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. If the ioengine is file based, you can specify a number of files by seperating the names with a ':' colon. So if you wanted a job to open /dev/sda and /dev/sdb as the two working files, you would use filename=/dev/sda:/dev/sdb opendir=str Tell fio to recursively add any file it can find in this directory and down the file system tree. 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. openfiles=int Number of files to keep open at the same time. Defaults to the same as nrfiles, can be set smaller to limit the number simultaneous opens. file_service_type=str Defines how fio decides which file from a job to service next. The following types are defined: random Just choose a file at random. roundrobin Round robin over open files. This is the default. The string can have a number appended, indicating how often to switch to a new file. So if option random:4 is given, fio will switch to a new random file after 4 ios have been issued. ioengine=str Defines how the job issues io to the file. The following types are defined: sync Basic read(2) or write(2) io. lseek(2) is used to position the io location. 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. cpu 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. external Prefix to specify loading an external IO engine object file. Append the engine filename, eg ioengine=external:/tmp/foo.o to load ioengine foo.o in /tmp. iodepth=int This defines how many io units to keep in flight against the file. The default is 1 for each file defined in this job, can be overridden with a larger value for higher concurrency. iodepth_batch=int This defines how many pieces of IO to submit at once. It defaults to the same as iodepth, but can be set lower if one so desires. iodepth_low=int The low water mark indicating when to start filling the queue again. Defaults to the same as iodepth, meaning that fio will attempt to keep the queue full at all times. If iodepth is set to eg 16 and iodepth_low is set to 4, then after fio has filled the queue of 16 requests, it will let the depth drain down to 4 before starting to fill it again. direct=bool If value is true, use non-buffered io. This is usually O_DIRECT. buffered=bool If value is true, use buffered io. This is the opposite of the 'direct' option. Defaults to true. offset=siint Start io at the given offset in the file. The data before the given offset will not be touched. This effectively caps the file size at real_size - offset. fsync=int If writing to a file, issue a sync of the dirty data for every number of blocks given. For example, if you give 32 as a parameter, fio will sync the file for every 32 writes issued. If fio is using non-buffered io, we may not sync the file. The exception is the sg io engine, which synchronizes the disk cache anyway. overwrite=bool If writing to a file, setup the file first and do overwrites. end_fsync=bool If true, fsync file contents when the job exits. fsync_on_close=bool If true, fio will fsync() a dirty file on close. This differs from end_fsync in that it will happen on every file close, not just at the end of the job. rwmixcycle=int Value in milliseconds describing how often to switch between reads and writes for a mixed workload. The default is 500 msecs. rwmixread=int How large a percentage of the mix should be reads. rwmixwrite=int How large a percentage of the mix should be writes. If both rwmixread and rwmixwrite is given and the values do not add up to 100%, the latter of the two will be used to override the first. norandommap Normally fio will cover every block of the file when doing random IO. If this option is given, fio will just get a new random offset without looking at past io history. This means that some blocks may not be read or written, and that some blocks may be read/written more than once. This option is mutually exclusive with verify= for that reason. 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. We regard that grouping of jobs as a specific group. group_reporting If 'numjobs' is set, it may be interesting to display statistics for the group as a whole instead of for each individual job. This is especially true of 'numjobs' is large, looking at individual thread/process output quickly becomes unwieldy. If 'group_reporting' is specified, fio will show the final report per-group instead of per-job. thread fio defaults to forking jobs, however if this option is given, fio will use pthread_create(3) to create threads instead. zonesize=siint Divide a file into zones of the specified size. See zoneskip. zoneskip=siint Skip the specified number of bytes when zonesize data has been read. The two zone options can be used to only do io on zones of a file. write_iolog=str Write the issued io patterns to the specified file. See read_iolog. read_iolog=str Open an iolog with the specified file name and replay the io patterns it contains. This can be used to store a workload and replay it sometime later. 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%, 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, 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, 20=12.8% means that 12.8% of the IO took more than 10 msecs, but less than (or equal to) 20 msecs. After each client has been listed, the group statistics are printed. They will look like this: Run status group 0 (all jobs): READ: io=64MiB, aggrb=22178, minb=11355, maxb=11814, mint=2840msec, maxt=2955msec WRITE: io=64MiB, aggrb=1302, minb=666, maxb=669, mint=50093msec, maxt=50320msec For each data direction, it prints: io= Number of megabytes io performed. aggrb= Aggregate bandwidth of threads in this group. minb= The minimum average bandwidth a thread saw. maxb= The maximum average bandwidth a thread saw. mint= The smallest runtime of the threads in that group. maxt= The longest runtime of the threads in that group. And finally, the disk statistics are printed. They will look like this: Disk stats (read/write): sda: ios=16398/16511, merge=30/162, ticks=6853/819634, in_queue=826487, util=100.00% Each value is printed for both reads and writes, with reads first. The numbers denote: ios= Number of ios performed by all groups. merge= Number of merges io the io scheduler. ticks= Number of ticks we kept the disk busy. io_queue= Total time spent in the disk queue. util= The disk utilization. A value of 100% means we kept the disk busy constantly, 50% would be a disk idling half of the time. 7.0 Terse output ---------------- For scripted usage where you typically want to generate tables or graphs of the results, fio can output the results in a semicolon separated format. The format is one long line of values, such as: client1;0;0;1906777;1090804;1790;0;0;0.000000;0.000000;0;0;0.000000;0.000000;929380;1152890;25.510151%;1078276.333333;128948.113404;0;0;0;0;0;0.000000;0.000000;0;0;0.000000;0.000000;0;0;0.000000%;0.000000;0.000000;100.000000%;0.000000%;324;100.0%;0.0%;0.0%;0.0%;0.0%;0.0%;0.0%;100.0%;0.0%;0.0%;0.0%;0.0%;0.0% ;0.0%;0.0%;0.0%;0.0%;0.0% Split up, the format is as follows: jobname, groupid, error READ status: KiB IO, bandwidth (KiB/sec), runtime (msec) Submission latency: min, max, mean, deviation Completion latency: min, max, mean, deviation Bw: min, max, aggregate percentage of total, mean, deviation WRITE status: KiB IO, bandwidth (KiB/sec), runtime (msec) Submission latency: min, max, mean, deviation Completion latency: min, max, mean, deviation Bw: min, max, aggregate percentage of total, mean, deviation CPU usage: user, system, context switches IO depths: <=1, 2, 4, 8, 16, 32, >=64 IO latencies: <=2, 4, 10, 20, 50, 100, 250, 500, 750, 1000, >=2000 Text description