Fix sync engine completion latency

[fio.git] / HOWTO

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

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


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

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

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

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

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

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

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

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

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

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

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


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

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

$ fio job_file

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

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

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


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

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

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

[job1]

[job2]

; -- end job file --

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

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


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

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

; -- end job file --

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

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

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


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

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

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

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

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

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

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

filename=str	Fio normally makes up a filename based on the job name,
		thread number, and file number. If you want to share
		files between threads in a job or several jobs, specify
		a filename for each of them to override the default. If
		the ioengine used is 'net', the filename is the host and
		port to connect to in the format of =host:port.

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.

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

size=siint	The total size of file io for this job. This may describe
		the size of the single file the job uses, or it may be
		divided between the number of files in the job. If the
		file already exists, the file size will be adjusted to this
		size if larger than the current file size. If this parameter
		is not given and the file exists, the file size will be used.

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

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=.

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.

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

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.

			libaio	Linux native asynchronous io.

			posixaio glibc posix asynchronous io.

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

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

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

			sg	SCSI generic sg v3 io. May either be
				synchronous using the SG_IO ioctl, or if
				the target is an sg character device
				we use read(2) and write(2) for asynchronous
				io.

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

ratemin=int	Tell fio to do whatever it can to maintain at least this
		bandwidth.

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. See man
		sched_setaffinity(2).

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

		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.

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.

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.

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

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

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

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

read_iolog=str	Open an iolog with the specified file name and replay the
		io patterns it contains. This can be used to store a
		workload and replay it sometime later.

write_bw_log	If given, write a bandwidth log of the jobs in this job
		file. Can be used to store data of the bandwidth of the
		jobs in their lifetime. The included fio_generate_plots
		script uses gnuplot to turn these text files into nice
		graphs.

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

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

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

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

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

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

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


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

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

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

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

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

The other values are fairly self explanatory - number of threads
currently running and doing io, rate of io since last check, 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
  IO depths    : 1=0.1%, 2=0.3%, 4=0.5%, 8=99.0%, 16=0.0%, 32=0.0%, >32=0.0%
     lat (msec): 2=1.6%, 4=0.0%, 8=3.2%, 16=12.8%, 32=38.4%, 64=24.8%, 128=15.2%
     lat (msec): 256=4.0%, 512=0.0%, 1024=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, dev 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.
	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.
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 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, 16=12.8% means that 12.8% of the IO
		took more than 8 msecs, but less than (or equal to) 16 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 comma separated format.
The format is one long line of values, such as:

client1,0,0,936,331,2894,0,0,0.000000,0.000000,1,170,22.115385,34.290410,16,714,84.252874%,366.500000,566.417819,3496,1237,2894,0,0,0.000000,0.000000,0,246,6.671625,21.436952,0,2534,55.465300%,1406.600000,2008.044216,0.000000%,0.431928%,1109

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