[fio.git] / HOWTO

Table of contents
-----------------

1. Overview
2. How fio works
3. Running fio
4. Job file format
5. Detailed list of parameters
6. Normal output
7. Terse output


1.0 Overview and history
------------------------
fio was originally written to save me the hassle of writing special test
case programs when I wanted to test a specific workload, either for
performance reasons or to find/reproduce a bug. The process of writing
such a test app can be tiresome, especially if you have to do it often.
Hence I needed a tool that would be able to simulate a given io workload
without resorting to writing a tailored test case again and again.

A test work load is difficult to define, though. There can be any number
of processes or threads involved, and they can each be using their own
way of generating io. You could have someone dirtying large amounts of
memory in an memory mapped file, or maybe several threads issuing
reads using asynchronous io. fio needed to be flexible enough to
simulate both of these cases, and many more.

2.0 How fio works
-----------------
The first step in getting fio to simulate a desired io workload, is
writing a job file describing that specific setup. A job file may contain
any number of threads and/or files - the typical contents of the job file
is a global section defining shared parameters, and one or more job
sections describing the jobs involved. When run, fio parses this file
and sets everything up as described. If we break down a job from top to
bottom, it contains the following basic parameters:

        IO type         Defines the io pattern issued to the file(s).
                        We may only be reading sequentially from this
                        file(s), or we may be writing randomly. Or even
                        mixing reads and writes, sequentially or randomly.

        Block size      In how large chunks are we issuing io? This may be
                        a single value, or it may describe a range of
                        block sizes.

        IO size         How much data are we going to be reading/writing.

        IO engine       How do we issue io? We could be memory mapping the
                        file, we could be using regular read/write, we
                        could be using splice, async io, syslet, or even
                        SG (SCSI generic sg).

        IO depth        If the io engine is async, how large a queuing
                        depth do we want to maintain?

        IO type         Should we be doing buffered io, or direct/raw io?

        Num files       How many files are we spreading the workload over.

        Num threads     How many threads or processes should we spread
                        this workload over.
        
The above are the basic parameters defined for a workload, in addition
there's a multitude of parameters that modify other aspects of how this
job behaves.


3.0 Running fio
---------------
See the README file for command line parameters, there are only a few
of them.

Running fio is normally the easiest part - you just give it the job file
(or job files) as parameters:

$ fio job_file

and it will start doing what the job_file tells it to do. You can give
more than one job file on the command line, fio will serialize the running
of those files. Internally that is the same as using the 'stonewall'
parameter described the the parameter section.

If the job file contains only one job, you may as well just give the
parameters on the command line. The command line parameters are identical
to the job parameters, with a few extra that control global parameters
(see README). For example, for the job file parameter iodepth=2, the
mirror command line option would be --iodepth 2 or --iodepth=2. You can
also use the command line for giving more than one job entry. For each
--name option that fio sees, it will start a new job with that name.
Command line entries following a --name entry will apply to that job,
until there are no more entries or a new --name entry is seen. This is
similar to the job file options, where each option applies to the current
job until a new [] job entry is seen.

fio does not need to run as root, except if the files or devices specified
in the job section requires that. Some other options may also be restricted,
such as memory locking, io scheduler switching, and decreasing the nice value.


4.0 Job file format
-------------------
As previously described, 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.
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. If the first character in a line is a ';' or a
'#', the entire line is discarded as a comment.

So lets look at a really simple job file that define to threads, each
randomly reading from a 128MiB file.

; -- start job file --
[global]
rw=randread
size=128m

[job1]

[job2]

; -- end job file --

As you can see, the job file sections themselves are empty as all the
described parameters are shared. As no filename= option is given, fio
makes up a filename for each of the jobs as it sees fit. On the command
line, this job would look as follows:

$ fio --name=global --rw=randread --size=128m --name=job1 --name=job2


Lets look at an example that have a number of processes writing randomly
to files.

; -- start job file --
[random-writers]
ioengine=libaio
iodepth=4
rw=randwrite
bs=32k
direct=0
size=64m
numjobs=4

; -- end job file --

Here we have no global section, as we only have one job defined anyway.
We want to use async io here, with a depth of 4 for each file. We also
increased the buffer size used to 32KiB and define numjobs to 4 to
fork 4 identical jobs. The result is 4 processes each randomly writing
to their own 64MiB file. Instead of using the above job file, you could
have given the parameters on the command line. For this case, you would
specify:

$ fio --name=random-writers --ioengine=libaio --iodepth=4 --rw=randwrite --bs=32k --direct=0 --size=64m --numjobs=4

fio ships with a few example job files, you can also look there for
inspiration.


5.0 Detailed list of parameters
-------------------------------

This section describes in details each parameter associated with a job.
Some parameters take an option of a given type, such as an integer or
a string. The following types are used:

str     String. This is a sequence of alpha characters.
int     Integer. A whole number value, can be negative. If prefixed with
        0x, the integer is assumed to be of base 16 (hexidecimal).
siint   SI integer. A whole number value, which may contain a postfix
        describing the base of the number. Accepted postfixes are k/m/g,
        meaning kilo, mega, and giga. So if you want to specify 4096,
        you could either write out '4096' or just give 4k. The postfixes
        signify base 2 values, so 1024 is 1k and 1024k is 1m and so on.
        If the option accepts an upper and lower range, use a colon ':'
        or minus '-' to seperate such values. See irange.
bool    Boolean. Usually parsed as an integer, however only defined for
        true and false (1 and 0).
irange  Integer range with postfix. Allows value range to be given, such
        as 1024-4096. A colon may also be used as the seperator, eg
        1k:4k. If the option allows two sets of ranges, they can be
        specified with a ',' or '/' delimiter: 1k-4k/8k-32k. Also see
        siint.

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

name=str        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.

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

directory=str   Prefix filenames with this directory. Used to places files
                in a different location than "./".

filename=str    Fio normally makes up a filename based on the job name,
                thread number, and file number. If you want to share
                files between threads in a job or several jobs, specify
                a filename for each of them to override the default. If
                the ioengine used is 'net', the filename is the host and
                port to connect to in the format of =host/port. If the
                ioengine is file based, you can specify a number of files
                by seperating 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. '-' is a reserved
                name, meaning stdin or stdout. Which of the two depends
                on the read/write direction set.

opendir=str     Tell fio to recursively add any file it can find in this
                directory and down the file system tree.

readwrite=str
rw=str          Type of io pattern. Accepted values are:

                        read            Sequential reads
                        write           Sequential writes
                        randwrite       Random writes
                        randread        Random reads
                        rw              Sequential mixed reads and writes
                        randrw          Random mixed reads and writes

                For the mixed io types, the default is to split them 50/50.
                For certain types of io the result may still be skewed a bit,
                since the speed may be different. It is possible to specify
                a number of IO's to do before getting a new offset - this
                is only useful for random IO, where fio would normally
                generate a new random offset for every IO. If you append
                eg 8 to randread, you would get a new random offset for
                every 8 IO's. The result would be a seek for only every 8
                IO's, instead of for every IO. Use rw=randread:8 to specify
                that.

randrepeat=bool For random IO workloads, seed the generator in a predictable
                way so that results are repeatable across repetitions.

fadvise_hint=bool By default, fio will use fadvise() to advise the kernel
                on what IO patterns it is likely to issue. Sometimes you
                want to test specific IO patterns without telling the
                kernel about it, in which case you can disable this option.
                If set, fio will use POSIX_FADV_SEQUENTIAL for sequential
                IO and POSIX_FADV_RANDOM for random IO.

size=siint      The total size of file io for this job. Fio will run until
                this many bytes has been transferred, unless runtime is
                limited by other options (such as 'runtime', for instance).
                Unless specific nr_files and filesize options are given,
                fio will divide this size between the available files
                specified by the job.

filesize=siint  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.

blocksize=siint
bs=siint        The block size used for the io units. Defaults to 4k. Values
                can be given for both read and writes. If a single siint is
                given, it will apply to both. If a second siint is specified
                after a comma, it will apply to writes only. In other words,
                the format is either bs=read_and_write or bs=read,write.
                bs=4k,8k will thus use 4k blocks for reads, and 8k blocks
                for writes. If you only wish to set the write size, you
                can do so by passing an empty read size - bs=,8k will set
                8k for writes and leave the read default value.

blocksize_range=irange
bsrange=irange  Instead of giving a single block size, specify a range
                and fio will mix the issued io block sizes. The issued
                io unit will always be a multiple of the minimum value
                given (also see bs_unaligned). Applies to both reads and
                writes, however a second range can be given after a comma.
                See bs=.

bssplit=str     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:

                        bssplit=blocksize/percentage:blocksize/percentage

                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:

                        bssplit=4k/10:64k/50:32k/40

                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:

                        bssplit=4k/50:1k/:32k/

                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.

blocksize_unaligned
bs_unaligned    If this option is given, any byte size value within bsrange
                may be used as a block range. This typically wont work with
                direct IO, as that normally requires sector alignment.

zero_buffers    If this option is given, fio will init the IO buffers to
                all zeroes. The default is to fill them with random data.

nrfiles=int     Number of files to use for this job. Defaults to 1.

openfiles=int   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.

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

                        random  Just choose a file at random.

                        roundrobin  Round robin over open files. This
                                is the default.

                The string can have a number appended, indicating how
                often to switch to a new file. So if option random:4 is
                given, fio will switch to a new random file after 4 ios
                have been issued.

ioengine=str    Defines how the job issues io to the file. The following
                types are defined:

                        sync    Basic read(2) or write(2) io. lseek(2) is
                                used to position the io location.

                        psync   Basic pread(2) or pwrite(2) io.

                        libaio  Linux native asynchronous io.

                        posixaio glibc posix asynchronous io.

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

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

                        syslet-rw Use the syslet system calls to make
                                regular read/write async.

                        sg      SCSI generic sg v3 io. 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
                                io.

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

                        net     Transfer over the network to given host:port.
                                'filename' must be set appropriately to
                                filename=host/port regardless of send
                                or receive, if the latter only the port
                                argument is used.

                        netsplice Like net, but uses splice/vmsplice to
                                map data and send/receive.

                        cpuio   Doesn't transfer any data, but burns CPU
                                cycles according to the cpuload= and
                                cpucycle= options. Setting cpuload=85
                                will cause that job to do nothing but burn
                                85% of the CPU.

                        guasi   The GUASI IO engine is the Generic Userspace
                                Asyncronous Syscall Interface approach
                                to async IO. See

                                http://www.xmailserver.org/guasi-lib.html

                                for more info on GUASI.

                        external Prefix to specify loading an external
                                IO engine object file. Append the engine
                                filename, eg ioengine=external:/tmp/foo.o
                                to load ioengine foo.o in /tmp.

iodepth=int     This defines how many io units to keep in flight against
                the file. The default is 1 for each file defined in this
                job, can be overridden with a larger value for higher
                concurrency.

iodepth_batch=int This defines how many pieces of IO to submit at once.
                It defaults to the same as iodepth, but can be set lower
                if one so desires.

iodepth_low=int 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 eg 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.

direct=bool     If value is true, use non-buffered io. This is usually
                O_DIRECT.

buffered=bool   If value is true, use buffered io. This is the opposite
                of the 'direct' option. Defaults to true.

offset=siint    Start io at the given offset in the file. The data before
                the given offset will not be touched. This effectively
                caps the file size at real_size - offset.

fsync=int       If writing to a file, issue a sync of the dirty data
                for every number of blocks given. For example, if you give
                32 as a parameter, fio will sync the file for every 32
                writes issued. If fio is using non-buffered io, we may
                not sync the file. The exception is the sg io engine, which
                synchronizes the disk cache anyway.

overwrite=bool  If writing to a file, setup the file first and do overwrites.

end_fsync=bool  If true, fsync file contents when the job exits.

fsync_on_close=bool     If true, fio will fsync() 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.

rwmixcycle=int  Value in milliseconds describing how often to switch between
                reads and writes for a mixed workload. The default is
                500 msecs.

rwmixread=int   How large a percentage of the mix should be reads.

rwmixwrite=int  How large a percentage of the mix 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.

norandommap     Normally fio will cover every block of the file when doing
                random IO. If this option is given, fio will just get a
                new random offset without looking at past io history. This
                means that some blocks may not be read or written, and that
                some blocks may be read/written more than once. This option
                is mutually exclusive with verify= for that reason, since
                fio doesn't track potential block rewrites which may alter
                the calculated checksum for that block.

nice=int        Run the job with the given nice value. See man nice(2).

prio=int        Set the io 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).

prioclass=int   Set the io priority class. See man ionice(1).

thinktime=int   Stall the job x microseconds after an io has completed before
                issuing the next. May be used to simulate processing being
                done by an application. See thinktime_blocks and
                thinktime_spin.

thinktime_spin=int
                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.

thinktime_blocks
                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.

rate=int        Cap the bandwidth used by this job to this number of KiB/sec.

ratemin=int     Tell fio to do whatever it can to maintain at least this
                bandwidth. Failing to meet this requirement, will cause
                the job to exit.

rate_iops=int   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.

rate_iops_min=int If fio doesn't meet this rate of IO, it will cause
                the job to exit.

ratecycle=int   Average bandwidth for 'rate' and 'ratemin' over this number
                of milliseconds.

cpumask=int     Set the CPU affinity of this job. The parameter given is a
                bitmask of allowed CPU's 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.

cpus_allowed=str Controls the same options as cpumask, but it allows a text
                setting of the permitted CPUs instead. So to use CPUs 1 and
                5, you would specify cpus_allowed=1,5.

startdelay=int  Start this job the specified number of seconds after fio
                has started. Only useful if the job file contains several
                jobs, and you want to delay starting some jobs to a certain
                time.

runtime=int     Tell fio to terminate processing after the specified number
                of seconds. It can be quite hard to determine for how long
                a specified job will run, so this parameter is handy to
                cap the total runtime to a given time.

time_based      If set, fio will run for the duration of the runtime
                specified even if the file(s) are completey read or
                written. It will simply loop over the same workload
                as many times as the runtime allows.

invalidate=bool Invalidate the buffer/page cache parts for this file prior
                to starting io. Defaults to true.

sync=bool       Use sync io for buffered writes. For the majority of the
                io engines, this means using O_SYNC.

iomem=str
mem=str         Fio can use various types of memory as the io unit buffer.
                The allowed values are:

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

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

                        shmhuge Same as shm, but use huge pages as backing.

                        mmap    Use mmap 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.

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

                The area allocated is a function of the maximum allowed
                bs size for the job, multiplied by the io 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 io 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.

hugepage-size=siint
                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.

exitall         When one job finishes, terminate the rest. The default is
                to wait for each job to finish, sometimes that is not the
                desired action.

bwavgtime=int   Average the calculated bandwidth over the given time. Value
                is specified in milliseconds.

create_serialize=bool   If true, serialize the file creating 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.

create_fsync=bool       fsync the data file after creation. This is the
                        default.

unlink=bool     Unlink the job files when done. Not the default, as repeated
                runs of that job would then waste time recreating the fileset
                again and again.

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

do_verify=bool  Run the verify phase after a write phase. Only makes sense if
                verify is set. Defaults to 1.

verify=str      If writing to a file, fio can verify the file contents
                after each iteration of the job. The allowed values are:

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

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

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

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

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

                        sha512  Use sha512 as the checksum function.

                        sha256  Use sha256 as the checksum function.

                        meta    Write extra information about each io
                                (timestamp, block number etc.). The block
                                number is verified.

                        pattern Fill the IO buffers with a specific pattern,
                                that we can use to verify. Depending on the
                                width of the pattern, fio will fill 1/2/3/4
                                bytes of the buffer at the time. The pattern
                                cannot be larger than a 32-bit quantity. The
                                given pattern is given as a postfix to this
                                option, ala: verify=pattern:0x5a. It accepts
                                both hex and dec values.

                        null    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.

verifysort=bool If set, 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 IO where the red-black tree sorting CPU time becomes
                significant.

verify_offset=siint     Swap the verification header with data somewhere else
                        in the block before writing. Its swapped back before
                        verifying.

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

verify_fatal=bool       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.
                
stonewall       Wait for preceeding 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.

new_group       Start a new reporting group. If this option isn't given,
                jobs in a file will be part of the same reporting group
                unless seperated by a stone wall (or if it's a group
                by itself, with the numjobs option).

numjobs=int     Create the specified number of clones of this job. May be
                used to setup a larger number of threads/processes doing
                the same thing. We regard that grouping of jobs as a
                specific group.

group_reporting If 'numjobs' is set, it may be interesting to display
                statistics for the group as a whole instead of for each
                individual job. This is especially true of 'numjobs' is
                large, looking at individual thread/process output quickly
                becomes unwieldy. If 'group_reporting' is specified, fio
                will show the final report per-group instead of per-job.

thread          fio defaults to forking jobs, however if this option is
                given, fio will use pthread_create(3) to create threads
                instead.

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

zoneskip=siint  Skip the specified number of bytes when zonesize data has
                been read. The two zone options can be used to only do
                io on zones of a file.

write_iolog=str Write the issued io patterns to the specified file. See
                read_iolog.

read_iolog=str  Open an iolog with the specified file name and replay the
                io 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
                for how to capture such logging data. For blktrace replay,
                the file needs to be turned into a blkparse binary data
                file first (blktrace <device> -d file_for_fio.bin).

write_bw_log    If given, write a bandwidth log of the jobs in this job
                file. 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.

write_lat_log   Same as write_bw_log, except that this option stores io
                completion latencies instead.

lockmem=siint   Pin down the specified amount of memory with mlock(2). Can
                potentially be used instead of removing memory or booting
                with less memory to simulate a smaller amount of memory.

exec_prerun=str Before running this job, issue the command specified
                through system(3).

exec_postrun=str After the job completes, issue the command specified
                 though system(3).

ioscheduler=str Attempt to switch the device hosting the file to the specified
                io scheduler before running.

cpuload=int     If the job is a CPU cycle eater, attempt to use the specified
                percentage of CPU cycles.

cpuchunks=int   If the job is a CPU cycle eater, split the load into
                cycles of the given time. In milliseconds.

disk_util=bool  Generate disk utilization statistics, if the platform
                supports it. Defaults to on.


6.0 Interpreting the output
---------------------------

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

Threads: 1: [_r] [24.8% done] [ 13509/  8334 kb/s] [eta 00h:01m:31s]

The characters inside the square brackets denote the current status of
each thread. The possible values (in typical life cycle order) are:

Idle    Run
----    ---
P               Thread setup, but not started.
C               Thread created.
I               Thread initialized, waiting.
        R       Running, doing sequential reads.
        r       Running, doing random reads.
        W       Running, doing sequential writes.
        w       Running, doing random writes.
        M       Running, doing mixed sequential reads/writes.
        m       Running, doing mixed random reads/writes.
        F       Running, currently waiting for fsync()
V               Running, doing verification of written data.
E               Thread exited, not reaped by main thread yet.
_               Thread reaped.

The other values are fairly self explanatory - number of threads
currently running and doing io, rate of io since last check (read speed
listed first, then write speed), and the estimated completion percentage
and time for the 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 data
direction, the output looks like:

Client1 (g=0): err= 0:
  write: io=    32MiB, bw=   666KiB/s, runt= 50320msec
    slat (msec): min=    0, max=  136, avg= 0.03, stdev= 1.92
    clat (msec): min=    0, max=  631, avg=48.50, stdev=86.82
    bw (KiB/s) : min=    0, max= 1196, per=51.00%, avg=664.02, stdev=681.68
  cpu        : usr=1.49%, sys=0.25%, ctx=7969, majf=0, minf=17
  IO depths    : 1=0.1%, 2=0.3%, 4=0.5%, 8=99.0%, 16=0.0%, 32=0.0%, >32=0.0%
     issued r/w: total=0/32768, short=0/0
     lat (msec): 2=1.6%, 4=0.0%, 10=3.2%, 20=12.8%, 50=38.4%, 100=24.8%,
     lat (msec): 250=15.2%, 500=0.0%, 750=0.0%, 1000=0.0%, >=2048=0.0%

The client number is printed, along with the group id and error of that
thread. Below is the io statistics, here for writes. In the order listed,
they denote:

io=             Number of megabytes io performed
bw=             Average bandwidth rate
runt=           The runtime of that thread
        slat=   Submission latency (avg being the average, stdev being the
                standard deviation). This is the time it took to submit
                the io. For sync io, the slat is really the completion
                latency, since queue/complete is one operation there. This
                value can be in miliseconds or microseconds, fio will choose
                the most appropriate base and print that. In the example
                above, miliseconds is the best scale.
        clat=   Completion latency. Same names as slat, this denotes the
                time from submission to completion of the io pieces. For
                sync io, clat will usually be equal (or very close) to 0,
                as the time from submit to complete is basically just
                CPU time (io has already been done, see slat explanation).
        bw=     Bandwidth. Same names as the xlat stats, but also includes
                an approximate percentage of total aggregate bandwidth
                this thread received in this group. 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.
cpu=            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.
IO depths=      The distribution of io depths over the job life time. The
                numbers are divided into powers of 2, so for example the
                16= entries includes depths up to that value but higher
                than the previous entry. In other words, it covers the
                range from 16 to 31.
IO issued=      The number of read/write requests issued, and how many
                of them were short.
IO latencies=   The distribution of IO completion latencies. This is the
                time from when IO leaves fio and when it gets completed.
                The numbers follow the same pattern as the IO depths,
                meaning that 2=1.6% means that 1.6% of the IO completed
                within 2 msecs, 20=12.8% means that 12.8% of the IO
                took more than 10 msecs, but less than (or equal to) 20 msecs.

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

Run status group 0 (all jobs):
   READ: io=64MiB, aggrb=22178, minb=11355, maxb=11814, mint=2840msec, maxt=2955msec
  WRITE: io=64MiB, aggrb=1302, minb=666, maxb=669, mint=50093msec, maxt=50320msec

For each data direction, it prints:

io=             Number of megabytes io performed.
aggrb=          Aggregate bandwidth of threads in this group.
minb=           The minimum average bandwidth a thread saw.
maxb=           The maximum average bandwidth a thread saw.
mint=           The smallest runtime of the threads in that group.
maxt=           The longest runtime of the threads in that group.

And finally, the disk statistics are printed. 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:

ios=            Number of ios performed by all groups.
merge=          Number of merges io the io scheduler.
ticks=          Number of ticks we kept the disk busy.
io_queue=       Total time spent in the disk queue.
util=           The disk utilization. A value of 100% means we kept the disk
                busy constantly, 50% would be a disk idling half of the time.


7.0 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:

client1;0;0;1906777;1090804;1790;0;0;0.000000;0.000000;0;0;0.000000;0.000000;929380;1152890;25.510151%;1078276.333333;128948.113404;0;0;0;0;0;0.000000;0.000000;0;0;0.000000;0.000000;0;0;0.000000%;0.000000;0.000000;100.000000%;0.000000%;324;100.0%;0.0%;0.0%;0.0%;0.0%;0.0%;0.0%;100.0%;0.0%;0.0%;0.0%;0.0%;0.0%
;0.0%;0.0%;0.0%;0.0%;0.0%

Split up, the format is as follows:

        jobname, groupid, error
        READ status:
                KiB IO, bandwidth (KiB/sec), runtime (msec)
                Submission latency: min, max, mean, deviation
                Completion latency: min, max, mean, deviation
                Bw: min, max, aggregate percentage of total, mean, deviation
        WRITE status:
                KiB IO, bandwidth (KiB/sec), runtime (msec)
                Submission latency: min, max, mean, deviation
                Completion latency: min, max, mean, deviation
                Bw: min, max, aggregate percentage of total, mean, deviation
        CPU usage: user, system, context switches, major faults, minor faults
        IO depths: <=1, 2, 4, 8, 16, 32, >=64
        IO latencies: <=2, 4, 10, 20, 50, 100, 250, 500, 750, 1000, >=2000
        Text description