[PATCH] 'null' ioengine
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1Table of contents
2-----------------
3
41. Overview
52. How fio works
63. Running fio
74. Job file format
85. Detailed list of parameters
96. Normal output
107. Terse output
11
12
131.0 Overview and history
14------------------------
15fio was originally written to save me the hassle of writing special test
16case programs when I wanted to test a specific workload, either for
17performance reasons or to find/reproduce a bug. The process of writing
18such a test app can be tiresome, especially if you have to do it often.
19Hence I needed a tool that would be able to simulate a given io workload
20without resorting to writing a tailored test case again and again.
21
22A test work load is difficult to define, though. There can be any number
23of processes or threads involved, and they can each be using their own
24way of generating io. You could have someone dirtying large amounts of
25memory in an memory mapped file, or maybe several threads issuing
26reads using asynchronous io. fio needed to be flexible enough to
27simulate both of these cases, and many more.
28
292.0 How fio works
30-----------------
31The first step in getting fio to simulate a desired io workload, is
32writing a job file describing that specific setup. A job file may contain
33any number of threads and/or files - the typical contents of the job file
34is a global section defining shared parameters, and one or more job
35sections describing the jobs involved. When run, fio parses this file
36and sets everything up as described. If we break down a job from top to
37bottom, it contains the following basic parameters:
38
39 IO type Defines the io pattern issued to the file(s).
40 We may only be reading sequentially from this
41 file(s), or we may be writing randomly. Or even
42 mixing reads and writes, sequentially or randomly.
43
44 Block size In how large chunks are we issuing io? This may be
45 a single value, or it may describe a range of
46 block sizes.
47
48 IO size How much data are we going to be reading/writing.
49
50 IO engine How do we issue io? We could be memory mapping the
51 file, we could be using regular read/write, we
52 could be using splice, async io, or even
53 SG (SCSI generic sg).
54
6c219763 55 IO depth If the io engine is async, how large a queuing
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56 depth do we want to maintain?
57
58 IO type Should we be doing buffered io, or direct/raw io?
59
60 Num files How many files are we spreading the workload over.
61
62 Num threads How many threads or processes should we spread
63 this workload over.
64
65The above are the basic parameters defined for a workload, in addition
66there's a multitude of parameters that modify other aspects of how this
67job behaves.
68
69
703.0 Running fio
71---------------
72See the README file for command line parameters, there are only a few
73of them.
74
75Running fio is normally the easiest part - you just give it the job file
76(or job files) as parameters:
77
78$ fio job_file
79
80and it will start doing what the job_file tells it to do. You can give
81more than one job file on the command line, fio will serialize the running
82of those files. Internally that is the same as using the 'stonewall'
83parameter described the the parameter section.
84
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85If the job file contains only one job, you may as well just give the
86parameters on the command line. The command line parameters are identical
87to the job parameters, with a few extra that control global parameters
88(see README). For example, for the job file parameter iodepth=2, the
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89mirror command line option would be --iodepth 2 or --iodepth=2. You can
90also use the command line for giving more than one job entry. For each
91--name option that fio sees, it will start a new job with that name.
92Command line entries following a --name entry will apply to that job,
93until there are no more entries or a new --name entry is seen. This is
94similar to the job file options, where each option applies to the current
95job until a new [] job entry is seen.
b4692828 96
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97fio does not need to run as root, except if the files or devices specified
98in the job section requires that. Some other options may also be restricted,
6c219763 99such as memory locking, io scheduler switching, and decreasing the nice value.
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100
101
1024.0 Job file format
103-------------------
104As previously described, fio accepts one or more job files describing
105what it is supposed to do. The job file format is the classic ini file,
106where the names enclosed in [] brackets define the job name. You are free
107to use any ascii name you want, except 'global' which has special meaning.
108A global section sets defaults for the jobs described in that file. A job
109may override a global section parameter, and a job file may even have
110several global sections if so desired. A job is only affected by a global
111section residing above it. If the first character in a line is a ';', the
112entire line is discarded as a comment.
113
114So lets look at a really simple job file that define to threads, each
115randomly reading from a 128MiB file.
116
117; -- start job file --
118[global]
119rw=randread
120size=128m
121
122[job1]
123
124[job2]
125
126; -- end job file --
127
128As you can see, the job file sections themselves are empty as all the
129described parameters are shared. As no filename= option is given, fio
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130makes up a filename for each of the jobs as it sees fit. On the command
131line, this job would look as follows:
132
133$ fio --name=global --rw=randread --size=128m --name=job1 --name=job2
134
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135
136Lets look at an example that have a number of processes writing randomly
137to files.
138
139; -- start job file --
140[random-writers]
141ioengine=libaio
142iodepth=4
143rw=randwrite
144bs=32k
145direct=0
146size=64m
147numjobs=4
148
149; -- end job file --
150
151Here we have no global section, as we only have one job defined anyway.
152We want to use async io here, with a depth of 4 for each file. We also
153increased the buffer size used to 32KiB and define numjobs to 4 to
154fork 4 identical jobs. The result is 4 processes each randomly writing
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155to their own 64MiB file. Instead of using the above job file, you could
156have given the parameters on the command line. For this case, you would
157specify:
158
159$ fio --name=random-writers --ioengine=libaio --iodepth=4 --rw=randwrite --bs=32k --direct=0 --size=64m --numjobs=4
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160
161fio ships with a few example job files, you can also look there for
162inspiration.
163
164
1655.0 Detailed list of parameters
166-------------------------------
167
168This section describes in details each parameter associated with a job.
169Some parameters take an option of a given type, such as an integer or
170a string. The following types are used:
171
172str String. This is a sequence of alpha characters.
173int Integer. A whole number value, may be negative.
174siint SI integer. A whole number value, which may contain a postfix
175 describing the base of the number. Accepted postfixes are k/m/g,
6c219763 176 meaning kilo, mega, and giga. So if you want to specify 4096,
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177 you could either write out '4096' or just give 4k. The postfixes
178 signify base 2 values, so 1024 is 1k and 1024k is 1m and so on.
179bool Boolean. Usually parsed as an integer, however only defined for
180 true and false (1 and 0).
181irange Integer range with postfix. Allows value range to be given, such
182 as 1024-4096. Also see siint.
183
184With the above in mind, here follows the complete list of fio job
185parameters.
186
187name=str ASCII name of the job. This may be used to override the
188 name printed by fio for this job. Otherwise the job
c2b1e753 189 name is used. On the command line this parameter has the
6c219763 190 special purpose of also signaling the start of a new
c2b1e753 191 job.
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192
193directory=str Prefix filenames with this directory. Used to places files
194 in a different location than "./".
195
196filename=str Fio normally makes up a filename based on the job name,
197 thread number, and file number. If you want to share
198 files between threads in a job or several jobs, specify
199 a filename for each of them to override the default.
200
201rw=str Type of io pattern. Accepted values are:
202
203 read Sequential reads
204 write Sequential writes
205 randwrite Random writes
206 randread Random reads
207 rw Sequential mixed reads and writes
208 randrw Random mixed reads and writes
209
210 For the mixed io types, the default is to split them 50/50.
211 For certain types of io the result may still be skewed a bit,
212 since the speed may be different.
213
214size=siint The total size of file io for this job. This may describe
215 the size of the single file the job uses, or it may be
216 divided between the number of files in the job. If the
217 file already exists, the file size will be adjusted to this
218 size if larger than the current file size. If this parameter
219 is not given and the file exists, the file size will be used.
220
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221bs=siint The block size used for the io units. Defaults to 4k. Values
222 can be given for both read and writes. If a single siint is
223 given, it will apply to both. If a second siint is specified
224 after a comma, it will apply to writes only. In other words,
225 the format is either bs=read_and_write or bs=read,write.
226 bs=4k,8k will thus use 4k blocks for reads, and 8k blocks
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227 for writes. If you only wish to set the write size, you
228 can do so by passing an empty read size - bs=,8k will set
229 8k for writes and leave the read default value.
a00735e6 230
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231bsrange=irange Instead of giving a single block size, specify a range
232 and fio will mix the issued io block sizes. The issued
233 io unit will always be a multiple of the minimum value
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234 given (also see bs_unaligned). Applies to both reads and
235 writes, however a second range can be given after a comma.
236 See bs=.
a00735e6 237
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238bs_unaligned If this option is given, any byte size value within bsrange
239 may be used as a block range. This typically wont work with
240 direct IO, as that normally requires sector alignment.
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241
242nrfiles=int Number of files to use for this job. Defaults to 1.
243
244ioengine=str Defines how the job issues io to the file. The following
245 types are defined:
246
247 sync Basic read(2) or write(2) io. lseek(2) is
248 used to position the io location.
249
250 libaio Linux native asynchronous io.
251
252 posixaio glibc posix asynchronous io.
253
254 mmap File is memory mapped and data copied
255 to/from using memcpy(3).
256
257 splice splice(2) is used to transfer the data and
258 vmsplice(2) to transfer data from user
259 space to the kernel.
260
261 sg SCSI generic sg v3 io. May either be
6c219763 262 synchronous using the SG_IO ioctl, or if
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263 the target is an sg character device
264 we use read(2) and write(2) for asynchronous
265 io.
266
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267 null Doesn't transfer any data, just pretends
268 to. This is mainly used to exercise fio
269 itself and for debugging/testing purposes.
270
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271iodepth=int This defines how many io units to keep in flight against
272 the file. The default is 1 for each file defined in this
273 job, can be overridden with a larger value for higher
274 concurrency.
275
276direct=bool If value is true, use non-buffered io. This is usually
277 O_DIRECT. Defaults to true.
278
279offset=siint Start io at the given offset in the file. The data before
280 the given offset will not be touched. This effectively
281 caps the file size at real_size - offset.
282
283fsync=int If writing to a file, issue a sync of the dirty data
284 for every number of blocks given. For example, if you give
285 32 as a parameter, fio will sync the file for every 32
286 writes issued. If fio is using non-buffered io, we may
287 not sync the file. The exception is the sg io engine, which
6c219763 288 synchronizes the disk cache anyway.
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289
290overwrite=bool If writing to a file, setup the file first and do overwrites.
291
292end_fsync=bool If true, fsync file contents when the job exits.
293
6c219763 294rwmixcycle=int Value in milliseconds describing how often to switch between
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295 reads and writes for a mixed workload. The default is
296 500 msecs.
297
298rwmixread=int How large a percentage of the mix should be reads.
299
300rwmixwrite=int How large a percentage of the mix should be writes. If both
301 rwmixread and rwmixwrite is given and the values do not add
302 up to 100%, the latter of the two will be used to override
303 the first.
304
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305norandommap Normally fio will cover every block of the file when doing
306 random IO. If this option is given, fio will just get a
307 new random offset without looking at past io history. This
308 means that some blocks may not be read or written, and that
309 some blocks may be read/written more than once. This option
310 is mutually exclusive with verify= for that reason.
311
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312nice=int Run the job with the given nice value. See man nice(2).
313
314prio=int Set the io priority value of this job. Linux limits us to
315 a positive value between 0 and 7, with 0 being the highest.
316 See man ionice(1).
317
318prioclass=int Set the io priority class. See man ionice(1).
319
320thinktime=int Stall the job x microseconds after an io has completed before
321 issuing the next. May be used to simulate processing being
322 done by an application.
323
324rate=int Cap the bandwidth used by this job to this number of KiB/sec.
325
326ratemin=int Tell fio to do whatever it can to maintain at least this
327 bandwidth.
328
329ratecycle=int Average bandwidth for 'rate' and 'ratemin' over this number
6c219763 330 of milliseconds.
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331
332cpumask=int Set the CPU affinity of this job. The parameter given is a
333 bitmask of allowed CPU's the job may run on. See man
334 sched_setaffinity(2).
335
336startdelay=int Start this job the specified number of seconds after fio
337 has started. Only useful if the job file contains several
338 jobs, and you want to delay starting some jobs to a certain
339 time.
340
341timeout=int Tell fio to terminate processing after the specified number
342 of seconds. It can be quite hard to determine for how long
343 a specified job will run, so this parameter is handy to
344 cap the total runtime to a given time.
345
346invalidate=bool Invalidate the buffer/page cache parts for this file prior
347 to starting io. Defaults to true.
348
349sync=bool Use sync io for buffered writes. For the majority of the
350 io engines, this means using O_SYNC.
351
352mem=str Fio can use various types of memory as the io unit buffer.
353 The allowed values are:
354
355 malloc Use memory from malloc(3) as the buffers.
356
357 shm Use shared memory as the buffers. Allocated
358 through shmget(2).
359
360 mmap Use anonymous memory maps as the buffers.
361 Allocated through mmap(2).
362
363 The area allocated is a function of the maximum allowed
364 bs size for the job, multiplied by the io depth given.
365
366exitall When one job finishes, terminate the rest. The default is
367 to wait for each job to finish, sometimes that is not the
368 desired action.
369
370bwavgtime=int Average the calculated bandwidth over the given time. Value
6c219763 371 is specified in milliseconds.
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372
373create_serialize=bool If true, serialize the file creating for the jobs.
374 This may be handy to avoid interleaving of data
375 files, which may greatly depend on the filesystem
376 used and even the number of processors in the system.
377
378create_fsync=bool fsync the data file after creation. This is the
379 default.
380
381unlink Unlink the job files when done. fio defaults to doing this,
382 if it created the file itself.
383
384loops=int Run the specified number of iterations of this job. Used
385 to repeat the same workload a given number of times. Defaults
386 to 1.
387
388verify=str If writing to a file, fio can verify the file contents
389 after each iteration of the job. The allowed values are:
390
391 md5 Use an md5 sum of the data area and store
392 it in the header of each block.
393
394 crc32 Use a crc32 sum of the data area and store
395 it in the header of each block.
396
6c219763 397 This option can be used for repeated burn-in tests of a
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398 system to make sure that the written data is also
399 correctly read back.
400
401stonewall Wait for preceeding jobs in the job file to exit, before
402 starting this one. Can be used to insert serialization
403 points in the job file.
404
405numjobs=int Create the specified number of clones of this job. May be
406 used to setup a larger number of threads/processes doing
407 the same thing.
408
409thread fio defaults to forking jobs, however if this option is
410 given, fio will use pthread_create(3) to create threads
411 instead.
412
413zonesize=siint Divide a file into zones of the specified size. See zoneskip.
414
415zoneskip=siint Skip the specified number of bytes when zonesize data has
416 been read. The two zone options can be used to only do
417 io on zones of a file.
418
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419write_iolog=str Write the issued io patterns to the specified file. See
420 read_iolog.
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076efc7c 422read_iolog=str Open an iolog with the specified file name and replay the
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423 io patterns it contains. This can be used to store a
424 workload and replay it sometime later.
425
426write_bw_log If given, write a bandwidth log of the jobs in this job
427 file. Can be used to store data of the bandwidth of the
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428 jobs in their lifetime. The included fio_generate_plots
429 script uses gnuplot to turn these text files into nice
430 graphs.
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431
432write_lat_log Same as write_bw_log, except that this option stores io
433 completion latencies instead.
434
435lockmem=siint Pin down the specified amount of memory with mlock(2). Can
436 potentially be used instead of removing memory or booting
437 with less memory to simulate a smaller amount of memory.
438
439exec_prerun=str Before running this job, issue the command specified
440 through system(3).
441
442exec_postrun=str After the job completes, issue the command specified
443 though system(3).
444
445ioscheduler=str Attempt to switch the device hosting the file to the specified
446 io scheduler before running.
447
448cpuload=int If the job is a CPU cycle eater, attempt to use the specified
449 percentage of CPU cycles.
450
451cpuchunks=int If the job is a CPU cycle eater, split the load into
6c219763 452 cycles of the given time. In milliseconds.
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453
454
4556.0 Interpreting the output
456---------------------------
457
458fio spits out a lot of output. While running, fio will display the
459status of the jobs created. An example of that would be:
460
6043c579 461Threads running: 1: [_r] [24.79% done] [ 13509/ 8334 kb/s] [eta 00h:01m:31s]
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462
463The characters inside the square brackets denote the current status of
464each thread. The possible values (in typical life cycle order) are:
465
466Idle Run
467---- ---
468P Thread setup, but not started.
469C Thread created.
470I Thread initialized, waiting.
471 R Running, doing sequential reads.
472 r Running, doing random reads.
473 W Running, doing sequential writes.
474 w Running, doing random writes.
475 M Running, doing mixed sequential reads/writes.
476 m Running, doing mixed random reads/writes.
477 F Running, currently waiting for fsync()
478V Running, doing verification of written data.
479E Thread exited, not reaped by main thread yet.
480_ Thread reaped.
481
482The other values are fairly self explanatory - number of threads
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483currently running and doing io, rate of io since last check, and the estimated
484completion percentage and time for the running group. It's impossible to
485estimate runtime of the following groups (if any).
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486
487When fio is done (or interrupted by ctrl-c), it will show the data for
488each thread, group of threads, and disks in that order. For each data
489direction, the output looks like:
490
491Client1 (g=0): err= 0:
492 write: io= 32MiB, bw= 666KiB/s, runt= 50320msec
493 slat (msec): min= 0, max= 136, avg= 0.03, dev= 1.92
494 clat (msec): min= 0, max= 631, avg=48.50, dev=86.82
495 bw (KiB/s) : min= 0, max= 1196, per=51.00%, avg=664.02, dev=681.68
496 cpu : usr=1.49%, sys=0.25%, ctx=7969
497
498The client number is printed, along with the group id and error of that
499thread. Below is the io statistics, here for writes. In the order listed,
500they denote:
501
502io= Number of megabytes io performed
503bw= Average bandwidth rate
504runt= The runtime of that thread
505 slat= Submission latency (avg being the average, dev being the
506 standard deviation). This is the time it took to submit
507 the io. For sync io, the slat is really the completion
508 latency, since queue/complete is one operation there.
509 clat= Completion latency. Same names as slat, this denotes the
510 time from submission to completion of the io pieces. For
511 sync io, clat will usually be equal (or very close) to 0,
512 as the time from submit to complete is basically just
513 CPU time (io has already been done, see slat explanation).
514 bw= Bandwidth. Same names as the xlat stats, but also includes
515 an approximate percentage of total aggregate bandwidth
516 this thread received in this group. This last value is
517 only really useful if the threads in this group are on the
518 same disk, since they are then competing for disk access.
519cpu= CPU usage. User and system time, along with the number
520 of context switches this thread went through.
521
522After each client has been listed, the group statistics are printed. They
523will look like this:
524
525Run status group 0 (all jobs):
526 READ: io=64MiB, aggrb=22178, minb=11355, maxb=11814, mint=2840msec, maxt=2955msec
527 WRITE: io=64MiB, aggrb=1302, minb=666, maxb=669, mint=50093msec, maxt=50320msec
528
529For each data direction, it prints:
530
531io= Number of megabytes io performed.
532aggrb= Aggregate bandwidth of threads in this group.
533minb= The minimum average bandwidth a thread saw.
534maxb= The maximum average bandwidth a thread saw.
535mint= The smallest runtime of the threads in that group.
536maxt= The longest runtime of the threads in that group.
537
538And finally, the disk statistics are printed. They will look like this:
539
540Disk stats (read/write):
541 sda: ios=16398/16511, merge=30/162, ticks=6853/819634, in_queue=826487, util=100.00%
542
543Each value is printed for both reads and writes, with reads first. The
544numbers denote:
545
546ios= Number of ios performed by all groups.
547merge= Number of merges io the io scheduler.
548ticks= Number of ticks we kept the disk busy.
549io_queue= Total time spent in the disk queue.
550util= The disk utilization. A value of 100% means we kept the disk
551 busy constantly, 50% would be a disk idling half of the time.
552
553
5547.0 Terse output
555----------------
556
557For scripted usage where you typically want to generate tables or graphs
6c219763 558of the results, fio can output the results in a comma separated format.
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559The format is one long line of values, such as:
560
561client1,0,0,936,331,2894,0,0,0.000000,0.000000,1,170,22.115385,34.290410,16,714,84.252874%,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
562
563Split up, the format is as follows:
564
565 jobname, groupid, error
566 READ status:
567 KiB IO, bandwidth (KiB/sec), runtime (msec)
568 Submission latency: min, max, mean, deviation
569 Completion latency: min, max, mean, deviation
6c219763 570 Bw: min, max, aggregate percentage of total, mean, deviation
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571 WRITE status:
572 KiB IO, bandwidth (KiB/sec), runtime (msec)
573 Submission latency: min, max, mean, deviation
574 Completion latency: min, max, mean, deviation
6c219763 575 Bw: min, max, aggregate percentage of total, mean, deviation
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576 CPU usage: user, system, context switches
577