Don't export state variable

[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
8. Trace file format
9. CPU idleness profiling

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 in 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 let's look at a really simple job file that defines two processes, each
randomly reading from a 128MB 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


Let's look at an example that has 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 32KB and define numjobs to 4 to
fork 4 identical jobs. The result is 4 processes each randomly writing
to their own 64MB 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

When fio is utilized as a basis of any reasonably large test suite, it might be
desirable to share a set of standardized settings across multiple job files.
Instead of copy/pasting such settings, any section may pull in an external
.fio file with 'include filename' directive, as in the following example:

; -- start job file including.fio --
[global]
filename=/tmp/test
filesize=1m
include glob-include.fio

[test]
rw=randread
bs=4k
time_based=1
runtime=10
include test-include.fio
; -- end job file including.fio --

; -- start job file glob-include.fio --
thread=1
group_reporting=1
; -- end job file glob-include.fio --

; -- start job file test-include.fio --
ioengine=libaio
iodepth=4
; -- end job file test-include.fio --

Settings pulled into a section apply to that section only (except global
section). Include directives may be nested in that any included file may
contain further include directive(s). Include files may not contain []
sections.


4.1 Environment variables
-------------------------

fio also supports environment variable expansion in job files. Any
substring of the form "${VARNAME}" as part of an option value (in other
words, on the right of the `='), will be expanded to the value of the
environment variable called VARNAME.  If no such environment variable
is defined, or VARNAME is the empty string, the empty string will be
substituted.

As an example, let's look at a sample fio invocation and job file:

$ SIZE=64m NUMJOBS=4 fio jobfile.fio

; -- start job file --
[random-writers]
rw=randwrite
size=${SIZE}
numjobs=${NUMJOBS}
; -- end job file --

This will expand to the following equivalent job file at runtime:

; -- start job file --
[random-writers]
rw=randwrite
size=64m
numjobs=4
; -- end job file --

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

4.2 Reserved keywords
---------------------

Additionally, fio has a set of reserved keywords that will be replaced
internally with the appropriate value. Those keywords are:

$pagesize       The architecture page size of the running system
$mb_memory      Megabytes of total memory in the system
$ncpus          Number of online available CPUs

These can be used on the command line or in the job file, and will be
automatically substituted with the current system values when the job
is run. Simple math is also supported on these keywords, so you can
perform actions like:

size=8*$mb_memory

and get that properly expanded to 8 times the size of memory in the
machine.


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. 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). The following types are used:

str     String. This is a sequence of alpha characters.
time    Integer with possible time suffix. In seconds unless otherwise
        specified, use eg 10m for 10 minutes. Accepts s/m/h for seconds,
        minutes, and hours, and accepts 'ms' (or 'msec') for milliseconds,
        and 'us' (or 'usec') for microseconds.
int     SI integer. A whole number value, which may contain a suffix
        describing the base of the number. Accepted suffixes are k/m/g/t/p,
        meaning kilo, mega, giga, tera, and peta. The suffix is not case
        sensitive, and you may also include trailing 'b' (eg 'kb' is the same
        as 'k'). So if you want to specify 4096, you could either write
        out '4096' or just give 4k. The suffixes signify base 2 values, so
        1024 is 1k and 1024k is 1m and so on, unless the suffix is explicitly
        set to a base 10 value using 'kib', 'mib', 'gib', etc. If that is the
        case, then 1000 is used as the multiplier. This can be handy for
        disks, since manufacturers generally use base 10 values when listing
        the capacity of a drive. If the option accepts an upper and lower
        range, use a colon ':' or minus '-' to separate such values.  May also
        include a prefix to indicate numbers base. If 0x is used, the number
        is assumed to be hexadecimal.  See irange.
bool    Boolean. Usually parsed as an integer, however only defined for
        true and false (1 and 0).
irange  Integer range with suffix. Allows value range to be given, such
        as 1024-4096. A colon may also be used as the separator, 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
        int.
float_list      A list of floating numbers, separated by a ':' character.

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 place files
                in a different location than "./". See the 'filename' option
                for escaping certain characters.

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, port,
                and protocol to use in the format of =host,port,protocol.
                See ioengine=net for more. 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. 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).
                If the wanted filename does need to include a colon, then
                escape that with a '\' character. For instance, if the filename
                is "/dev/dsk/foo@3,0:c", then you would use
                filename="/dev/dsk/foo@3,0\:c". '-' is a reserved name, meaning
                stdin or stdout. Which of the two depends on the read/write
                direction set.

filename_format=str
                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:

                $jobname
                        The name of the worker thread or process.

                $jobnum
                        The incremental number of the worker thread or
                        process.

                $filenum
                        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.

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

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

                        none            No locking. The default.
                        exclusive       Only one thread/process may do IO,
                                        excluding all others.
                        readwrite       Read-write locking on the file. Many
                                        readers may access the file at the
                                        same time, but writes get exclusive
                                        access.

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

                        read            Sequential reads
                        write           Sequential writes
                        randwrite       Random writes
                        randread        Random reads
                        rw,readwrite    Sequential mixed reads and writes
                        randrw          Random mixed reads and writes
                        trimwrite       Mixed trims and writes. Blocks will be
                                        trimmed first, then written to.

                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
                done by appending a ':<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 IO pattern, then the value
                specified will be added to the generated offset for each IO.
                For instance, using rw=write:4k will skip 4k for every
                write. It turns sequential IO into sequential IO with holes.
                See the 'rw_sequencer' option.

rw_sequencer=str If an offset modifier is given by appending a number to
                the rw=<str> line, then this option controls how that
                number modifies the IO offset being generated. Accepted
                values are:

                        sequential      Generate sequential offset
                        identical       Generate the same offset

                'sequential' 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. As sequential IO 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.

kb_base=int     The base unit for a kilobyte. The defacto base is 2^10, 1024.
                Storage manufacturers like to use 10^3 or 1000 as a base
                ten unit instead, for obvious reasons. Allow values are
                1024 or 1000, with 1024 being the default.

unified_rw_reporting=bool       Fio normally reports statistics on a per
                data direction basis, meaning that read, write, and trim are
                accounted and reported separately. If this option is set,
                the fio will sum the results and report them as "mixed"
                instead.

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

randseed=int    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.

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

                        none            Do not pre-allocate space
                        posix           Pre-allocate via posix_fallocate()
                        keep            Pre-allocate via fallocate() with
                                        FALLOC_FL_KEEP_SIZE set
                        0               Backward-compatible alias for 'none'
                        1               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 must be set to
                'none' because ZFS doesn't support it. Default: 'posix'.

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.

fadvise_stream=int Notify the kernel what write stream ID to place these
                writes under. Only supported on Linux. Note, this option
                may change going forward.

size=int        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,
                or increased/decreased by 'io_size'). Unless specific nrfiles
                and filesize options are given, fio will divide this size
                between the available files specified by the job. If not set,
                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.

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
                IO to be performed. Sometimes that is not what you want. With
                this option, it is possible to define just the amount of IO
                that fio should do. For instance, if 'size' is set to 20G and
                'io_size' is set to 5G, fio will perform IO within the first
                20G but exit when 5G have been done. The opposite is also
                possible - if 'size' is set to 20G, and 'io_size' is set to
                40G, then fio will do 40G of IO within the 0..20G region.

filesize=int    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.

file_append=bool        Perform IO 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 IO 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.

blocksize=int
bs=int          The block size used for the io units. Defaults to 4k. Values
                can be given for both read and writes. If a single int is
                given, it will apply to both. If a second int 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,trim.
                bs=4k,8k will thus use 4k blocks for reads, 8k blocks for
                writes, and 8k for trims. You can terminate the list with
                a trailing comma. bs=4k,8k, would use the default value for
                trims.. 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.

blockalign=int
ba=int          At what boundary to align random IO offsets. Defaults to
                the same as 'blocksize' the minimum blocksize given.
                Minimum alignment is typically 512b for using direct IO,
                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.

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.

                bssplit also supports giving separate splits to reads and
                writes. The format is identical to what bs= accepts. You
                have to separate the read and write parts with a comma. So
                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:

                bssplit=2k/50:4k/50,4k/90:8k/10

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.

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

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.

refill_buffers  If this option is given, fio will refill the IO 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.

scramble_buffers=bool   If refill_buffers is too costly and the target is
                using data deduplication, then setting this option will
                slightly modify the IO 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.

buffer_compress_percentage=int  If this is set, then fio will attempt to
                provide IO buffer content (on WRITEs) that compress to
                the specified level. Fio does this by providing a mix of
                random data and a fixed pattern. The fixed pattern is either
                zeroes, 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.

buffer_compress_chunk=int       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 IO
                buffer.

buffer_pattern=str      If set, fio will fill the io buffers with this
                pattern. If not set, the contents of io buffers is 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 "".

dedupe_percentage=int   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.

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.

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

                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.

                        vsync   Basic readv(2) or writev(2) IO.

                        psyncv  Basic preadv(2) or pwritev(2) IO.

                        libaio  Linux native asynchronous io. Note that Linux
                                may only support queued behaviour with
                                non-buffered IO (set direct=1 or buffered=0).
                                This engine defines engine specific options.

                        posixaio glibc posix asynchronous io.

                        solarisaio Solaris native asynchronous io.

                        windowsaio Windows native 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.
                                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.

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

                        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. In case of SMP machines,
                                use numjobs=<no_of_cpu> to get desired CPU
                                usage, as the cpuload only loads a single
                                CPU at the desired rate.

                        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.

                        rdma    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.

                        falloc  IO engine that does regular fallocate to
                                simulate data transfer as fio ioengine.
                                DDIR_READ  does fallocate(,mode = keep_size,)
                                DDIR_WRITE does fallocate(,mode = 0)
                                DDIR_TRIM  does fallocate(,mode = punch_hole)

                        e4defrag IO engine that does regular EXT4_IOC_MOVE_EXT
                                ioctls to simulate defragment activity in
                                request to DDIR_WRITE event

                        rbd     IO 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.

                        gfapi   Using Glusterfs libgfapi sync interface to
                                direct access to Glusterfs volumes without
                                options.

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

                        libhdfs 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, libhdfs engine
                                expects bunch of small files to be created
                                over HDFS, and engine will randomly pick a
                                file out of those files based on the offset
                                generated by fio backend. (see the example
                                job file to create such files, use rw=write
                                option). Please note, you might want to set
                                necessary environment variables to work with
                                hdfs/libhdfs properly.

                        mtd     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 writetrim mode works well
                                for this constraint.

                        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. Note that increasing iodepth beyond 1 will not
                affect synchronous ioengines (except for small degress 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 IO is not async on that OS. Keep an
                eye on the IO 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 IO to submit at once.
                It defaults to 1 which means that we submit each IO
                as soon as it is available, but can be raised to submit
                bigger batches of IO at the time.

iodepth_batch_complete=int This defines how many pieces of IO to retrieve
                at once. It defaults to 1 which means that we'll ask
                for a minimum of 1 IO in the retrieval process from
                the kernel. The IO 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 IO. This helps reduce
                IO latency, at the cost of more retrieval system calls.

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. Note that ZFS on Solaris doesn't support direct io.
                On Windows the synchronous ioengines don't support direct io.

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

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

offset=int      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.

offset_increment=int    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.

number_ios=int  Fio will normally perform IOs 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 IOs to
                perform. When fio reaches this number, it will exit normally
                and report status. Note that this does not extend the amount
                of IO that will be done, it will only stop fio if this
                condition is met before other end-of-job criteria.

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.

fdatasync=int   Like fsync= but uses fdatasync() to only sync data and not
                metadata blocks.
                In FreeBSD and Windows there is no fdatasync(), this falls back to
                using fsync()

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

                wait_before     SYNC_FILE_RANGE_WAIT_BEFORE
                write           SYNC_FILE_RANGE_WRITE
                wait_after      SYNC_FILE_RANGE_WAIT_AFTER

                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.

overwrite=bool  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.

end_fsync=bool  If true, fsync file contents when a write stage has completed.

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.

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. 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.

random_distribution=str:float   By default, fio will use a completely uniform
                random distribution when asked to perform random IO. 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:

                random          Uniform random distribution
                zipf            Zipf distribution
                pareto          Pareto 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, 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.

percentage_random=int   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 IO, at the given percentages. It is possible to
                set different values for reads, writes, and trim. To do so,
                simply use a comma separated list. See blocksize.
        
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. If this option
                is used with verify= and multiple blocksizes (via bsrange=),
                only intact blocks are verified, i.e., partially-overwritten
                blocks are ignored.

softrandommap=bool 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.

random_generator=str    Fio supports the following engines for generating
                IO offsets for random IO:

                tausworthe      Strong 2^88 cycle random number generator
                lfsr            Linear feedback shift register 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 IO 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.

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=int
                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 IO 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.

rate=int        Cap the bandwidth used by this job. The number is in bytes/sec,
                the normal suffix rules apply. You can use rate=500k to limit
                reads and writes to 500k each, or you can specify read and
                writes separately. Using rate=1m,500k would limit reads to
                1MB/sec and writes to 500KB/sec. Capping only reads or
                writes can be done with rate=,500k or rate=500k,. The former
                will only limit writes (to 500KB/sec), the latter will only
                limit reads.

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. The same format as rate is used for
                read vs write separation.

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. The same format
                as rate is used for read vs write separation.

rate_iops_min=int If fio doesn't meet this rate of IO, it will cause
                the job to exit. The same format as rate is used for read vs
                write separation.

latency_target=int      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. The values is given in microseconds.
                See latency_window and latency_percentile

latency_window=int      Used with latency_target to specify the sample window
                that the job is run at varying queue depths to test the
                performance. The value is given in microseconds.

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

max_latency=int If set, fio will exit the job if it exceeds this maximum
                latency. It will exit with an ETIME error.

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. 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.

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. This options also
                allows a range of CPUs. Say you wanted a binding to CPUs
                1, 5, and 8-15, you would set cpus_allowed=1,5,8-15.

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

                shared  All jobs will share the CPU set specified.
                split   Each job will get a unique CPU from the CPU set.

                'shared' is the default behaviour, 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.

numa_cpu_nodes=str Set this job running on spcified 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.

numa_mem_policy=str Set this job's memory policy and corresponding NUMA
                nodes. Format of the argements:
                        <mode>[:<nodelist>]
                `mode' is one of the following memory policy:
                        default, prefer, bind, interleave, local
                For `default' and `local' memory policy, no node is
                needed to be specified.
                For `prefer', only one node is allowed.
                For `bind' and `interleave', it allow comma delimited
                list of numbers, A-B ranges, or 'all'.

startdelay=time 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=time    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 completely read or
                written. It will simply loop over the same workload
                as many times as the runtime allows.

ramp_time=time  If set, fio will run the specified workload for this amount
                of time before logging any performance numbers. Useful for
                letting performance settle before logging results, thus
                minimizing the runtime required for stable results. Note
                that the ramp_time is considered lead in time for a job,
                thus it will increase the total runtime if a special timeout
                or runtime is specified.

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 4MB 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.

iomem_align=int This indiciates the memory alignment of the IO memory buffers.
                Note that the given alignment is applied to the first IO 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 IO memory buffers is the
                sum of the iomem_align and bs used.

hugepage-size=int
                Defines the size of a huge page. Must at least be equal
                to the system setting, see /proc/meminfo. Defaults to 4MB.
                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.

iopsavgtime=int Average the calculated IOPS 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.

create_on_open=bool     Don't pre-setup the files for IO, just create open()
                        when it's time to do IO to that file.

create_only=bool        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.

pre_read=bool   If this is given, files will be pre-read into memory before
                starting the given IO 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 IO engines
                that are seekable, since they allow you to read the same data
                multiple times. Thus it will not work on eg network or splice
                IO.

unlink=bool     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.

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.

verify_only     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.

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.

                        crc32c  Use a crc32c sum of the data area and store
                                it in the header of each block.

                        crc32c-intel Use hardware assisted crc32c calcuation
                                provided on SSE4.2 enabled processors. Falls
                                back to regular software crc32c, if not
                                supported by the system.

                        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.

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

                        sha512  Use sha512 as the checksum function.

                        sha256  Use sha256 as the checksum function.

                        sha1    Use optimized sha1 as the checksum function.

                        meta    Write extra information about each io
                                (timestamp, block number etc.). The block
                                number is verified. The io sequence number is
                                verified for workloads that write data.
                                See also verify_pattern.

                        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. 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.

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

verify_interval=int     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_pattern=str      If set, fio will fill the io buffers with this
                pattern. Fio defaults to filling with totally random
                bytes, but sometimes it's interesting to fill with a known
                pattern for io 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=meta.

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.

verify_dump=bool        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.

verify_async=int        Fio will normally verify IO inline from the submitting
                thread. This option takes an integer describing how many
                async offload threads to create for IO verification instead,
                causing fio to offload the duty of verifying IO contents
                to one or more separate threads. If using this offload
                option, even sync IO engines can benefit from using an
                iodepth setting higher than 1, as it allows them to have
                IO in flight while verifies are running.

verify_async_cpus=str   Tell fio to set the given CPU affinity on the
                async IO verification threads. See cpus_allowed for the
                format used.

verify_backlog=int      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 IO 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.

verify_backlog_batch=int        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.

verify_state_save=bool  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>-<jobname>-<jobindex>-verify.state. <type> is "local"
                for a local run, "sock" for a client/server socket connection,
                and "ip" (192.168.0.1, for instance) for a networked
                client/server connection.

verify_state_load=bool  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.

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.

new_group       Start a new reporting group. See: group_reporting.

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. Each thread is reported separately; to see
                statistics for all clones as a whole, use group_reporting in
                conjunction with new_group.

group_reporting 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'.

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

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

zoneskip=int    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.  Specify a separate file for each job, otherwise
                the iologs will be interspersed and the file may be corrupt.

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 (blkparse <device> -o /dev/null -d file_for_fio.bin).

replay_no_stall=int When replaying I/O with read_iolog the default behavior
                is to attempt to respect the time stamps 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.

replay_redirect=str 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 IOPS 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
                IO in the blktrace to be replayed onto /dev/sdc.  This means
                multiple devices will be replayed onto a single, 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.  Unfortuantely this also breaks
                the strict time ordering between multiple device accesses.

write_bw_log=str 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. See write_lat_log for behaviour of given
                filename. For this option, the suffix is _bw.x.log, where
                x is the index of the job (1..N, where N is the number of
                jobs).

write_lat_log=str Same as write_bw_log, except that this option stores io
                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

                write_lat_log=foo

                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_plot
                fine the logs automatically.

write_iops_log=str 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.

log_avg_msec=int By default, fio will log an entry in the iops, latency,
                or bw log for every IO 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.
                Defaults to 0.

log_offset=int  If this is set, the iolog options will include the byte
                offset for the IO entry as well as the other data values.

log_compression=int     If this is set, fio will compress the IO 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 IO 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 IO 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.

log_store_compressed=bool       If set, and log_compression is also 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.

block_error_percentiles=bool    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.

lockmem=int     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.
                The amount specified is per worker.

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

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

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

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

disable_lat=bool Disable measurements of total latency numbers. Useful
                only for cutting back the number of calls to gettimeofday,
                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 as well.

disable_clat=bool Disable measurements of completion latency numbers. See
                disable_lat.

disable_slat=bool Disable measurements of submission latency numbers. See
                disable_slat.

disable_bw=bool Disable measurements of throughput/bandwidth numbers. See
                disable_lat.

clat_percentiles=bool Enable the reporting of percentiles of
                 completion latencies.

percentile_list=float_list 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.

clocksource=str Use the given clocksource as the base of timing. The
                supported options are:

                        gettimeofday    gettimeofday(2)

                        clock_gettime   clock_gettime(2)

                        cpu             Internal CPU clock source

                cpu is the preferred clocksource if it is reliable, as it
                is very fast (and fio is heavy on time calls). Fio will
                automatically use this clocksource if it's supported and
                considered reliable on the system it is running on, unless
                another clocksource is specifically set. For x86/x86-64 CPUs,
                this means supporting TSC Invariant.

gtod_reduce=bool Enable all of the gettimeofday() reducing options
                (disable_clat, disable_slat, disable_bw) plus reduce
                precision of the timeout somewhat to really shrink
                the gettimeofday() call count. With this option enabled,
                we only do about 0.4% of the gtod() calls we would have
                done if all time keeping was enabled.

gtod_cpu=int    Sometimes it's cheaper to dedicate a single thread of
                execution to just getting the current time. Fio (and
                databases, for instance) are very intensive on gettimeofday()
                calls. With this option, you can set one CPU aside for
                doing nothing but logging current time to a shared memory
                location. Then the other threads/processes that run IO
                workloads need only copy that segment, instead of entering
                the kernel with a gettimeofday() call. The CPU set aside
                for doing these time calls will be excluded from other
                uses. Fio will manually clear it from the CPU mask of other
                jobs.

continue_on_error=str   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:

                        none    Exit on any IO or verify errors.

                        read    Continue on read errors, exit on all others.

                        write   Continue on write errors, exit on all others.

                        io      Continue on any IO error, exit on all others.

                        verify  Continue on verify errors, exit on all others.

                        all     Continue on all errors.

                        0               Backward-compatible alias for 'none'.

                        1               Backward-compatible alias for 'all'.

ignore_error=str Sometimes you want to ignore some errors during test
                 in that case you can specify error list for each error type.
                 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:
                        ignore_error=EAGAIN,ENOSPC:122
                 This option will ignore EAGAIN from READ, and ENOSPC and
                 122(EDQUOT) from WRITE.

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

cgroup=str      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

cgroup_weight=int       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.

cgroup_nodelete=bool 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

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

gid=int         Set group ID, see uid.

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

flow=int        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.

flow_watermark=int      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.

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

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.

[libaio] userspace_reap Normally, with the libaio engine in use, fio will use
                the io_getevents 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 (eg when
                iodepth_batch_complete=0).

[cpu] cpuload=int Attempt to use the specified percentage of CPU cycles.

[cpu] cpuchunks=int Split the load into cycles of the given time. In
                microseconds.

[cpu] exit_on_io_done=bool Detect when IO threads are done, then exit.

[netsplice] hostname=str
[net] hostname=str The host name or IP address to use for TCP or UDP based IO.
                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.

[netsplice] port=int
[net] port=int  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.

[netsplice] interface=str
[net] interface=str  The IP address of the network interface used to send or
                receive UDP multicast

[netsplice] ttl=int
[net] ttl=int   Time-to-live value for outgoing UDP multicast packets.
                Default: 1

[netsplice] nodelay=bool
[net] nodelay=bool      Set TCP_NODELAY on TCP connections.

[netsplice] protocol=str
[netsplice] proto=str
[net] protocol=str
[net] proto=str The network protocol to use. Accepted values are:

                        tcp     Transmission control protocol
                        tcpv6   Transmission control protocol V6
                        udp     User datagram protocol
                        udpv6   User datagram protocol V6
                        unix    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.

[net] listen    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.

[net] pingpong  Normaly 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.

[net] window_size       Set the desired socket buffer size for the connection.

[net] mss       Set the TCP maximum segment size (TCP_MAXSEG).

[e4defrag] donorname=str
                File will be used as a block donor(swap extents between files)
[e4defrag] inplace=int
                Configure donor file blocks allocation strategy
                0(default): Preallocate donor's file on init
                1         : allocate space immidietly inside defragment event,
                            and free right after event

[mtd] skip_bad=bool     Skip operations against known bad blocks.


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 or generating necessary data.
        p       Thread running pre-reading file(s).
        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()
        f       Running, finishing up (writing IO logs, etc)
        V       Running, doing verification of written data.
E               Thread exited, not reaped by main thread yet.
_               Thread reaped, or
X               Thread reaped, exited with an error.
K               Thread reaped, exited due to signal.

Fio will condense the thread string as not to take up more space on the
command line as is 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% done] [2103MB/0KB/0KB /s] [538K/0/0 iops] [eta 57m:36s]

Fio will still maintain the ordering, though. So the above 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 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). Note that the string is displayed in order,
so it's possible to tell which of the jobs are currently doing what. The
first character is the first job defined in the job file, and so forth.

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=    32MB, bw=   666KB/s, iops=89 , 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 (KB/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%
     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 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
iops=           Average IOs performed per second
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 milliseconds or microseconds, fio will choose
                the most appropriate base and print that. In the example
                above, milliseconds is the best scale. Note: in --minimal mode
                latencies are always expressed in microseconds.
        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 submit=      How many pieces of IO were submitting in a single submit
                call. Each entry denotes that amount and below, until
                the previous entry - eg, 8=100% mean that we submitted
                anywhere in between 5-8 ios per submit call.
IO complete=    Like the above submit number, but for completions instead.
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=64MB, aggrb=22178, minb=11355, maxb=11814, mint=2840msec, maxt=2955msec
  WRITE: io=64MB, 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.

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.


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:

2;card0;0;0;7139336;121836;60004;1;10109;27.932460;116.933948;220;126861;3495.446807;1085.368601;226;126864;3523.635629;1089.012448;24063;99944;50.275485%;59818.274627;5540.657370;7155060;122104;60004;1;8338;29.086342;117.839068;388;128077;5032.488518;1234.785715;391;128085;5061.839412;1236.909129;23436;100928;50.287926%;59964.832030;5644.844189;14.595833%;19.394167%;123706;0;7313;0.1%;0.1%;0.1%;0.1%;0.1%;0.1%;100.0%;0.00%;0.00%;0.00%;0.00%;0.00%;0.00%;0.01%;0.02%;0.05%;0.16%;6.04%;40.40%;52.68%;0.64%;0.01%;0.00%;0.01%;0.00%;0.00%;0.00%;0.00%;0.00%
A description of this job goes here.

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

To enable terse output, use the --minimal command line option. 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:

        terse version, fio version, jobname, groupid, error
        READ status:
                Total IO (KB), bandwidth (KB/sec), IOPS, runtime (msec)
                Submission latency: min, max, mean, deviation (usec)
                Completion latency: min, max, mean, deviation (usec)
                Completion latency percentiles: 20 fields (see below)
                Total latency: min, max, mean, deviation (usec)
                Bw (KB/s): min, max, aggregate percentage of total, mean, deviation
        WRITE status:
                Total IO (KB), bandwidth (KB/sec), IOPS, runtime (msec)
                Submission latency: min, max, mean, deviation (usec)
                Completion latency: min, max, mean, deviation (usec)
                Completion latency percentiles: 20 fields (see below)
                Total latency: min, max, mean, deviation (usec)
                Bw (KB/s): 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 microseconds: <=2, 4, 10, 20, 50, 100, 250, 500, 750, 1000
        IO latencies milliseconds: <=2, 4, 10, 20, 50, 100, 250, 500, 750, 1000, 2000, >=2000
        Disk utilization: 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:

        1.00%=6112

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.


8.0 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.


8.1 Trace file format v1
------------------------
Each line represents a single io 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.


8.2 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          Add the given filename to the trace
open         Open the file with the given filename. The filename has to have
             been added with the add action before.
close        Close the file with the given filename. The file has to have been
             opened before.


The file io 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       Wait for 'offset' microseconds. Everything below 100 is discarded.
read       Read 'length' bytes beginning from 'offset'
write      Write 'length' bytes beginning from 'offset'
sync       fsync() the file
datasync   fdatasync() the file
trim       trim the given file from the given 'offset' for 'length' bytes


9.0 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.


10.0 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 IO 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.

10.1 Verification trigger example
---------------------------------
Lets say we want to run a powercut test on the remote 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. Lets 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.

10.1 Loading verify state
-------------------------
To load store write state, 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.