Skip to main content

Fio Output Explained


Previously, I blogged about setting up my benchmarking machine. Now that it's up and running, I've started exploring the fio benchmarking tool.

fio - the Flexible IO Tester is an application written by Jens Axboe, who may be better known as the maintainer of the Linux kernel's block IO subsystem. It resembles the older ffsb tool in a few ways, but doesn't seem to have any relation to it. As power tools go, it's capable of generating pretty much arbitrary load. The tradeoff is that it's difficult to learn and that's exactly what I've been doing.

Here's a section-by-section breakdown of the default output. I'll look at other output options in future posts. The data displayed is from a Samsung 840 Pro SSD.

The explanation for each section can be found below the output text.

  read : io=10240MB, bw=63317KB/s, iops=15829, runt=165607msec

The first line is pretty easy to read. fio did a total of 10GB of IO at 63.317MB/s for a total of 15829 IOPS (at the default 4k block size), and ran for 2 minutes and 45 seconds.

The first latency metric you'll see is the 'slat' or submission latency. It is pretty much what it sounds like, meaning "how long did it take to submit this IO to the kernel for processing?"

    slat (usec): min=3, max=335, avg= 9.73, stdev= 5.76

I originally thought that submission latency would be useless for tuning, but the numbers below changed my mind. 269usec or 1/4 of a millisecond seems to be noise, but check it out. I haven't tuned anything yet, so I suspect that changing the scheduler and telling the kernel it's not a rotating device will help.

Here are some more examples from the other devices:

    slat (usec): min=3, max=335, avg= 9.73, stdev= 5.76 (SATA SSD)
    slat (usec): min=5, max=68,  avg=26.21, stdev= 5.97 (SAS 7200)
    slat (usec): min=5, max=63,  avg=25.86, stdev= 6.12 (SATA 7200)
    slat (usec): min=3, max=269, avg= 9.78, stdev= 2.85 (SATA SSD)
    slat (usec): min=6, max=66,  avg=27.74, stdev= 6.12 (MDRAID0/SAS)

    clat (usec): min=1, max=18600, avg=51.29, stdev=16.79

Next up is completion latency. This is the time that passes between submission to the kernel and when the IO is complete, not including submission latency. In older versions of fio, this was the best metric for approximating application-level latency.

     lat (usec): min=44, max=18627, avg=61.33, stdev=17.91

From what I can see, the 'lat' metric is fairly new. It's not documented in the man page or docs. Looking at the C code, it seems that this metric starts the moment the IO struct is created in fio and is completed right after clat, making this the one that best represents what applications will experience. This is the one that I will graph.

    clat percentiles (usec):
     |  1.00th=[   42],  5.00th=[   45], 10.00th=[   45], 20.00th=[   46],
     | 30.00th=[   47], 40.00th=[   47], 50.00th=[   49], 60.00th=[   51],
     | 70.00th=[   53], 80.00th=[   56], 90.00th=[   60], 95.00th=[   67],
     | 99.00th=[   78], 99.50th=[   81], 99.90th=[   94], 99.95th=[  101],
     | 99.99th=[  112]

Completion latency percentiles are fairly self-explanatory and probably the most useful bit of info in the output. I looked at the source code and this is not slat + clat; it is tracked in its own struct.

The buckets are configurable in the config file. In the terse output, this is 20 fields of %f=%d;%f=%d;... which makes parsing more fun than it should be.

For comparison, here's the same section from a 7200 RPM SAS drive running the exact same load.

    clat percentiles (usec):
     |  1.00th=[ 3952],  5.00th=[ 5792], 10.00th=[ 7200], 20.00th=[ 8896],
     | 30.00th=[10304], 40.00th=[11456], 50.00th=[12608], 60.00th=[13760],
     | 70.00th=[15168], 80.00th=[16768], 90.00th=[18816], 95.00th=[20608],
     | 99.00th=[23424], 99.50th=[24192], 99.90th=[26752], 99.95th=[28032],
     | 99.99th=[30080]

    bw (KB  /s): min=52536, max=75504, per=67.14%, avg=63316.81, stdev=4057.09

Bandwidth is pretty self-explanatory except for the per= part. The docs say it's meant for testing a single device with multiple workloads, so you can see how much of the IO was consumed by each process. When fio is run against multiple devices, as I did for this output, it doesn't provide much meaning but is amusing when SSDs are mixed with spinning rust.

And here's the SAS drive again with 0.36% of the total IO out of 4 devices being tested.

    bw (KB  /s): min=   71, max=  251, per=0.36%, avg=154.84, stdev=18.29

    lat (usec) :   2= 0.01%,   4=0.01%,  10=0.01%,   20=0.01%, 50=51.41%
    lat (usec) : 100=48.53%, 250=0.06%, 500=0.01%, 1000=0.01%
    lat (msec) :   2= 0.01%,   4=0.01%,  10=0.01%,   20=0.01%

The latency distribution section took me a couple passes to understand. This is one series of metrics. Instead of using the same units for all three lines, the third line switches to milliseconds to keep the text width under control. Read the last line as 2000, 4000, 10,000, and 20,000usec and it makes more sense.

As this is a latency distribution, it's saying that 51.41% of requests took less than 50usec, 48.53% took less than 100usec and so on.

    lat (msec) : 4=1.07%, 10=27.04%, 20=65.43%, 50=6.46%, 100=0.01%

In case you were thinking of parsing this madness with a quick script, you might want to know that the lat section will omit entries and whole lines if there is no data. For example, the SAS drive I've been referencing didn't manage to do any IO faster than a millisecond, so this is the only line.

  cpu          : usr=5.32%, sys=21.95%, ctx=2829095, majf=0, minf=21

Here's the user/system CPU percentages followed by context switches then major and minor page faults. Since the test is configured to use direct IO, there should be very few page faults.

  IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%

Fio has an iodepth setting that controls how many IOs it issues to the OS at any given time. This is entirely application-side, meaning it is not the same thing as the device's IO queue. In this case, iodepth was set to 1 so the IO depth was always 1 100% of the time.

     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%

submit and complete represent the number of submitted IOs at a time by fio and the number completed at a time. In the case of the thrashing test used to generate this output, the iodepth is at the default value of 1, so 100% of IOs were submitted 1 at a time placing the results in the 1-4 bucket. Basically these only matter if iodepth is greater than 1.

These will get much more interesting when I get around to testing the various schedulers.

     issued    : total=r=2621440/w=0/d=0, short=r=0/w=0/d=0

The number of IOs issued. Something is weird here since this was a 50/50 read/write load, so there should have been an equal number of writes. I suspect having unified_rw_reporting enabled is making fio count all IOs as reads.

If you see short IOs in a direct IO test something has probably gone wrong. The reference I found in the Linux kernel indicates that this happens at EOF and likely end of device.

     latency   : target=0, window=0, percentile=100.00%, depth=1

Fio can be configured with a latency target, which will cause it to adjust throughput until it can consistently hit the configured latency target. I haven't messed with this much yet. In time or size-based tests, this line will always look the same. All four of these values represent the configuration settings latency_target, latency_window, latency_percentile, and iodepth.

Run status group 0 (all jobs):

fio supports grouping different tests for aggregation. For example, I can have one config for SSDs and HDDs mixed in the same file, but set up groups to report the IO separately. I'm not doing this for now, but future configs will need this functionality.

  MIXED: io=12497MB, aggrb=42653KB/s, minb=277KB/s, maxb=41711KB/s, mint=300000msec, maxt=300012msec

And finally, the total throughput and time. io= indicates the amount of IO done in total. It will be variable for timed tests and should match the size parameter for sized tests. aggrb is the aggregate bandwidth across all processes / devices. minb/maxb show minimum/maximum observed bandwidth. mint/maxt show the shortest & longest times for tests. Similar to the io= parameter, these should match the runtime parameter for time-based tests and will vary in size-based tests.

Since I ran this test with unified_rw_reporting enabled, we only see a line for MIXED. If it's disabled there will be separate lines for READ and WRITE.

Simple, right? I'll be spending a lot more time with fio for the next few weeks and will post more examples of configs, output, and graphing code.