Fix problem with too many opened files

[fio.git] / HOWTO

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

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


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

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

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

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

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

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

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

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

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

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

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


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

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

$ fio job_file

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

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

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


4.0 Job file format
-------------------
As previously described, fio accepts one or more job files describing
what it is supposed to do. The job file format is the classic ini file,
where the names enclosed in [] brackets define the job name. You are free
to use any ascii name you want, except 'global' which has special meaning.
A global section sets defaults for the jobs described in that file. A job
may override a global section parameter, and a job file may even have
several global sections if so desired. A job is only affected by a global
section residing above it. If the first character in a line is a ';' or a
'#', the entire line is discarded as a comment.

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

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

[job1]

[job2]

; -- end job file --

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

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


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 32KiB and define numjobs to 4 to
fork 4 identical jobs. The result is 4 processes each randomly writing
to their own 64MiB file. Instead of using the above job file, you could
have given the parameters on the command line. For this case, you would
specify:

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

fio 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.


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

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

str	String. This is a sequence of alpha characters.
time	Integer with possible time postfix. In seconds unless otherwise
	specified, use eg 10m for 10 minutes. Accepts s/m/h for seconds,
	minutes, and hours.
int	SI integer. A whole number value, which may contain a postfix
	describing the base of the number. Accepted postfixes are k/m/g,
	meaning kilo, mega, and giga. So if you want to specify 4096,
	you could either write out '4096' or just give 4k. The postfixes
	signify base 2 values, so 1024 is 1k and 1024k is 1m and so on.
	If the option accepts an upper and lower range, use a colon ':'
	or minus '-' to 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 postfix. 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.

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 "./".

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

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

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.

		The option may be post-fixed with a lock batch number. If
		set, then each thread/process may do that amount of IOs to
		the file before giving up the lock. Since lock acquisition is
		expensive, batching the lock/unlocks will speed up IO.

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

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

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

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

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

size=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).
		Unless specific nrfiles and filesize options are given,
		fio will divide this size between the available files
		specified by the job.

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.

fill_device=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.

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.
		bs=4k,8k will thus use 4k blocks for reads, and 8k blocks
		for writes. If you only wish to set the write size, you
		can do so by passing an empty read size - bs=,8k will set
		8k for writes and leave the read default value.

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.

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.

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.

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

			posixaio glibc posix asynchronous io.

			solarisaio Solaris 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.
				'filename' must be set appropriately to
				filename=host/port/protocol regardless of send
				or receive, if the latter only the port
				argument is used. 'host' may be an IP address
				or hostname, port is the port number to be used,
				and protocol may be 'udp' or 'tcp'. If no
				protocol is given, TCP is used.

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

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

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

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

iodepth_batch_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.

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.

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

overwrite=bool	If 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 the job exits.

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

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

rwmixwrite=int	How large a percentage of the mix should be writes. If both
		rwmixread and rwmixwrite is given and the values do not add
		up to 100%, the latter of the two will be used to override
		the first.

norandommap	Normally fio will cover every block of the file when doing
		random IO. If this option is given, fio will just get a
		new random offset without looking at past io history. This
		means that some blocks may not be read or written, and that
		some blocks may be read/written more than once. This option
		is mutually exclusive with verify= if and only if multiple
		blocksizes (via bsrange=) are used, since fio only tracks
		complete rewrites of blocks.

softrandommap	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.

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

prio=int	Set the io priority value of this job. Linux limits us to
		a positive value between 0 and 7, with 0 being the highest.
		See man ionice(1).

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

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

thinktime_spin=int
		Only valid if thinktime is set - pretend to spend CPU time
		doing something with the data received, before falling back
		to sleeping for the rest of the period specified by
		thinktime.

thinktime_blocks
		Only valid if thinktime is set - control how many blocks
		to issue, before waiting 'thinktime' usecs. If not set,
		defaults to 1 which will make fio wait 'thinktime' usecs
		after every block.

rate=int	Cap the bandwidth used by this job. The number is in bytes/sec,
		the normal postfix 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 seperation.

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 seperation.

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.

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 4MiB in size. So
		to calculate the number of huge pages you need for a given
		job file, add up the io depth of all jobs (normally one unless
		iodepth= is used) and multiply by the maximum bs set. Then
		divide that number by the huge page size. You can see the
		size of the huge pages in /proc/meminfo. If no huge pages
		are allocated by having a non-zero number in nr_hugepages,
		using mmaphuge or shmhuge will fail. Also see hugepage-size.

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

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

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

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

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

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

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.

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.

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.

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.

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

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

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

			sha512	Use sha512 as the checksum function.

			sha256	Use sha256 as the checksum function.

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

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

		This option can be used for repeated burn-in tests of a
		system to make sure that the written data is also
		correctly read back.

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

verify_offset=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=int	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. The verify_pattern cannot be larger than
		a 32-bit quantity.

verify_fatal=bool	Normally fio will keep checking the entire contents
		before quitting on a block verification failure. If this
		option is set, fio will exit the job on the first observed
		failure.
		
stonewall	Wait for preceeding jobs in the job file to exit, before
		starting this one. Can be used to insert serialization
		points in the job file. A stone wall also implies starting
		a new reporting group.

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

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

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

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

zonesize=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.

read_iolog=str	Open an iolog with the specified file name and replay the
		io patterns it contains. This can be used to store a
		workload and replay it sometime later. The iolog given
		may also be a blktrace binary file, which allows fio
		to replay a workload captured by blktrace. See blktrace
		for how to capture such logging data. For blktrace replay,
		the file needs to be turned into a blkparse binary data
		file first (blktrace <device> -d file_for_fio.bin).

write_bw_log=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_log_log for behaviour of given
		filename. For this option, the postfix is _bw.log.

write_lat_log=str Same as write_bw_log, except that this option stores io
		completion 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_clat.log and foo_slat.log.
		This helps fio_generate_plot fine the logs automatically.

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.

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

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

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

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

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

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

disable_clat=bool Disable measurements of completion 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_slat=bool Disable measurements of submission latency numbers. See
		disable_clat.

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

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.


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.
	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()
	V	Running, doing verification of written data.
E		Thread exited, not reaped by main thread yet.
_		Thread reaped.

The other values are fairly self explanatory - number of threads
currently running and doing io, rate of io since last check (read speed
listed first, then write speed), and the estimated completion percentage
and time for the running group. It's impossible to estimate runtime of
the following groups (if any).

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

Client1 (g=0): err= 0:
  write: io=    32MiB, bw=   666KiB/s, runt= 50320msec
    slat (msec): min=    0, max=  136, avg= 0.03, stdev= 1.92
    clat (msec): min=    0, max=  631, avg=48.50, stdev=86.82
    bw (KiB/s) : min=    0, max= 1196, per=51.00%, avg=664.02, stdev=681.68
  cpu        : usr=1.49%, sys=0.25%, ctx=7969, majf=0, minf=17
  IO depths    : 1=0.1%, 2=0.3%, 4=0.5%, 8=99.0%, 16=0.0%, 32=0.0%, >32=0.0%
     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
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.
	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=64MiB, aggrb=22178, minb=11355, maxb=11814, mint=2840msec, maxt=2955msec
  WRITE: io=64MiB, aggrb=1302, minb=666, maxb=669, mint=50093msec, maxt=50320msec

For each data direction, it prints:

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

And finally, the disk statistics are printed. They will look like this:

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

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

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


7.0 Terse output
----------------

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

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

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

Split up, the format is as follows:

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