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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, 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 ';', 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.
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.
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%
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.
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