., partially-overwritten blocks are ignored.


See norandommap. If fio runs with the random block map enabled and it fails to allocate the map, if this option is set it will continue without a random block map. As coverage will not be as complete as with random maps, this option is disabled by default.


Fio supports the following engines for generating I/O offsets for random I/O:


Strong 2^88 cycle random number generator.


Linear feedback shift register generator.


Strong 64-bit 2^258 cycle random number generator.

tausworthe is a strong random number generator, but it requires tracking on the side if we want to ensure that blocks are only read or written once. lfsr guarantees that we never generate the same offset twice, and it's also less computationally expensive. It's not a true random generator, however, though for I/O purposes it's typically good enough. lfsr only works with single block sizes, not with workloads that use multiple block sizes. If used with such a workload, fio may read or write some blocks multiple times. The default value is tausworthe, unless the required space exceeds 2^32 blocks. If it does, then tausworthe64 is selected automatically.

Block size

blocksize=int[,int][,int], bs=int[,int][,int]

The block size in bytes used for I/O units. Default: 4096. A single value applies to reads, writes, and trims. Comma-separated values may be specified for reads, writes, and trims. A value not terminated in a comma applies to subsequent types. Examples:

bs=256k        means 256k for reads, writes and trims.

bs=8k,32k      means 8k for reads, 32k for writes and trims.

bs=8k,32k,     means 8k for reads, 32k for writes, and default for trims.

bs=,8k         means default for reads, 8k for writes and trims.

bs=,8k,        means default for reads, 8k for writes, and default for trims.

blocksize_range=irange[,irange][,irange], bsrange=irange[,irange][,irange]

A range of block sizes in bytes for I/O units. The issued I/O unit will always be a multiple of the minimum size, unless blocksize_unaligned is set. Comma-separated ranges may be specified for reads, writes, and trims as described in blocksize. Example:



Sometimes you want even finer grained control of the block sizes issued, not just an even split between them. This option allows you to weight various block sizes, so that you are able to define a specific amount of block sizes issued. The format for this option is:


for as many block sizes as needed. So if you want to define a workload that has 50% 64k blocks, 10% 4k blocks, and 40% 32k blocks, you would write:


Ordering does not matter. If the percentage is left blank, fio will fill in the remaining values evenly. So a bssplit option like this one:


would have 50% 4k ios, and 25% 1k and 32k ios. The percentages always add up to 100, if bssplit is given a range that adds up to more, it will error out.

Comma-separated values may be specified for reads, writes, and trims as described in blocksize.

If you want a workload that has 50% 2k reads and 50% 4k reads, while having 90% 4k writes and 10% 8k writes, you would specify:


blocksize_unaligned, bs_unaligned

If set, fio will issue I/O units with any size within blocksize_range, not just multiples of the minimum size. This typically won't work with direct I/O, as that normally requires sector alignment.


If this option is set, fio will use the normal read,write blocksize settings as sequential,random blocksize settings instead. Any random read or write will use the WRITE blocksize settings, and any sequential read or write will use the READ blocksize settings.

blockalign=int[,int][,int], ba=int[,int][,int]

Boundary to which fio will align random I/O units. Default: blocksize. Minimum alignment is typically 512b for using direct I/O, though it usually depends on the hardware block size. This option is mutually exclusive with using a random map for files, so it will turn off that option. Comma-separated values may be specified for reads, writes, and trims as described in blocksize.

Buffers and memory


Initialize buffers with all zeros. Default: fill buffers with random data.


If this option is given, fio will refill the I/O buffers on every submit. The default is to only fill it at init time and reuse that data. Only makes sense if zero_buffers isn't specified, naturally. If data verification is enabled, refill_buffers is also automatically enabled.


If refill_buffers is too costly and the target is using data deduplication, then setting this option will slightly modify the I/O buffer contents to defeat normal de-dupe attempts. This is not enough to defeat more clever block compression attempts, but it will stop naive dedupe of blocks. Default: true.


If this is set, then fio will attempt to provide I/O buffer content (on WRITEs) that compresses to the specified level. Fio does this by providing a mix of random data and a fixed pattern. The fixed pattern is either zeros, or the pattern specified by buffer_pattern. If the pattern option is used, it might skew the compression ratio slightly. Note that this is per block size unit, for file/disk wide compression level that matches this setting, you'll also want to set refill_buffers.


See buffer_compress_percentage. This setting allows fio to manage how big the ranges of random data and zeroed data is. Without this set, fio will provide buffer_compress_percentage of blocksize random data, followed by the remaining zeroed. With this set to some chunk size smaller than the block size, fio can alternate random and zeroed data throughout the I/O buffer.


If set, fio will fill the I/O buffers with this pattern or with the contents of a file. If not set, the contents of I/O buffers are defined by the other options related to buffer contents. The setting can be any pattern of bytes, and can be prefixed with 0x for hex values. It may also be a string, where the string must then be wrapped with "". Or it may also be a filename, where the filename must be wrapped with '' in which case the file is opened and read. Note that not all the file contents will be read if that would cause the buffers to overflow. So, for example:








Also you can combine everything together in any order:



If set, fio will generate this percentage of identical buffers when writing. These buffers will be naturally dedupable. The contents of the buffers depend on what other buffer compression settings have been set. It's possible to have the individual buffers either fully compressible, or not at all. This option only controls the distribution of unique buffers.


Invalidate the buffer/page cache parts of the files to be used prior to starting I/O if the platform and file type support it. Defaults to true. This will be ignored if pre_read is also specified for the same job.


Use synchronous I/O for buffered writes. For the majority of I/O engines, this means using O_SYNC. Default: false.

iomem=str, mem=str

Fio can use various types of memory as the I/O unit buffer. The allowed values are:


Use memory from malloc(3) as the buffers. Default memory type.


Use shared memory as the buffers. Allocated through shmget(2).


Same as shm, but use huge pages as backing.


Use mmap(2) to allocate buffers. May either be anonymous memory, or can be file backed if a filename is given after the option. The format is `mem=mmap:/path/to/file'.


Use a memory mapped huge file as the buffer backing. Append filename after mmaphuge, ala `mem=mmaphuge:/hugetlbfs/file'.


Same as mmap, but use a MMAP_SHARED mapping.


Use GPU memory as the buffers for GPUDirect RDMA benchmark. The ioengine must be rdma.

The area allocated is a function of the maximum allowed bs size for the job, multiplied by the I/O depth given. Note that for shmhuge and mmaphuge to work, the system must have free huge pages allocated. This can normally be checked and set by reading/writing `/proc/sys/vm/nr_hugepages' on a Linux system. Fio assumes a huge page is 4MiB in size. So to calculate the number of huge pages you need for a given job file, add up the I/O depth of all jobs (normally one unless iodepth is used) and multiply by the maximum bs set. Then divide that number by the huge page size. You can see the size of the huge pages in `/proc/meminfo'. If no huge pages are allocated by having a non-zero number in `nr_hugepages', using mmaphuge or shmhuge will fail. Also see hugepage-size.

mmaphuge also needs to have hugetlbfs mounted and the file location should point there. So if it's mounted in `/huge', you would use `mem=mmaphuge:/huge/somefile'.

iomem_align=int, mem_align=int

This indicates the memory alignment of the I/O memory buffers. Note that the given alignment is applied to the first I/O unit buffer, if using iodepth the alignment of the following buffers are given by the bs used. In other words, if using a bs that is a multiple of the page sized in the system, all buffers will be aligned to this value. If using a bs that is not page aligned, the alignment of subsequent I/O memory buffers is the sum of the iomem_align and bs used.


Defines the size of a huge page. Must at least be equal to the system setting, see `/proc/meminfo'. Defaults to 4MiB. Should probably always be a multiple of megabytes, so using `hugepage-size=Xm' is the preferred way to set this to avoid setting a non-pow-2 bad value.


Pin the specified amount of memory with mlock(2). Can be used to simulate a smaller amount of memory. The amount specified is per worker.

I/O size


The total size of file I/O for each thread of this job. Fio will run until this many bytes has been transferred, unless runtime is limited by other options (such as runtime, for instance, or increased/decreased by io_size). Fio will divide this size between the available files determined by options such as nrfiles, filename, unless filesize is specified by the job. If the result of division happens to be 0, the size is set to the physical size of the given files or devices if they exist. If this option is not specified, fio will use the full size of the given files or devices. If the files do not exist, size must be given. It is also possible to give size as a percentage between 1 and 100. If `size=20%' is given, fio will use 20% of the full size of the given files or devices. Can be combined with offset to constrain the start and end range that I/O will be done within.

io_size=int, io_limit=int

Normally fio operates within the region set by size, which means that the size option sets both the region and size of I/O to be performed. Sometimes that is not what you want. With this option, it is possible to define just the amount of I/O that fio should do. For instance, if size is set to 20GiB and io_size is set to 5GiB, fio will perform I/O within the first 20GiB but exit when 5GiB have been done. The opposite is also possible -- if size is set to 20GiB, and io_size is set to 40GiB, then fio will do 40GiB of I/O within the 0..20GiB region.


Individual file sizes. May be a range, in which case fio will select sizes for files at random within the given range and limited to size in total (if that is given). If not given, each created file is the same size. This option overrides size in terms of file size, which means this value is used as a fixed size or possible range of each file.


Perform I/O after the end of the file. Normally fio will operate within the size of a file. If this option is set, then fio will append to the file instead. This has identical behavior to setting offset to the size of a file. This option is ignored on non-regular files.

fill_device=bool, fill_fs=bool

Sets size to something really large and waits for ENOSPC (no space left on device) as the terminating condition. Only makes sense with sequential write. For a read workload, the mount point will be filled first then I/O started on the result. This option doesn't make sense if operating on a raw device node, since the size of that is already known by the file system. Additionally, writing beyond end-of-device will not return ENOSPC there.

I/O engine


Defines how the job issues I/O to the file. The following types are defined:


Basic read(2) or write(2) I/O. lseek(2) is used to position the I/O location. See fsync and fdatasync for syncing write I/Os.


Basic pread(2) or pwrite(2) I/O. Default on all supported operating systems except for Windows.


Basic readv(2) or writev(2) I/O. Will emulate queuing by coalescing adjacent I/Os into a single submission.


Basic preadv(2) or pwritev(2) I/O.


Basic preadv2(2) or pwritev2(2) I/O.


Linux native asynchronous I/O. Note that Linux may only support queued behavior with non-buffered I/O (set `direct=1' or `buffered=0'). This engine defines engine specific options.


POSIX asynchronous I/O using aio_read(3) and aio_write(3).


Solaris native asynchronous I/O.


Windows native asynchronous I/O. Default on Windows.


File is memory mapped with mmap(2) and data copied to/from using memcpy(3).


splice(2) is used to transfer the data and vmsplice(2) to transfer data from user space to the kernel.


SCSI generic sg v3 I/O. May either be synchronous using the SG_IO ioctl, or if the target is an sg character device we use read(2) and write(2) for asynchronous I/O. Requires filename option to specify either block or character devices.


Doesn't transfer any data, just pretends to. This is mainly used to exercise fio itself and for debugging/testing purposes.


Transfer over the network to given `host:port'. Depending on the protocol used, the hostname, port, listen and filename options are used to specify what sort of connection to make, while the protocol option determines which protocol will be used. This engine defines engine specific options.


Like net, but uses splice(2) and vmsplice(2) to map data and send/receive. This engine defines engine specific options.


Doesn't transfer any data, but burns CPU cycles according to the cpuload and cpuchunks options. Setting cpuload=85 will cause that job to do nothing but burn 85% of the CPU. In case of SMP machines, use `numjobs=<nr_of_cpu>' to get desired CPU usage, as the cpuload only loads a single CPU at the desired rate. A job never finishes unless there is at least one non-cpuio job.


The GUASI I/O engine is the Generic Userspace Asyncronous Syscall Interface approach to async I/O. See for more info on GUASI.


The RDMA I/O engine supports both RDMA memory semantics (RDMA_WRITE/RDMA_READ) and channel semantics (Send/Recv) for the InfiniBand, RoCE and iWARP protocols.


I/O engine that does regular fallocate to simulate data transfer as fio ioengine.

DDIR_READ      does fallocate(,mode = FALLOC_FL_KEEP_SIZE,).

DIR_WRITE      does fallocate(,mode = 0).



I/O engine that sends ftruncate(2) operations in response to write (DDIR_WRITE) events. Each ftruncate issued sets the file's size to the current block offset. blocksize is ignored.


I/O engine that does regular EXT4_IOC_MOVE_EXT ioctls to simulate defragment activity in request to DDIR_WRITE event.


I/O engine supporting direct access to Ceph Rados Block Devices (RBD) via librbd without the need to use the kernel rbd driver. This ioengine defines engine specific options.


Using GlusterFS libgfapi sync interface to direct access to GlusterFS volumes without having to go through FUSE. This ioengine defines engine specific options.


Using GlusterFS libgfapi async interface to direct access to GlusterFS volumes without having to go through FUSE. This ioengine defines engine specific options.


Read and write through Hadoop (HDFS). The filename option is used to specify host,port of the hdfs name-node to connect. This engine interprets offsets a little differently. In HDFS, files once created cannot be modified so random writes are not possible. To imitate this the libhdfs engine expects a bunch of small files to be created over HDFS and will randomly pick a file from them based on the offset generated by fio backend (see the example job file to create such files, use `rw=write' option). Please note, it may be necessary to set environment variables to work with HDFS/libhdfs properly. Each job uses its own connection to HDFS.


Read, write and erase an MTD character device (e.g., `/dev/mtd0'). Discards are treated as erases. Depending on the underlying device type, the I/O may have to go in a certain pattern, e.g., on NAND, writing sequentially to erase blocks and discarding before overwriting. The trimwrite mode works well for this constraint.


Read and write using filesystem DAX to a file on a filesystem mounted with DAX on a persistent memory device through the NVML libpmemblk library.


Read and write using device DAX to a persistent memory device (e.g., /dev/dax0.0) through the NVML libpmem library.


Prefix to specify loading an external I/O engine object file. Append the engine filename, e.g. `ioengine=external:/tmp/foo.o' to load ioengine `foo.o' in `/tmp'.

I/O engine specific parameters

In addition, there are some parameters which are only valid when a specific ioengine is in use. These are used identically to normal parameters, with the caveat that when used on the command line, they must come after the ioengine that defines them is selected.


Normally, with the libaio engine in use, fio will use the io_getevents(3) system call to reap newly returned events. With this flag turned on, the AIO ring will be read directly from user-space to reap events. The reaping mode is only enabled when polling for a minimum of 0 events (e.g. when `iodepth_batch_complete=0').


Set RWF_HIPRI on I/O, indicating to the kernel that it's of higher priority than normal.


When hipri is set this determines the probability of a pvsync2 I/O being high priority. The default is 100%.


Attempt to use the specified percentage of CPU cycles. This is a mandatory option when using cpuio I/O engine.


Split the load into cycles of the given time. In microseconds.


Detect when I/O threads are done, then exit.


The hostname or IP address of a HDFS cluster namenode to contact.


The listening port of the HFDS cluster namenode.


The TCP or UDP port to bind to or connect to. If this is used with numjobs to spawn multiple instances of the same job type, then this will be the starting port number since fio will use a range of ports.


The hostname or IP address to use for TCP or UDP based I/O. If the job is a TCP listener or UDP reader, the hostname is not used and must be omitted unless it is a valid UDP multicast address.


The IP address of the network interface used to send or receive UDP multicast.


Time-to-live value for outgoing UDP multicast packets. Default: 1.


Set TCP_NODELAY on TCP connections.

(netsplice,net)protocol=str, proto=str

The network protocol to use. Accepted values are:


Transmission control protocol.


Transmission control protocol V6.


User datagram protocol.


User datagram protocol V6.


UNIX domain socket.

When the protocol is TCP or UDP, the port must also be given, as well as the hostname if the job is a TCP listener or UDP reader. For unix sockets, the normal filename option should be used and the port is invalid.


For TCP network connections, tell fio to listen for incoming connections rather than initiating an outgoing connection. The hostname must be omitted if this option is used.


Normally a network writer will just continue writing data, and a network reader will just consume packages. If `pingpong=1' is set, a writer will send its normal payload to the reader, then wait for the reader to send the same payload back. This allows fio to measure network latencies. The submission and completion latencies then measure local time spent sending or receiving, and the completion latency measures how long it took for the other end to receive and send back. For UDP multicast traffic `pingpong=1' should only be set for a single reader when multiple readers are listening to the same address.


Set the desired socket buffer size for the connection.


Set the TCP maximum segment size (TCP_MAXSEG).


File will be used as a block donor (swap extents between files).


Configure donor file blocks allocation strategy:


Default. Preallocate donor's file on init.


Allocate space immediately inside defragment event, and free right after event.


Specifies the name of the Ceph cluster.


Specifies the name of the RBD.


Specifies the name of the Ceph pool containing RBD.


Specifies the username (without the 'client.' prefix) used to access the Ceph cluster. If the clustername is specified, the clientname shall be the full ** string. If no type. prefix is given, fio will add 'client.' by default.


Skip operations against known bad blocks.


libhdfs will create chunk in this HDFS directory.


The size of the chunk to use for each file.

I/O depth


Number of I/O units to keep in flight against the file. Note that increasing iodepth beyond 1 will not affect synchronous ioengines (except for small degrees when verify_async is in use). Even async engines may impose OS restrictions causing the desired depth not to be achieved. This may happen on Linux when using libaio and not setting `direct=1', since buffered I/O is not async on that OS. Keep an eye on the I/O depth distribution in the fio output to verify that the achieved depth is as expected. Default: 1.

iodepth_batch_submit=int, iodepth_batch=int

This defines how many pieces of I/O to submit at once. It defaults to 1 which means that we submit each I/O as soon as it is available, but can be raised to submit bigger batches of I/O at the time. If it is set to 0 the iodepth value will be used.

iodepth_batch_complete_min=int, iodepth_batch_complete=int

This defines how many pieces of I/O to retrieve at once. It defaults to 1 which means that we'll ask for a minimum of 1 I/O in the retrieval process from the kernel. The I/O retrieval will go on until we hit the limit set by iodepth_low. If this variable is set to 0, then fio will always check for completed events before queuing more I/O. This helps reduce I/O latency, at the cost of more retrieval system calls.


This defines maximum pieces of I/O to retrieve at once. This variable should be used along with iodepth_batch_complete_min=int variable, specifying the range of min and max amount of I/O which should be retrieved. By default it is equal to iodepth_batch_complete_min value. Example #1:



which means that we will retrieve at least 1 I/O and up to the whole submitted queue depth. If none of I/O has been completed yet, we will wait. Example #2:



which means that we can retrieve up to the whole submitted queue depth, but if none of I/O has been completed yet, we will NOT wait and immediately exit the system call. In this example we simply do polling.


The low water mark indicating when to start filling the queue again. Defaults to the same as iodepth, meaning that fio will attempt to keep the queue full at all times. If iodepth is set to e.g. 16 and iodepth_low is set to 4, then after fio has filled the queue of 16 requests, it will let the depth drain down to 4 before starting to fill it again.


Serialize in-flight I/Os that might otherwise cause or suffer from data races. When two or more I/Os are submitted simultaneously, there is no guarantee that the I/Os will be processed or completed in the submitted order. Further, if two or more of those I/Os are writes, any overlapping region between them can become indeterminate/undefined on certain storage. These issues can cause verification to fail erratically when at least one of the racing I/Os is changing data and the overlapping region has a non-zero size. Setting serialize_overlap tells fio to avoid provoking this behavior by explicitly serializing in-flight I/Os that have a non-zero overlap. Note that setting this option can reduce both performance and the iodepth achieved. Additionally this option does not work when io_submit_mode is set to offload. Default: false.


This option controls how fio submits the I/O to the I/O engine. The default is `inline', which means that the fio job threads submit and reap I/O directly. If set to `offload', the job threads will offload I/O submission to a dedicated pool of I/O threads. This requires some coordination and thus has a bit of extra overhead, especially for lower queue depth I/O where it can increase latencies. The benefit is that fio can manage submission rates independently of the device completion rates. This avoids skewed latency reporting if I/O gets backed up on the device side (the coordinated omission problem).

I/O rate


Stall the job for the specified period of time after an I/O has completed before issuing the next. May be used to simulate processing being done by an application. When the unit is omitted, the value is interpreted in microseconds. See thinktime_blocks and thinktime_spin.


Only valid if thinktime is set - pretend to spend CPU time doing something with the data received, before falling back to sleeping for the rest of the period specified by thinktime. When the unit is omitted, the value is interpreted in microseconds.


Only valid if thinktime is set - control how many blocks to issue, before waiting thinktime usecs. If not set, defaults to 1 which will make fio wait thinktime usecs after every block. This effectively makes any queue depth setting redundant, since no more than 1 I/O will be queued before we have to complete it and do our thinktime. In other words, this setting effectively caps the queue depth if the latter is larger.


Cap the bandwidth used by this job. The number is in bytes/sec, the normal suffix rules apply. Comma-separated values may be specified for reads, writes, and trims as described in blocksize.

For example, using `rate=1m,500k' would limit reads to 1MiB/sec and writes to 500KiB/sec. Capping only reads or writes can be done with `rate=,500k' or `rate=500k,' where the former will only limit writes (to 500KiB/sec) and the latter will only limit reads.


Tell fio to do whatever it can to maintain at least this bandwidth. Failing to meet this requirement will cause the job to exit. Comma-separated values may be specified for reads, writes, and trims as described in blocksize.


Cap the bandwidth to this number of IOPS. Basically the same as rate, just specified independently of bandwidth. If the job is given a block size range instead of a fixed value, the smallest block size is used as the metric. Comma-separated values may be specified for reads, writes, and trims as described in blocksize.


If fio doesn't meet this rate of I/O, it will cause the job to exit. Comma-separated values may be specified for reads, writes, and trims as described in blocksize.


This option controls how fio manages rated I/O submissions. The default is `linear', which submits I/O in a linear fashion with fixed delays between I/Os that gets adjusted based on I/O completion rates. If this is set to `poisson', fio will submit I/O based on a more real world random request flow, known as the Poisson process ( The lambda will be 10^6 / IOPS for the given workload.

I/O latency


If set, fio will attempt to find the max performance point that the given workload will run at while maintaining a latency below this target. When the unit is omitted, the value is interpreted in microseconds. See latency_window and latency_percentile.


Used with latency_target to specify the sample window that the job is run at varying queue depths to test the performance. When the unit is omitted, the value is interpreted in microseconds.


The percentage of I/Os that must fall within the criteria specified by latency_target and latency_window. If not set, this defaults to 100.0, meaning that all I/Os must be equal or below to the value set by latency_target.


If set, fio will exit the job with an ETIMEDOUT error if it exceeds this maximum latency. When the unit is omitted, the value is interpreted in microseconds.


Average bandwidth for rate and rate_min over this number of milliseconds. Defaults to 1000.

I/O replay


Write the issued I/O patterns to the specified file. See read_iolog. Specify a separate file for each job, otherwise the iologs will be interspersed and the file may be corrupt.


Open an iolog with the specified filename and replay the I/O patterns it contains. This can be used to store a workload and replay it sometime later. The iolog given may also be a blktrace binary file, which allows fio to replay a workload captured by blktrace. See blktrace(8) for how to capture such logging data. For blktrace replay, the file needs to be turned into a blkparse binary data file first (`blkparse <device> -o /dev/null -d file_for_fio.bin').


When replaying I/O with read_iolog the default behavior is to attempt to respect the timestamps within the log and replay them with the appropriate delay between IOPS. By setting this variable fio will not respect the timestamps and attempt to replay them as fast as possible while still respecting ordering. The result is the same I/O pattern to a given device, but different timings.


While replaying I/O patterns using read_iolog the default behavior is to replay the IOPS onto the major/minor device that each IOP was recorded from. This is sometimes undesirable because on a different machine those major/minor numbers can map to a different device. Changing hardware on the same system can also result in a different major/minor mapping. replay_redirect causes all I/Os to be replayed onto the single specified device regardless of the device it was recorded from. i.e. `replay_redirect=/dev/sdc' would cause all I/O in the blktrace or iolog to be replayed onto `/dev/sdc'. This means multiple devices will be replayed onto a single device, if the trace contains multiple devices. If you want multiple devices to be replayed concurrently to multiple redirected devices you must blkparse your trace into separate traces and replay them with independent fio invocations. Unfortunately this also breaks the strict time ordering between multiple device accesses.


Force alignment of I/O offsets and lengths in a trace to this power of 2 value.


Scale sector offsets down by this factor when replaying traces.

Threads, processes and job synchronization


Fio defaults to creating jobs by using fork, however if this option is given, fio will create jobs by using POSIX Threads' function pthread_create(3) to create threads instead.


If set, the current job won't be started until all workers of the specified waitee job are done. wait_for operates on the job name basis, so there are a few limitations. First, the waitee must be defined prior to the waiter job (meaning no forward references). Second, if a job is being referenced as a waitee, it must have a unique name (no duplicate waitees).


Run the job with the given nice value. See man nice(2). On Windows, values less than -15 set the process class to "High"; -1 through -15 set "Above Normal"; 1 through 15 "Below Normal"; and above 15 "Idle" priority class.


Set the I/O priority value of this job. Linux limits us to a positive value between 0 and 7, with 0 being the highest. See man ionice(1). Refer to an appropriate manpage for other operating systems since meaning of priority may differ.


Set the I/O priority class. See man ionice(1).


Set the CPU affinity of this job. The parameter given is a bit mask of allowed CPUs the job may run on. So if you want the allowed CPUs to be 1 and 5, you would pass the decimal value of (1 << 1 | 1 << 5), or 34. See man sched_setaffinity(2). This may not work on all supported operating systems or kernel versions. This option doesn't work well for a higher CPU count than what you can store in an integer mask, so it can only control cpus 1-32. For boxes with larger CPU counts, use cpus_allowed.


Controls the same options as cpumask, but accepts a textual specification of the permitted CPUs instead. So to use CPUs 1 and 5 you would specify `cpus_allowed=1,5'. This option also allows a range of CPUs to be specified -- say you wanted a binding to CPUs 1, 5, and 8 to 15, you would set `cpus_allowed=1,5,8-15'.


Set the policy of how fio distributes the CPUs specified by cpus_allowed or cpumask. Two policies are supported:


All jobs will share the CPU set specified.


Each job will get a unique CPU from the CPU set.

shared is the default behavior, if the option isn't specified. If split is specified, then fio will will assign one cpu per job. If not enough CPUs are given for the jobs listed, then fio will roundrobin the CPUs in the set.


Set this job running on specified NUMA nodes' CPUs. The arguments allow comma delimited list of cpu numbers, A-B ranges, or `all'. Note, to enable NUMA options support, fio must be built on a system with libnuma-dev(el) installed.


Set this job's memory policy and corresponding NUMA nodes. Format of the arguments:


`mode' is one of the following memory poicies: `default', `prefer', `bind', `interleave' or `local'. For `default' and `local' memory policies, no node needs to be specified. For `prefer', only one node is allowed. For `bind' and `interleave' the `nodelist' may be as follows: a comma delimited list of numbers, A-B ranges, or `all'.


Add job to this control group. If it doesn't exist, it will be created. The system must have a mounted cgroup blkio mount point for this to work. If your system doesn't have it mounted, you can do so with:

# mount -t cgroup -o blkio none /cgroup


Set the weight of the cgroup to this value. See the documentation that comes with the kernel, allowed values are in the range of 100..1000.


Normally fio will delete the cgroups it has created after the job completion. To override this behavior and to leave cgroups around after the job completion, set `cgroup_nodelete=1'. This can be useful if one wants to inspect various cgroup files after job completion. Default: false.


The ID of the flow. If not specified, it defaults to being a global flow. See flow.


Weight in token-based flow control. If this value is used, then there is a 'flow counter' which is used to regulate the proportion of activity between two or more jobs. Fio attempts to keep this flow counter near zero. The flow parameter stands for how much should be added or subtracted to the flow counter on each iteration of the main I/O loop. That is, if one job has `flow=8' and another job has `flow=-1', then there will be a roughly 1:8 ratio in how much one runs vs the other.


The maximum value that the absolute value of the flow counter is allowed to reach before the job must wait for a lower value of the counter.


The period of time, in microseconds, to wait after the flow watermark has been exceeded before retrying operations.

stonewall, wait_for_previous

Wait for preceding jobs in the job file to exit, before starting this one. Can be used to insert serialization points in the job file. A stone wall also implies starting a new reporting group, see group_reporting.


By default, fio will continue running all other jobs when one job finishes but sometimes this is not the desired action. Setting exitall will instead make fio terminate all other jobs when one job finishes.


Before running this job, issue the command specified through system(3). Output is redirected in a file called `jobname.prerun.txt'.


After the job completes, issue the command specified though system(3). Output is redirected in a file called `jobname.postrun.txt'.


Instead of running as the invoking user, set the user ID to this value before the thread/process does any work.


Set group ID, see uid.



Do not perform specified workload, only verify data still matches previous invocation of this workload. This option allows one to check data multiple times at a later date without overwriting it. This option makes sense only for workloads that write data, and does not support workloads with the time_based option set.


Run the verify phase after a write phase. Only valid if verify is set. Default: true.


If writing to a file, fio can verify the file contents after each iteration of the job. Each verification method also implies verification of special header, which is written to the beginning of each block. This header also includes meta information, like offset of the block, block number, timestamp when block was written, etc. verify can be combined with verify_pattern option. The allowed values are:


Use an md5 sum of the data area and store it in the header of each block.


Use an experimental crc64 sum of the data area and store it in the header of each block.


Use a crc32c sum of the data area and store it in the header of each block. This will automatically use hardware acceleration (e.g. SSE4.2 on an x86 or CRC crypto extensions on ARM64) but will fall back to software crc32c if none is found. Generally the fatest checksum fio supports when hardware accelerated.


Synonym for crc32c.


Use a crc32 sum of the data area and store it in the header of each block.


Use a crc16 sum of the data area and store it in the header of each block.


Use a crc7 sum of the data area and store it in the header of each block.


Use xxhash as the checksum function. Generally the fastest software checksum that fio supports.


Use sha512 as the checksum function.


Use sha256 as the checksum function.


Use optimized sha1 as the checksum function.


Use optimized sha3-224 as the checksum function.


Use optimized sha3-256 as the checksum function.


Use optimized sha3-384 as the checksum function.


Use optimized sha3-512 as the checksum function.


This option is deprecated, since now meta information is included in generic verification header and meta verification happens by default. For detailed information see the description of the verify setting. This option is kept because of compatibility's sake with old configurations. Do not use it.


Verify a strict pattern. Normally fio includes a header with some basic information and checksumming, but if this option is set, only the specific pattern set with verify_pattern is verified.


Only pretend to verify. Useful for testing internals with `ioengine=null', not for much else.

This option can be used for repeated burn-in tests of a system to make sure that the written data is also correctly read back. If the data direction given is a read or random read, fio will assume that it should verify a previously written file. If the data direction includes any form of write, the verify will be of the newly written data.


If true, fio will sort written verify blocks when it deems it faster to read them back in a sorted manner. This is often the case when overwriting an existing file, since the blocks are already laid out in the file system. You can ignore this option unless doing huge amounts of really fast I/O where the red-black tree sorting CPU time becomes significant. Default: true.


Pre-load and sort verify blocks for a read workload.


Swap the verification header with data somewhere else in the block before writing. It is swapped back before verifying.


Write the verification header at a finer granularity than the blocksize. It will be written for chunks the size of verify_interval. blocksize should divide this evenly.


If set, fio will fill the I/O buffers with this pattern. Fio defaults to filling with totally random bytes, but sometimes it's interesting to fill with a known pattern for I/O verification purposes. Depending on the width of the pattern, fio will fill 1/2/3/4 bytes of the buffer at the time (it can be either a decimal or a hex number). The verify_pattern if larger than a 32-bit quantity has to be a hex number that starts with either "0x" or "0X". Use with verify. Also, verify_pattern supports %o format, which means that for each block offset will be written and then verified back, e.g.:


Or use combination of everything:



Normally fio will keep checking the entire contents before quitting on a block verification failure. If this option is set, fio will exit the job on the first observed failure. Default: false.


If set, dump the contents of both the original data block and the data block we read off disk to files. This allows later analysis to inspect just what kind of data corruption occurred. Off by default.


Fio will normally verify I/O inline from the submitting thread. This option takes an integer describing how many async offload threads to create for I/O verification instead, causing fio to offload the duty of verifying I/O contents to one or more separate threads. If using this offload option, even sync I/O engines can benefit from using an iodepth setting higher than 1, as it allows them to have I/O in flight while verifies are running. Defaults to 0 async threads, i.e. verification is not asynchronous.


Tell fio to set the given CPU affinity on the async I/O verification threads. See cpus_allowed for the format used.


Fio will normally verify the written contents of a job that utilizes verify once that job has completed. In other words, everything is written then everything is read back and verified. You may want to verify continually instead for a variety of reasons. Fio stores the meta data associated with an I/O block in memory, so for large verify workloads, quite a bit of memory would be used up holding this meta data. If this option is enabled, fio will write only N blocks before verifying these blocks.


Control how many blocks fio will verify if verify_backlog is set. If not set, will default to the value of verify_backlog (meaning the entire queue is read back and verified). If verify_backlog_batch is less than verify_backlog then not all blocks will be verified, if verify_backlog_batch is larger than verify_backlog, some blocks will be verified more than once.


When a job exits during the write phase of a verify workload, save its current state. This allows fio to replay up until that point, if the verify state is loaded for the verify read phase. The format of the filename is, roughly:


<type> is "local" for a local run, "sock" for a client/server socket connection, and "ip" (, for instance) for a networked client/server connection. Defaults to true.


If a verify termination trigger was used, fio stores the current write state of each thread. This can be used at verification time so that fio knows how far it should verify. Without this information, fio will run a full verification pass, according to the settings in the job file used. Default false.


Number of verify blocks to discard/trim.


Verify that trim/discarded blocks are returned as zeros.


Verify that trim/discarded blocks are returned as zeros.


Trim this number of I/O blocks.


Enable experimental verification.

Steady state

steadystate=str:float, ss=str:float

Define the criterion and limit for assessing steady state performance. The first parameter designates the criterion whereas the second parameter sets the threshold. When the criterion falls below the threshold for the specified duration, the job will stop. For example, `iops_slope:0.1%' will direct fio to terminate the job when the least squares regression slope falls below 0.1% of the mean IOPS. If group_reporting is enabled this will apply to all jobs in the group. Below is the list of available steady state assessment criteria. All assessments are carried out using only data from the rolling collection window. Threshold limits can be expressed as a fixed value or as a percentage of the mean in the collection window.


Collect IOPS data. Stop the job if all individual IOPS measurements are within the specified limit of the mean IOPS (e.g., `iops:2' means that all individual IOPS values must be within 2 of the mean, whereas `iops:0.2%' means that all individual IOPS values must be within 0.2% of the mean IOPS to terminate the job).


Collect IOPS data and calculate the least squares regression slope. Stop the job if the slope falls below the specified limit.


Collect bandwidth data. Stop the job if all individual bandwidth measurements are within the specified limit of the mean bandwidth.


Collect bandwidth data and calculate the least squares regression slope. Stop the job if the slope falls below the specified limit.

steadystate_duration=time, ss_dur=time

A rolling window of this duration will be used to judge whether steady state has been reached. Data will be collected once per second. The default is 0 which disables steady state detection. When the unit is omitted, the value is interpreted in seconds.

steadystate_ramp_time=time, ss_ramp=time

Allow the job to run for the specified duration before beginning data collection for checking the steady state job termination criterion. The default is 0. When the unit is omitted, the value is interpreted in seconds.

Measurements and reporting


If set, this generates bw/clat/iops log with per file private filenames. If not set, jobs with identical names will share the log filename. Default: true.


It may sometimes be interesting to display statistics for groups of jobs as a whole instead of for each individual job. This is especially true if numjobs is used; looking at individual thread/process output quickly becomes unwieldy. To see the final report per-group instead of per-job, use group_reporting. Jobs in a file will be part of the same reporting group, unless if separated by a stonewall, or by using new_group.


Start a new reporting group. See: group_reporting. If not given, all jobs in a file will be part of the same reporting group, unless separated by a stonewall.


By default, fio collects and shows final output results for all jobs that run. If this option is set to 0, then fio will ignore it in the final stat output.


If given, write a bandwidth log for this job. Can be used to store data of the bandwidth of the jobs in their lifetime. The included fio_generate_plots script uses gnuplot to turn these text files into nice graphs. See write_lat_log for behavior of given filename. For this option, the postfix is `_bw.x.log', where `x' is the index of the job (1..N, where N is the number of jobs). If per_job_logs is false, then the filename will not include the job index. See Log File Formats section.


Same as write_bw_log, except that this option stores I/O submission, completion, and total latencies instead. If no filename is given with this option, the default filename of `jobname_type.log' is used. Even if the filename is given, fio will still append the type of log. So if one specifies:


The actual log names will be `foo_slat.x.log', `foo_clat.x.log', and `foo_lat.x.log', where `x' is the index of the job (1..N, where N is the number of jobs). This helps fio_generate_plots find the logs automatically. If per_job_logs is false, then the filename will not include the job index. See Log File Formats section.


Same as write_lat_log, but writes I/O completion latency histograms. If no filename is given with this option, the default filename of `jobname_clat_hist.x.log' is used, where `x' is the index of the job (1..N, where N is the number of jobs). Even if the filename is given, fio will still append the type of log. If per_job_logs is false, then the filename will not include the job index. See Log File Formats section.


Same as write_bw_log, but writes IOPS. If no filename is given with this option, the default filename of `jobname_type.x.log' is used, where `x' is the index of the job (1..N, where N is the number of jobs). Even if the filename is given, fio will still append the type of log. If per_job_logs is false, then the filename will not include the job index. See Log File Formats section.


By default, fio will log an entry in the iops, latency, or bw log for every I/O that completes. When writing to the disk log, that can quickly grow to a very large size. Setting this option makes fio average the each log entry over the specified period of time, reducing the resolution of the log. See log_max_value as well. Defaults to 0, logging all entries. Also see Log File Formats section.


Same as log_avg_msec, but logs entries for completion latency histograms. Computing latency percentiles from averages of intervals using log_avg_msec is inaccurate. Setting this option makes fio log histogram entries over the specified period of time, reducing log sizes for high IOPS devices while retaining percentile accuracy. See log_hist_coarseness as well. Defaults to 0, meaning histogram logging is disabled.


Integer ranging from 0 to 6, defining the coarseness of the resolution of the histogram logs enabled with log_hist_msec. For each increment in coarseness, fio outputs half as many bins. Defaults to 0, for which histogram logs contain 1216 latency bins. See Log File Formats section.


If log_avg_msec is set, fio logs the average over that window. If you instead want to log the maximum value, set this option to 1. Defaults to 0, meaning that averaged values are logged.


If this is set, the iolog options will include the byte offset for the I/O entry as well as the other data values. Defaults to 0 meaning that offsets are not present in logs. Also see Log File Formats section.


If this is set, fio will compress the I/O logs as it goes, to keep the memory footprint lower. When a log reaches the specified size, that chunk is removed and compressed in the background. Given that I/O logs are fairly highly compressible, this yields a nice memory savings for longer runs. The downside is that the compression will consume some background CPU cycles, so it may impact the run. This, however, is also true if the logging ends up consuming most of the system memory. So pick your poison. The I/O logs are saved normally at the end of a run, by decompressing the chunks and storing them in the specified log file. This feature depends on the availability of zlib.


Define the set of CPUs that are allowed to handle online log compression for the I/O jobs. This can provide better isolation between performance sensitive jobs, and background compression work.


If set, fio will store the log files in a compressed format. They can be decompressed with fio, using the --inflate-log command line parameter. The files will be stored with a `.fz' suffix.


If set, fio will log Unix timestamps to the log files produced by enabling write_type_log for each log type, instead of the default zero-based timestamps.


If set, record errors in trim block-sized units from writes and trims and output a histogram of how many trims it took to get to errors, and what kind of error was encountered.


Average the calculated bandwidth over the given time. Value is specified in milliseconds. If the job also does bandwidth logging through write_bw_log, then the minimum of this option and log_avg_msec will be used. Default: 500ms.


Average the calculated IOPS over the given time. Value is specified in milliseconds. If the job also does IOPS logging through write_iops_log, then the minimum of this option and log_avg_msec will be used. Default: 500ms.


Generate disk utilization statistics, if the platform supports it. Default: true.


Disable measurements of total latency numbers. Useful only for cutting back the number of calls to gettimeofday(2), as that does impact performance at really high IOPS rates. Note that to really get rid of a large amount of these calls, this option must be used with disable_slat and disable_bw_measurement as well.


Disable measurements of completion latency numbers. See disable_lat.


Disable measurements of submission latency numbers. See disable_lat.

disable_bw_measurement=bool, disable_bw=bool

Disable measurements of throughput/bandwidth numbers. See disable_lat.


Enable the reporting of percentiles of completion latencies.


Overwrite the default list of percentiles for completion latencies and the block error histogram. Each number is a floating number in the range (0,100], and the maximum length of the list is 20. Use ':' to separate the numbers, and list the numbers in ascending order. For example, `--percentile_list=99.5:99.9' will cause fio to report the values of completion latency below which 99.5% and 99.9% of the observed latencies fell, respectively.

Error handling


When one job finishes in error, terminate the rest. The default is to wait for each job to finish.


Normally fio will exit the job on the first observed failure. If this option is set, fio will continue the job when there is a 'non-fatal error' (EIO or EILSEQ) until the runtime is exceeded or the I/O size specified is completed. If this option is used, there are two more stats that are appended, the total error count and the first error. The error field given in the stats is the first error that was hit during the run. The allowed values are:


Exit on any I/O or verify errors.


Continue on read errors, exit on all others.


Continue on write errors, exit on all others.


Continue on any I/O error, exit on all others.


Continue on verify errors, exit on all others.


Continue on all errors.


Backward-compatible alias for 'none'.


Backward-compatible alias for 'all'.


Sometimes you want to ignore some errors during test in that case you can specify error list for each error type, instead of only being able to ignore the default 'non-fatal error' using continue_on_error. `ignore_error=READ_ERR_LIST,WRITE_ERR_LIST,VERIFY_ERR_LIST' errors for given error type is separated with ':'. Error may be symbol ('ENOSPC', 'ENOMEM') or integer. Example:


This option will ignore EAGAIN from READ, and ENOSPC and 122(EDQUOT) from WRITE. This option works by overriding continue_on_error with the list of errors for each error type if any.


If set dump every error even if it is non fatal, true by default. If disabled only fatal error will be dumped.

Running predefined workloads

Fio includes predefined profiles that mimic the I/O workloads generated by other tools.


The predefined workload to run. Current profiles are:


Threaded I/O bench (tiotest/tiobench) like workload.


Aerospike Certification Tool (ACT) like workload.

To view a profile's additional options use --cmdhelp after specifying the profile. For example:

$ fio --profile=act --cmdhelp

Act profile options


Devices to use.


ACT load multiplier. Default: 1.


How long the entire test takes to run. When the unit is omitted, the value is given in seconds. Default: 24h.


Number of read I/O threads per device. Default: 8.


Number of 512B blocks to read at the time. Default: 3.


Size of large block ops in KiB (writes). Default: 131072.


Set to run ACT prep phase.

Tiobench profile options


Size in MiB.


Block size in bytes. Default: 4096.


Number of runs.


Test directory.


Number of threads.


Fio spits out a lot of output. While running, fio will display the status of the jobs created. An example of that would be:

		Jobs: 1 (f=1): [_(1),M(1)][24.8%][r=20.5MiB/s,w=23.5MiB/s][r=82,w=94 IOPS][eta 01m:31s]

The characters inside the first set of square brackets denote the current status of each thread. The first character is the first job defined in the job file, and so forth. The possible values (in typical life cycle order) are:


Thread setup, but not started.


Thread created.


Thread initialized, waiting or generating necessary data.


Thread running pre-reading file(s).


Thread is in ramp period.


Running, doing sequential reads.


Running, doing random reads.


Running, doing sequential writes.


Running, doing random writes.


Running, doing mixed sequential reads/writes.


Running, doing mixed random reads/writes.


Running, doing sequential trims.


Running, doing random trims.


Running, currently waiting for fsync(2).


Running, doing verification of written data.


Thread finishing.


Thread exited, not reaped by main thread yet.


Thread reaped.


Thread reaped, exited with an error.


Thread reaped, exited due to signal.

Fio will condense the thread string as not to take up more space on the command line than needed. For instance, if you have 10 readers and 10 writers running, the output would look like this:

		Jobs: 20 (f=20): [R(10),W(10)][4.0%][r=20.5MiB/s,w=23.5MiB/s][r=82,w=94 IOPS][eta 57m:36s]

Note that the status string is displayed in order, so it's possible to tell which of the jobs are currently doing what. In the example above this means that jobs 1--10 are readers and 11--20 are writers.

The other values are fairly self explanatory -- number of threads currently running and doing I/O, the number of currently open files (f=), the estimated completion percentage, the rate of I/O since last check (read speed listed first, then write speed and optionally trim speed) in terms of bandwidth and IOPS, and time to completion for the current running group. It's impossible to estimate runtime of the following groups (if any).

When fio is done (or interrupted by Ctrl-C), it will show the data for each thread, group of threads, and disks in that order. For each overall thread (or group) the output looks like:

		Client1: (groupid=0, jobs=1): err= 0: pid=16109: Sat Jun 24 12:07:54 2017
		  write: IOPS=88, BW=623KiB/s (638kB/s)(30.4MiB/50032msec)
		    slat (nsec): min=500, max=145500, avg=8318.00, stdev=4781.50
		    clat (usec): min=170, max=78367, avg=4019.02, stdev=8293.31
		     lat (usec): min=174, max=78375, avg=4027.34, stdev=8291.79
		    clat percentiles (usec):
		     |  1.00th=[  302],  5.00th=[  326], 10.00th=[  343], 20.00th=[  363],
		     | 30.00th=[  392], 40.00th=[  404], 50.00th=[  416], 60.00th=[  445],
		     | 70.00th=[  816], 80.00th=[ 6718], 90.00th=[12911], 95.00th=[21627],
		     | 99.00th=[43779], 99.50th=[51643], 99.90th=[68682], 99.95th=[72877],
		     | 99.99th=[78119]
		   bw (  KiB/s): min=  532, max=  686, per=0.10%, avg=622.87, stdev=24.82, samples=  100
		   iops        : min=   76, max=   98, avg=88.98, stdev= 3.54, samples=  100
		  lat (usec)   : 250=0.04%, 500=64.11%, 750=4.81%, 1000=2.79%
		  lat (msec)   : 2=4.16%, 4=1.84%, 10=4.90%, 20=11.33%, 50=5.37%
		  lat (msec)   : 100=0.65%
		  cpu          : usr=0.27%, sys=0.18%, ctx=12072, majf=0, minf=21
		  IO depths    : 1=85.0%, 2=13.1%, 4=1.8%, 8=0.1%, 16=0.0%, 32=0.0%, >=64=0.0%
		     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
		     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
		     issued rwt: total=0,4450,0, short=0,0,0, dropped=0,0,0
		     latency   : target=0, window=0, percentile=100.00%, depth=8

The job name (or first job's name when using group_reporting) is printed, along with the group id, count of jobs being aggregated, last error id seen (which is 0 when there are no errors), pid/tid of that thread and the time the job/group completed. Below are the I/O statistics for each data direction performed (showing writes in the example above). In the order listed, they denote:


The string before the colon shows the I/O direction the statistics are for. IOPS is the average I/Os performed per second. BW is the average bandwidth rate shown as: value in power of 2 format (value in power of 10 format). The last two values show: (total I/O performed in power of 2 format / runtime of that thread).


Submission latency (min being the minimum, max being the maximum, avg being the average, stdev being the standard deviation). This is the time it took to submit the I/O. For sync I/O this row is not displayed as the slat is really the completion latency (since queue/complete is one operation there). This value can be in nanoseconds, microseconds or milliseconds --- fio will choose the most appropriate base and print that (in the example above nanoseconds was the best scale). Note: in --minimal mode latencies are always expressed in microseconds.


Completion latency. Same names as slat, this denotes the time from submission to completion of the I/O pieces. For sync I/O, clat will usually be equal (or very close) to 0, as the time from submit to complete is basically just CPU time (I/O has already been done, see slat explanation).


Total latency. Same names as slat and clat, this denotes the time from when fio created the I/O unit to completion of the I/O operation.


Bandwidth statistics based on samples. Same names as the xlat stats, but also includes the number of samples taken (samples) and an approximate percentage of total aggregate bandwidth this thread received in its group (per). This last value is only really useful if the threads in this group are on the same disk, since they are then competing for disk access.


IOPS statistics based on samples. Same names as bw.

lat (nsec/usec/msec)

The distribution of I/O completion latencies. This is the time from when I/O leaves fio and when it gets completed. Unlike the separate read/write/trim sections above, the data here and in the remaining sections apply to all I/Os for the reporting group. 250=0.04% means that 0.04% of the I/Os completed in under 250us. 500=64.11% means that 64.11% of the I/Os required 250 to 499us for completion.


CPU usage. User and system time, along with the number of context switches this thread went through, usage of system and user time, and finally the number of major and minor page faults. The CPU utilization numbers are averages for the jobs in that reporting group, while the context and fault counters are summed.

IO depths

The distribution of I/O depths over the job lifetime. The numbers are divided into powers of 2 and each entry covers depths from that value up to those that are lower than the next entry -- e.g., 16= covers depths from 16 to 31. Note that the range covered by a depth distribution entry can be different to the range covered by the equivalent submit/complete distribution entry.

IO submit

How many pieces of I/O were submitting in a single submit call. Each entry denotes that amount and below, until the previous entry -- e.g., 16=100% means that we submitted anywhere between 9 to 16 I/Os per submit call. Note that the range covered by a submit distribution entry can be different to the range covered by the equivalent depth distribution entry.

IO complete

Like the above submit number, but for completions instead.

IO issued rwt

The number of read/write/trim requests issued, and how many of them were short or dropped.

IO latency

These values are for latency-target and related options. When these options are engaged, this section describes the I/O depth required to meet the specified latency target.

After each client has been listed, the group statistics are printed. They will look like this:

		Run status group 0 (all jobs):
		   READ: bw=20.9MiB/s (21.9MB/s), 10.4MiB/s-10.8MiB/s (10.9MB/s-11.3MB/s), io=64.0MiB (67.1MB), run=2973-3069msec
		  WRITE: bw=1231KiB/s (1261kB/s), 616KiB/s-621KiB/s (630kB/s-636kB/s), io=64.0MiB (67.1MB), run=52747-53223msec

For each data direction it prints:


Aggregate bandwidth of threads in this group followed by the minimum and maximum bandwidth of all the threads in this group. Values outside of brackets are power-of-2 format and those within are the equivalent value in a power-of-10 format.


Aggregate I/O performed of all threads in this group. The format is the same as bw.


The smallest and longest runtimes of the threads in this group.

And finally, the disk statistics are printed. This is Linux specific. They will look like this:

		  Disk stats (read/write):
		    sda: ios=16398/16511, merge=30/162, ticks=6853/819634, in_queue=826487, util=100.00%

Each value is printed for both reads and writes, with reads first. The numbers denote:


Number of I/Os performed by all groups.


Number of merges performed by the I/O scheduler.


Number of ticks we kept the disk busy.


Total time spent in the disk queue.


The disk utilization. A value of 100% means we kept the disk busy constantly, 50% would be a disk idling half of the time.

It is also possible to get fio to dump the current output while it is running, without terminating the job. To do that, send fio the USR1 signal. You can also get regularly timed dumps by using the --status-interval parameter, or by creating a file in `/tmp' named `fio-dump-status'. If fio sees this file, it will unlink it and dump the current output status.

Terse Output

For scripted usage where you typically want to generate tables or graphs of the results, fio can output the results in a semicolon separated format. The format is one long line of values, such as:

		A description of this job goes here.

The job description (if provided) follows on a second line.

To enable terse output, use the --minimal or `--output-format=terse' command line options. The first value is the version of the terse output format. If the output has to be changed for some reason, this number will be incremented by 1 to signify that change.

Split up, the format is as follows (comments in brackets denote when a field was introduced or whether it's specific to some terse version):

			terse version, fio version [v3], jobname, groupid, error

READ status:

			Total IO (KiB), bandwidth (KiB/sec), IOPS, runtime (msec)
			Submission latency: min, max, mean, stdev (usec)
			Completion latency: min, max, mean, stdev (usec)
			Completion latency percentiles: 20 fields (see below)
			Total latency: min, max, mean, stdev (usec)
			Bw (KiB/s): min, max, aggregate percentage of total, mean, stdev, number of samples [v5]
			IOPS [v5]: min, max, mean, stdev, number of samples

WRITE status:

			Total IO (KiB), bandwidth (KiB/sec), IOPS, runtime (msec)
			Submission latency: min, max, mean, stdev (usec)
			Completion latency: min, max, mean, stdev (usec)
			Completion latency percentiles: 20 fields (see below)
			Total latency: min, max, mean, stdev (usec)
			Bw (KiB/s): min, max, aggregate percentage of total, mean, stdev, number of samples [v5]
			IOPS [v5]: min, max, mean, stdev, number of samples

TRIM status [all but version 3]:

			Fields are similar to READ/WRITE status.

CPU usage:

			user, system, context switches, major faults, minor faults

I/O depths:

			<=1, 2, 4, 8, 16, 32, >=64

I/O latencies microseconds:

			<=2, 4, 10, 20, 50, 100, 250, 500, 750, 1000

I/O latencies milliseconds:

			<=2, 4, 10, 20, 50, 100, 250, 500, 750, 1000, 2000, >=2000

Disk utilization [v3]:

			disk name, read ios, write ios, read merges, write merges, read ticks, write ticks, time spent in queue, disk utilization percentage

Additional Info (dependent on continue_on_error, default off):

			total # errors, first error code

Additional Info (dependent on description being set):

			Text description

Completion latency percentiles can be a grouping of up to 20 sets, so for the terse output fio writes all of them. Each field will look like this:


which is the Xth percentile, and the `usec' latency associated with it.

For Disk utilization, all disks used by fio are shown. So for each disk there will be a disk utilization section.

Below is a single line containing short names for each of the fields in the minimal output v3, separated by semicolons:


JSON+ Output

The json+ output format is identical to the json output format except that it adds a full dump of the completion latency bins. Each bins object contains a set of (key, value) pairs where keys are latency durations and values count how many I/Os had completion latencies of the corresponding duration. For example, consider:

"bins" : { "87552" : 1, "89600" : 1, "94720" : 1, "96768" : 1, "97792" : 1, "99840" : 1, "100864" : 2, "103936" : 6, "104960" : 534, "105984" : 5995, "107008" : 7529, ... }

This data indicates that one I/O required 87,552ns to complete, two I/Os required 100,864ns to complete, and 7529 I/Os required 107,008ns to complete.

Also included with fio is a Python script fio_jsonplus_clat2csv that takes json+ output and generates CSV-formatted latency data suitable for plotting.

The latency durations actually represent the midpoints of latency intervals. For details refer to `stat.h' in the fio source.

Trace File Format

There are two trace file format that you can encounter. The older (v1) format is unsupported since version 1.20-rc3 (March 2008). It will still be described below in case that you get an old trace and want to understand it.

In any case the trace is a simple text file with a single action per line.

Trace file format v1

Each line represents a single I/O action in the following format:

rw, offset, length

where `rw=0/1' for read/write, and the `offset' and `length' entries being in bytes.

This format is not supported in fio versions >= 1.20-rc3.

Trace file format v2

The second version of the trace file format was added in fio version 1.17. It allows to access more then one file per trace and has a bigger set of possible file actions.

The first line of the trace file has to be:

"fio version 2 iolog"

Following this can be lines in two different formats, which are described below.

The file management format:

filename action

The `filename' is given as an absolute path. The `action' can be one of these:


Add the given `filename' to the trace.


Open the file with the given `filename'. The `filename' has to have been added with the add action before.


Close the file with the given `filename'. The file has to have been opened before.

The file I/O action format:

filename action offset length

The `filename' is given as an absolute path, and has to have been added and opened before it can be used with this format. The `offset' and `length' are given in bytes. The `action' can be one of these:


Wait for `offset' microseconds. Everything below 100 is discarded. The time is relative to the previous `wait' statement.


Read `length' bytes beginning from `offset'.


Write `length' bytes beginning from `offset'.


fsync(2) the file.


fdatasync(2) the file.


Trim the given file from the given `offset' for `length' bytes.

Cpu Idleness Profiling

In some cases, we want to understand CPU overhead in a test. For example, we test patches for the specific goodness of whether they reduce CPU usage. Fio implements a balloon approach to create a thread per CPU that runs at idle priority, meaning that it only runs when nobody else needs the cpu. By measuring the amount of work completed by the thread, idleness of each CPU can be derived accordingly.

An unit work is defined as touching a full page of unsigned characters. Mean and standard deviation of time to complete an unit work is reported in "unit work" section. Options can be chosen to report detailed percpu idleness or overall system idleness by aggregating percpu stats.

Verification and Triggers

Fio is usually run in one of two ways, when data verification is done. The first is a normal write job of some sort with verify enabled. When the write phase has completed, fio switches to reads and verifies everything it wrote. The second model is running just the write phase, and then later on running the same job (but with reads instead of writes) to repeat the same I/O patterns and verify the contents. Both of these methods depend on the write phase being completed, as fio otherwise has no idea how much data was written.

With verification triggers, fio supports dumping the current write state to local files. Then a subsequent read verify workload can load this state and know exactly where to stop. This is useful for testing cases where power is cut to a server in a managed fashion, for instance.

A verification trigger consists of two things:

1) Storing the write state of each job.

2) Executing a trigger command.

The write state is relatively small, on the order of hundreds of bytes to single kilobytes. It contains information on the number of completions done, the last X completions, etc.

A trigger is invoked either through creation ('touch') of a specified file in the system, or through a timeout setting. If fio is run with `--trigger-file=/tmp/trigger-file', then it will continually check for the existence of `/tmp/trigger-file'. When it sees this file, it will fire off the trigger (thus saving state, and executing the trigger command).

For client/server runs, there's both a local and remote trigger. If fio is running as a server backend, it will send the job states back to the client for safe storage, then execute the remote trigger, if specified. If a local trigger is specified, the server will still send back the write state, but the client will then execute the trigger.

Verification trigger example

Let's say we want to run a powercut test on the remote Linux machine 'server'. Our write workload is in `write-test.fio'. We want to cut power to 'server' at some point during the run, and we'll run this test from the safety or our local machine, 'localbox'. On the server, we'll start the fio backend normally:

server# fio --server

and on the client, we'll fire off the workload:

localbox$ fio --client=server --trigger-file=/tmp/my-trigger --trigger-remote="bash -c "echo b > /proc/sysrq-triger""

We set `/tmp/my-trigger' as the trigger file, and we tell fio to execute:

echo b > /proc/sysrq-trigger

on the server once it has received the trigger and sent us the write state. This will work, but it's not really cutting power to the server, it's merely abruptly rebooting it. If we have a remote way of cutting power to the server through IPMI or similar, we could do that through a local trigger command instead. Let's assume we have a script that does IPMI reboot of a given hostname, ipmi-reboot. On localbox, we could then have run fio with a local trigger instead:

localbox$ fio --client=server --trigger-file=/tmp/my-trigger --trigger="ipmi-reboot server"

For this case, fio would wait for the server to send us the write state, then execute `ipmi-reboot server' when that happened.

Loading verify state

To load stored write state, a read verification job file must contain the verify_state_load option. If that is set, fio will load the previously stored state. For a local fio run this is done by loading the files directly, and on a client/server run, the server backend will ask the client to send the files over and load them from there.

Log File Formats

Fio supports a variety of log file formats, for logging latencies, bandwidth, and IOPS. The logs share a common format, which looks like this:

time (msec), value, data direction, block size (bytes), offset (bytes)

`Time' for the log entry is always in milliseconds. The `value' logged depends on the type of log, it will be one of the following:

Latency log

Value is latency in usecs

Bandwidth log

Value is in KiB/sec

IOPS log

Value is IOPS

`Data direction' is one of the following:


I/O is a READ


I/O is a WRITE


I/O is a TRIM

The entry's `block size' is always in bytes. The `offset' is the offset, in bytes, from the start of the file, for that particular I/O. The logging of the offset can be toggled with log_offset.

Fio defaults to logging every individual I/O. When IOPS are logged for individual I/Os the `value' entry will always be 1. If windowed logging is enabled through log_avg_msec, fio logs the average values over the specified period of time. If windowed logging is enabled and log_max_value is set, then fio logs maximum values in that window instead of averages. Since `data direction', `block size' and `offset' are per-I/O values, if windowed logging is enabled they aren't applicable and will be 0.

Client / Server

Normally fio is invoked as a stand-alone application on the machine where the I/O workload should be generated. However, the backend and frontend of fio can be run separately i.e., the fio server can generate an I/O workload on the "Device Under Test" while being controlled by a client on another machine.

Start the server on the machine which has access to the storage DUT:

$ fio --server=args

where `args' defines what fio listens to. The arguments are of the form `type,hostname' or `IP,port'. `type' is either `ip' (or ip4) for TCP/IP v4, `ip6' for TCP/IP v6, or `sock' for a local unix domain socket. `hostname' is either a hostname or IP address, and `port' is the port to listen to (only valid for TCP/IP, not a local socket). Some examples:

1) fio --server

Start a fio server, listening on all interfaces on the default port (8765).

2) fio --server=ip:hostname,4444

Start a fio server, listening on IP belonging to hostname and on port 4444.

3) fio --server=ip6:::1,4444

Start a fio server, listening on IPv6 localhost ::1 and on port 4444.

4) fio --server=,4444

Start a fio server, listening on all interfaces on port 4444.

5) fio --server=

Start a fio server, listening on IP on the default port.

6) fio --server=sock:/tmp/fio.sock

Start a fio server, listening on the local socket `/tmp/fio.sock'.

Once a server is running, a "client" can connect to the fio server with:

$ fio <local-args> --client=<server> <remote-args> <job file(s)>

where `local-args' are arguments for the client where it is running, `server' is the connect string, and `remote-args' and `job file(s)' are sent to the server. The `server' string follows the same format as it does on the server side, to allow IP/hostname/socket and port strings.

Fio can connect to multiple servers this way:

$ fio --client=<server1> <job file(s)> --client=<server2> <job file(s)>

If the job file is located on the fio server, then you can tell the server to load a local file as well. This is done by using --remote-config:

$ fio --client=server --remote-config /path/to/file.fio

Then fio will open this local (to the server) job file instead of being passed one from the client.

If you have many servers (example: 100 VMs/containers), you can input a pathname of a file containing host IPs/names as the parameter value for the --client option. For example, here is an example `host.list' file containing 2 hostnames:



The fio command would then be:

$ fio --client=host.list <job file(s)>

In this mode, you cannot input server-specific parameters or job files -- all servers receive the same job file.

In order to let `fio --client' runs use a shared filesystem from multiple hosts, `fio --client' now prepends the IP address of the server to the filename. For example, if fio is using the directory `/mnt/nfs/fio' and is writing filename `fileio.tmp', with a --client `hostfile' containing two hostnames `h1' and `h2' with IP addresses and, then fio will create two files:




fio was written by Jens Axboe <>, now Jens Axboe <>.
This man page was written by Aaron Carroll <> based on documentation by Jens Axboe.
This man page was rewritten by Tomohiro Kusumi <> based on documentation by Jens Axboe.

Reporting Bugs

Report bugs to the fio mailing list <>.


See Also

For further documentation see HOWTO and README.
Sample jobfiles are available in the `examples/' directory.
These are typically located under `/usr/share/doc/fio'.


Referenced By

ioping(1), iowatcher(1).

August 2017 User Manual